Do Keto and Fasting Wreck Your Thyroid and Metabolism?

I’m kicking this post off with a disclosure: what you read today is heavy on speculation and light on evidence because…well…the evidence in this space is thin, at best. The fact that many of you read this blog for its scientific debunking of internet myths is not lost on me. But ever since receiving a reader’s email about effects of the ketogenic diet (KD) on the thyroid and basal metabolic rate (BMR), I just can’t get this issue out of my head. As a result, I’ve been spending wayyyyy too much time researching this issue, diving down all manner of rabbit holes. I’m hoping that laying out my thoughts on the blog will be cathartic, so I can get back to the work of writing about stuff that actually has clearer answers.

Below is a cut-up version of several emails I received from Clarice*, a reader with initial success on KD, who now is having problems with weight regain, hair loss, and “low” T3 levels. I’ll present her case study in chunks, so that we can address and digest the different points she raises:

I’ve eaten keto for nearly 2.5 years.  After 40 lbs weight loss and landing at a beautiful weight just with the diet change, I stayed there and maintained effortlessly for 6 months.  Then, with absolutely no change in diet after 1.5yrs, I crept up…pound, 2 pounds, 5 pounds, etc. The weight gain has been very stubborn and I have never been able to take a pound back off, and around this time also began general hair loss.  The keto ‘guru’ I now follow has a non-standard approach within the keto community and preaches that things like hair loss and weight regain happen because we low-carbers often undereat and ‘damage metabolism and hurt thyroid function’ by eating too few calories…then regain as per recent media discussions of Biggest Loser contestants regaining weight after low-calorie dieting.  I was probably consistently eating quite low-calorie (as keto is very appetite-suppressing and, hey, one only wants to eat so much steak and butter).

Weight Loss, Keto, and Your Metabolism

Let’s unpack a couple of things here. First, is it possible to damage one’s metabolism by eating too few calories? As with many things in Medicine, the answer is, “It’s complicated.” There are plenty of studies in the literature looking at the effects of starvation and hypocaloric diets on BMR. Overall, the evidence is fairly clear that BMR will drop, which is a whole lot of obvious. If you are not ingesting much of a calorie load, you don’t need to expend much energy to handle that load. Not only that, but you’ll be losing weight, so there will be less metabolically active tissue to support, resulting in a lower BMR. Plus, the body really hates to lose weight, so it will often ratchet your BMR down out of proportion to the reduction in the amount of lean tissue it is required to support. So far, I’m sure none of this is news to you.

But would you really call this “damaging” the metabolism, given that a lower BMR is a normal physiologic response to dieting and weight loss? I suppose if you’re gaining weight while trying to lose weight, you might consider a lower BMR to be damage, so let’s just stipulate that point as an issue of semantics and move on. The real question is: is the lower BMR permanent, i.e. are you screwed for the rest of your life because you consumed too few calories in your attempt to lose weight and get healthier?

The answer to that question appears to be: maybe. As there are plenty of bloggers who have written in detail about this issue, I will summarize. Most data I came across suggest that a lower BMR can return to its expected level over time – whether one is battling obesity, recovering from an eating disorder, or post-gastric bypass. However, “The Biggest Loser” study showed persistence of a lower-than-expected BMR in the contestants 6 years after wrapping the television show, despite significant weight regain.

Unfortunately, we don’t know why this suppression of BMR persisted. Some have speculated it was the rapidity of weight loss that shocked the participants’ bodies into permanently low-BMR territory; but, I read two randomized human studies that argue against this hypothesis – here and here. These trials showed that the rate of weight loss did not impact the likelihood of weight regain. Others think that there must have been hormonal or other metabolite abnormalities that account for the lower BMR, but none of the measured variables in the study bears that out (though the study wasn’t powered to detect changes of this nature). Bottom line: we just don’t know.

Getting back to Clarice, does she need to worry that her BMR is too low, resulting in weight regain? Perhaps. She has lost a fair amount of weight, which would result in a lowering of her BMR (again, that process of metabolic adaptation is normal). Let’s also assume that she adheres to KD strictly, meaning she’s in nutritional ketosis. If we buy into the theory that ketones (and dietary fats) are an excellent appetite suppressant, then she may not be ingesting enough calories to signal her body that it needs to raise BMR.

My coach has recently had the most success with ‘reverse dieting’ and keto, where he puts his clients on hypercaloric diets (high fat, low carb, just enough protein) and then drops them to their ‘TDEE’ calorie level and sees weight loss.  He advocates this as a better way to lose weight without sacrificing metabolism than calorie-cutting, with more lasting weight loss.

On his plan, I have upped calories (very hard to be consistent!), and have not been able to lose an ounce, but have also not gained.  Hair still coming out.

Reverse Dieting and Your Metabolism

I’ll admit, when I first read that her coach recommends eating more calories to lose weight, it raised my eyebrows. I don’t spend much time in the internet weight management space, so “reverse dieting” was a new term for me. In my world, where most people are not strictly following any healthy eating plan, eating more calories is a surefire path to weight gain.

When I looked at the medical literature, though, I discovered data showing that BMR can increase with an increased calorie load. This, of course, makes sense. What I haven’t been able to find is any non-anecdotal evidence that short-term increased caloric intake, followed by reversion to baseline intake, results in persistence of the higher BMR and weight loss.

If any of you employ this reverse-dieting technique regularly, I’m genuinely interested in your feedback. It seems to me that this would be a difficult needle to thread: you would need to increase calories just enough – for just long enough – to get the BMR up by enough to actually make a significant difference in overall energy expenditure. Then, you would have to cut calories back down to a level that results in enough of an energy deficit to lose weight. Then, as further metabolic adaptation occurs over time (BMR lowers again), you need to repeat the process – even though the targets you achieved last time may not work as well this time.

As a brief aside, those of you who have thrown the “calories-in, calories out” (CICO) model of obesity in the trash and replaced it with the insulin-carbohydrate model probably object to my focus on caloric intake at the end of the preceding paragraph. While I agree that the macronutrient composition of one’s diet is important, and I even buy into the idea that KD is one of the more effective extreme** eating strategies (sorry, vegans!), I don’t believe that “calories don’t matter.” If you consume an overabundance of calories compared to your output, even if they’re mostly from fat and protein, you’ll gain weight – simple. I honestly don’t understand how anyone can claim that calories don’t matter.

If my coach’s opinions about fasting are accurate, the LCHF (low-carb high-fat) community needs voices to explain the opposition opinion.  Fasting is so trendy in the keto community now, and many, many women I know are chronically undereating all the time–one meal a day,  and/or multiday fasts every week.  I wonder if they’ll eventually start regaining as I have. 

Intermittent Fasting Should be Intermittent

Intermittent Fasting (IF) is, indeed, gaining in popularity. I’ve even done a bit of Time Restricted Feeding (TRF) myself. Unlike Clarice’s keto guru, I think IF can be a useful tool in the weight management toolkit. Although clinical trials so far have not yielded impressive results compared to hypocaloric dieting, there is a suggestion that the weight lost with IF is mostly fat, whereas traditional diets lead to loss of both fat and muscle. Regardless of whether IF is a good thing to do, I suspect that Clarice’s coach and I would agree: if one chooses to do IF, it should be intermittent.

Why am I italicizing intermittent? I have noticed what seems to be a trend among IF enthusiasts – they eat progressively fewer meals. They may begin their IF journey with an eating window of 14-16 hours per day. Then, they move on to fasting until dinner a few days per week. Then, they’re fasting from dinner-dinner every day. Finally, they toss in a multi-day fast every month or so on top of that. This is purely speculation, but I think that too much fasting may be detrimental to BMR. It is my contention that this will ultimately cause many fasting enthusiasts to gain weight, particularly those who gradually move back towards a regular eating schedule, consuming more calories than they can burn.

Let me emphatically state that this is my opinion, and it is 100% possible that I am wrong. I am not the Yoda of obesity. And I’ve got news for you – neither is anyone else. That’s why there are scores of theories and hundreds if not thousands of books out there about The Way to lose weight – nobody has it all figured out.

So, my wonders about thyroid and the effect of the diet.  TSH and T4 for me are normal, but free T3 is very low, rT3 is high (I went down the alt. med. rabbit hole far enough that I asked for these tests).  Endocrinology says I’m fine, alt. med. says I’m severely hypo ;}  With no real fatigue, not feeling cold, etc. it doesn’t feel right that I am truly hypothyroid, but the hair loss and weight changes are VERY frustrating and stubborn.  So, I’ve been trying to figure it out.  My guru/coach does believe in the T3/rT3 stuff, and says he’s seen the ratio improve in his clients when they increase calories (even without increasing carbs).   After going to an Endo who dismissed T3/rT3 practically with an eye roll, I’ve been suspicious, which is why I found your articles so valuable. 

Keto and Your Thyroid

So now we’re getting down to the good stuff. Does keto really wreck your thyroid? If you read the internet, it’s littered with personal stories of low T3 and high reverse T3 (rT3) on KD. You know my position on the general utility of rT3 levels, but has anyone actually studied this phenomenon? It turns out the answer is, yes.

The role of dietary fat in peripheral thyroid hormone metabolism, a human study published in Metabolism in 1980, is a good representation of what I found about this topic. Instead of going through the study in excruciating detail here, I’ll simply highlight the salient results. First, subjects eating a 100% fat, 1500 calorie diet exhibited a big drop in T3 and a big increase in rT3. In fact, these changes were about equivalent to what we see during total starvation. Wow, right? Does this mean that the thyroid is literally starving – that it can’t use fat to function?

Before tackling that question…in a different study, an 800 calorie diet of 100% carbs didn’t show any change in T3 or rT3, whereas the isocaloric, no-carb subjects showed a significant drop in T3 (with no change in rT3). Isn’t that fascinating? Eating very few calories (800!), as long as those calories were from carbs, didn’t do a whit of damage to thyroid levels. But eating 800 calories on a no-carb diet? T3 tanked.

So far, it’s not looking too good for KD. Think about it: the 100% fat diet from the first study – containing twice the number of calories as the 100% carb diet from the second study – resulted in low T3, while that measly 800 calories of carbs allowed the thyroid to happily chug along. I realize we’re comparing subjects from different studies, which isn’t kosher, but I can tell you that I found several studies all showing fairly similar results.

Does Dietary Fat Lower T3, Even in the Presence of Carbs?

Now, here’s the really interesting part of the puzzle: going back to that first study, subjects eating 1500 calories comprised of 50% fat and 50% carbs showed a significant rise in rT3 with a non-statistically significant 24% drop in T3. Though the drop in T3 failed to reach significance, it seemed to be close, and keep in mind that the study was small.

Given that the second study showed no effect on T3 or rT3 with an 800 calorie all-carb diet, you might have expected the subjects eating 750 calories of carbs and 750 calories of fat to show the same thing. After all, don’t you just need a minimum number of carbs to keep the thyroid humming? Maybe not, because the presence of 50% fat in the diet appeared to cause a trend toward lower T3 levels.

So, does KD cause hypothyroidism? A 2017 study in children treated with KD for refractory epilepsy reported that 20/120 patients developed subclinical hypothyroidism (slightly elevated TSH, normal FT4, no symptoms). Unfortunately, this study had so many weaknesses that I’m not sure how much weight to give the findings. And it’s very difficult to argue that we can apply data from kids on antiepileptic drugs to healthy adults on KD. Nonetheless, it’s provocative that such a high percentage of their subjects developed thyroid dysfunction.

HD, we’re begging you…imploring you…does KD cause hypothyroidism or not?

The Ketogenic Diet does not Cause Hypothyroidism

You really want to know my opinion? I’m willing to share it with you, as long as you understand that this is my opinion, and I could be wrong. I agree with Dr. Stephen Phinney of Virta Health, who wrote a great, evidence-based post in 2017, postulating that low T3 in KD does not reflect hypothyroidism. You should read that, as I’m not going to rehash his entire argument here. What I will do, however, is propose an alternate explanation for the low T3 from the one Dr. Phinney proposed.

Dr. Phinney wrote that KD likely increases tissue sensitivity to T3, much like it increases insulin sensitivity. Therefore, it takes a lot less T3 to get the job done. This is certainly plausible – even probable – but I’d like to suggest an additional mechanism that may also play a role. Could it be that consuming a predominance of fat in the diet is simply more metabolically efficient than consuming a predominance of carbs and protein? After all, 1 gram of fat has twice the potential energy (9 cal) of 1 gram of carb or 1 gram of protein (4 cal).

I realize that there are a ridiculous number of inputs when it comes to how nutrients are metabolized, which is why we generally measure BMR and not individual cellular metabolic processes. But is it possible that, when it comes to thyroid hormone’s role in the metabolism of nutrients, it simply takes less T3 to liberate energy from fat than from carbs/protein? In addition, there may be other metabolic pathways that are more prominently involved in the breakdown of fats, while T3 could be a more important regulator of carb and protein metabolism (again, this is speculative on my part and I could be wrong).

Remember the study I mentioned earlier – the one showing a substantial (but not statistically significant) drop in T3 in the subjects on a 1500 calorie diet comprised of 50% fat and 50% carbs? The study duration was very short and the carbs were too high for the subjects to be in ketosis. Therefore, Dr. Phinney’s opinion about ketosis leading to enhanced T3 sensitivity would not hold up in this particular situation. But my theory – as simplistic as it is – would better explain the lab “abnormalities.”

It is my contention – presumably in agreement with Dr. Phinney – that the normal reference range for T3 was never designed to apply to people on KD. How could it, since most people don’t restrict carbs to that degree? The lower levels seen in these people are likely normal for people on KD, especially in the context of normal TSH, normal T4, and a lack of signs/symptoms of overt hypothyroidism. I really can’t stress that enough, by the way – if everyone on KD had hypothyroidism, we’d be seeing a lot more overtly hypothyroid people with numerous symptoms, and their TSHs would be high.

So, KD enthusiasts, enjoy your steak, your avocado, your cheese, and your full-fat yogurt. Let nary a crumb of baguette, a grain of rice, nor a slippery noodle pass your lips in the pursuit of higher T3 levels – you don’t need them.

*Clarice is a pseudonym.

**My good friend Dr. Scher, the Low Carb Cardiologist, hates when I use the “E” word (extreme) to describe the ketogenic diet. But I love to needle him, and I can say whatever I want on my blog, so…tough noogies, Bret.

I welcome your comments below, but note that I will moderate the Comments section more aggressively than usual. I’m not interested in litigating the validity of KD as a dietary strategy; we already did that in the Comments section of Ketogenic Diet – Diabetes Cure? Similarly, if you’re a proud, card-carrying vegan, please refrain from trying to persuade us that your way is the best way. I agree with you that vegetables, fruits, nuts, seeds, beans, and all the other good stuff that comes out of the earth is awesome – but it’s not quite enough to make a nutritionally complete diet. If it was, you wouldn’t have to take supplements like B12, omega-3’s, iron, etc. You may disagree with me, and that’s fine, but send me an email instead of making your points here. I want us to focus on how keto and fasting affect metabolism and thyroid function. Thanks to everyone for reading and commenting.

By reading this site and interacting with me in the Comments below, you agree to abide by my Disclaimer.

Image Credit: Photo by Toa Heftiba on Unsplash

71 Replies to “Do Keto and Fasting Wreck Your Thyroid and Metabolism?”

  1. Dear HD, I really enjoyed your talk at AACE in LA and I am now a regular reader of your blog. It helps me talking with patients, esp the rT3 issue. Thank you! And excuse any mistakes, I am German:)
    About low T3 on KD: the only information we have are serum levels of T3 – if we are not dealing with a Low T3 illness, that is, we have serious symptoms, I do not really worry about it, as long as TSH and T4 are normal. Since we know that T4 is deiodinized to T3 in the target tissues as needed and we do not have any lab information about T3 levels in different target tissues- the serum T3 level has NULL value whether KD causes low T3.
    As long as patients eat PUFAs as fat intake and high quality proteins, I prefer KD over vegan diets for weight loss, because vegan or even vegetarian doesn’t mean it’s healthy-we all know about veganism, but that’s religion, so I don’t even try to argue about it-People tend to consume too many carbs instead of fiber. My advice for weight loss is eat Mediterranean without the carbs.

    1. Thanks, Anette. +1 to your point about not being able to measure T3 in target tissues. I’ve made that point in my posts about T3 and rT3, but I may not have emphasized that enough in this post.

  2. As someone who just wants to be as healthy as possible for as long as possible and doesn’t have any actual problems (touch wood) to fix, I love your articles. I keep a vague eye on what and when I eat, nominally 8 hour eating window, nominally low-carb highish fat. (I struggle when I run out avocados, butter wants to be applied to carbs) I eat enough protein for gluconeogenesis to kick me out of ketosis fairly regularly. My personal experience is that messing with my insulin secretion ( frequency of eating and insulinogenic index of foods) leads to quicker significant weight changes while changing my energy input (+/- 1000 cal) makes me hot and tiggerish or cold and stupid but (apart from an initial change of about 1kg) doesn’t really change my weight. I have experimented a lot with lower cal, I did a month of eating 3500cal/day of my version of LCHF (eat meat, fish, eggs and anything green that grows above ground) and came out like half a kg heavier. It was by far the hardest diet I have ever done. I was never hungry, felt like gavage. Wow! I set out to ask a question and haven’t even got there yet. Sorry about the long-winded comment. Question; Have you encountered any studies that looked at the levels and effects of micronutrients in KD or LCHF adherents?

  3. Fascinating subject, HD! Though not inclined to diet for weight loss, I enjoyed one day a week 24-hour water fasting, and occasional one-week fasts. My endo told me that plays hell with Hashimoto’s so I gave it up. Intermittent fasting may not be such a drastic leap.

  4. I can’t believe you used the E word! (extreme). Shame on you HD! But I guess I will forgive you since you wrote a wonderful and thought provoking article. This is a very important topic for patients and clinicians without a strong body of evidence. Sometimes it is difficult to separate the effects of calorie restriction from carbohydrate restriction when they occur simultaneously. Another interesting question is what happens to T4 levels over time, once weight loss stabilizes, or BMR resets, or if calories gradually increase overt time? Would temporary thyroid replacement be beneficial? I don’t know the answer to these questions, but I sure hope you are the guy to help me figure them out! Thanks again for a great post.

    1. Thanks, Bret. As long as we are always looking at the T4 in the context of TSH, we should typically have a pretty good idea of whether a thyroid problem is brewing, but I get your point. However, I think if thyroid hormone levels are truly low, then it’s more a result of low BMR than the cause (my opinion).

      1. I seem to remember reading somewhere that giving thyroid supplementation in the context of low BMR/low T3/low calories increased muscle loss instead of fat loss. Does this sound plausible?

        1. If someone without true hypothyroidsm is treated with thyroid hormone in the context of low BMR/low T3, I would not be surprised to see a fair amount of muscle catabolism.

          1. So for someone from the alt-med world claiming they are tired, sluggish, hair falling out, low T3 and normal TSH (demanding thyroid meds) they might be on low-calorie or low carb diet perhaps?

    2. I am certain that something in the unrestricted US diet is damaging to metabolism. Does the KD wreck it even further? I am doubtful. Continuous bombardment of the metabolism with concentrated carbs and sugar which are impossible to find outside of a society with agriculture and food refining would be a more plausible theory in my opinion. Ketosis is a state one would expect humans to be in a lot more frequently as carbohydrate scarcity is more the rule when deprived of the luxury of agriculture. ALL of the people who resort to the “extreme” KD in my neck of the woods have been damaged by SAD already. BMR decreases in all people in the reduced state. Dr Ludwig showed that the KD resulted in a higher BMR than people on control diet at least during that study period. In the long run… I agree, we are operating in a data vacuum here.

  5. Interesting discussion here. I am in Dr. Phinney’s camp regarding the lack of necessity in fasting over 24 hours for the average person. I practice time-restricted feeding most days, mostly because I don’t have to consciously do it (comes natural)… I just eat when I am hungry, with first meal of day typically 2-3 hours post-morning workout. I have found that eating too LITTLE is not good for my body. In fact, I once took bloods during a protein sparing modified fast and the liver test results were shocking…after that experience, in an effort to strategically reduce body fat for a competition (from an already lean state), I increased my protein macros and reduced (slightly, not eliminating) my fat macros. This seems to have me dropping weight slowly, but I will check my body fat % in a couple of weeks to see if I have lost body fat, which is the target. Bottom line: I am not a fan of (true) IF too often, especially if you do NOT need to lose weight. I think we should eat when we are hungry and not peer pressure each other to “not be hungry.” I feel that the media glamorizes a contrived “silicon valley” culture which shuns eating pretty much altogether. I am quite pleased that after 4.5 years of LCHF, Keto, and initially losing 30 pounds, I can eat “whatever” I want (LCHF). My body tells me when and how much to eat, not the clock.

  6. Anecdotally on hashimoto’s support groups I have followed, both Keto and Intermediate Fasting are generally discouraged. Though some people really like them. People (with Hashimoto’s) who don’t feel good on Keto will sometimes feel better by adding some carbs. Maybe they won’t be in total Ketosis at that point though. The vast majority of Hashimoto’s patients follow more of a Paleo style diet. I suppose that’s pretty much what I follow, but I still have grains such as GF oats and rice. IF is discouraged because people with Hashimoto’s often have blood sugar regulation issues and supposedly that doesn’t help with that (probably skipping breakfast is why).

    As to low calorie diets, I personally find that is the worst thing for Hypothyroidism. In fact, I think that really set me back in my progress. I didn’t realize I was low calorie until I did a food diary for a dietitian. I was only getting around 1200 a day. When I was just starting to take Levo I adjusted my diet (based on a Endo’s suggestion and others) to cut out processed foods. However, most of the food didn’t have many calories. I’d imagine my body was slowing things down more to conserve energy and I was likely becoming nutrient deficient. I did lose about 20 pounds after starting levo and my diet…but I think my thyroid levels finally getting better is what helped (71 TSH when I was diagnosed).

    I now get around 2200 calories a day and that improved things quite a bit. I make sure to eat a lot of nuts, seeds, avocados, oatmeal, protein shakes (healthy ones), nut butters, etc. I’ve stabilized my weight and I am feeling better. But we are still adjusting my Synthroid dosages and slowly are getting things better. I seem to be very sensitive to dosage changes. I wonder if that’s a sign of anything (maybe a very damaged thyroid)?

    1. Mike, while I can’t speak to your sensitivity to dose changes, I can say that there are certainly some people who have a very narrow therapeutic window where they feel best.

      As for Hashimoto’s patients having “blood sugar issues,” that is a common myth. While people may feel as if they have “hypoglycemia,” the vast majority of them will not be hypoglycemic. Rather, they may have symptoms that feel like hypoglycemic symptoms but aren’t.

      1. Ahh. I did seem to have weird reactions to sugar and cutting out added sugar has helped me. I think when your thyroid levels are really bad your whole body is not working as it should.

        I think you are correct that my therapeutic window is super narrow. I think me and my endo are dialing it in. Because of it, I’m extremely diligent on taking my medicine the same and avoiding anything that may interfere with Synthroid. Eating one hour after etc. Ran into an issue with walnuts recently and remembered something on the Synthroid website on it.

      2. Rather, they may have symptoms that feel like hypoglycemic symptoms but aren’t.

        Those symptoms, what are their aetiology?


        1. I know for me there is no doubt my body was having trouble with sugar. My A1C didn’t show too bad (5.7), but I’d have these weird spikes of blood sugar levels. I believe cortisol and insulin are possible related origins of this. I never had these issues before. I believe trauma and stress accelerated my Hashimoto’s to show itself (though likely had it for many years). Early on when my TSH was really high 71, 17, 8 something as simple as a candy bar gave me diarrhea 20 minutes after having it. I had an eye hemorrhage right before my Hypothyroidism diagnosis. All of these symptoms lessened as I got this more under control and was more careful about regulating blood sugars. Even now, I had some weird cases where my blood sugar (I don’t test it often) would be high after a high carb meal 160 after 1 hr 170 after 2 and stayed high. But in the morning it was 87. My body just seems to be a bit confused. During those times I’d have insomnia issues and such. But when my thryoid levels are improving, I’m seeing less and less of these issues. At least from my experience, there seems to be a connection. My dad (who also has Hashis) has Type 2 diabetes and a lot of pain from Neuropothy. I’m hoping to avoid that by being much more careful. Maybe a good topic to cover is blood sugar and Hashis.

          1. Sorry…A1C was 5.4 back when I was in bad shape and is 5.2 now. But even with that, something was wrong with my body’s handling of sugars. I’m wondering if it’s related to your article on HI and IR. My weight is really good right now (154 and I’m a 41 year old male). But I was a bit overweight when my health went to poo (176). Interesting stuff. I’m wondering if my issues were HI (I had vertigo issues, numbness in fingers/toes (still have some), etc). Could explain why I’m feeling better cutting out processed sugars. But again, never had issues before all this thyroid craziness.

          2. Ok, I see what you’re saying. With your original comment about blood sugar and the thyroid, I thought you were talking about hypoglycemia. My mistake.

        2. It’s hard to answer that question, as it could be anything. Could be as simple as that’s the way that particular person’s body senses hunger, sending the signal that it’s time to eat.

  7. I can only share my experience with IF, keto, and CICO. I have Hashimoto’s, but my blood sugar levels have always been normal. I have lost 35 pounds twice, going to the bottom of my healthy weight range, with many smaller losses on various diets. I’m overweight now by 30-40 pounds, 50 over my lowest healthy BMI. This is mainly due to non-hunger-related overeating. I’m active and muscular.

    With IF, I fasted for 24 hours twice a week (Mondays and Thursdays) for three months. I lost 15 pounds. However, I suffered from feeling extremely cold, especially in the hands and feet, and foggy at the end of the day. I never had any problems like that following a CICO diet. One thing that concerns me is that after the diet, I regained some of the weight, but in different places than I had lost it. I’ve always been very pear-shaped, but I regained fat in my belly and arms. Combined with the cold feeling, I have wondered if IF changed my cortisol levels? This was around the time I turned 30, so it could also just be age.

    I did a ketogenic diet for six months when I was 19. I lost 35 pounds, but I never felt well. I was extremely tired. I felt hungry in a weak, low-energy way, even when my stomach was full. I took up running in that time, and after trying every other day for six months, I couldn’t even run a mile. The day I went off the diet, I had a granola bar. I ran a mile and a half effortlessly. It was wonderful.

    Of all the diets I’ve tried, CICO is by far the easiest for me and was just as succcessful. I am not a big fan of sweets, but I have come to realise I’m a starch-driven animal. Potatoes, oatmeal, whole-grain toast– these are always the foods that give me the most sateity and energy. I don’t know if it has anything to do with my thyroid, but I do know I wouldn’t try keto or IF again, because the side effects are just way too extreme. CICO is a pain in terms of tracking and watching macros, but I feel normal doing it.

    1. Thanks for sharing a real-world, non-rosy perspective about KD and IF. I think it’s helpful for people to hear stories from others who tried these strategies and didn’t like them.

  8. HD I’m really enjoying your blog and it’s pulling back from the clutches of alt-Med 🙂

    I thought I’d chip in with my keto experience. I’ve had a total thyroidectomy so maybe not applicable to everyone.

    I lost weight doing quite low carb (I didn’t exclude starches veges) but also it was quite low calorie. When I naturally started to go into ketosis (huh what’s this funny smell in my pee?) I started down that rabbit hole. I found reducing my carbs further and going into ketosis brought on hypo symptoms (cold, dry skin, weight loss stalled then weight started creeping back on). The more it stalled the deeper I dug and I started thinking about IF etc etc.

    Eventually I decided I need to stop as it was getting awful and my thyroid labs showed increased TSH (from .31 to 3.18) and T4 dropped a little (same dose of levo) and T3 plummeted.

    So when I stopped and ate “normally” I regained pretty much everything which was sad.

    Anyway sorry for the wall of text I wonder if there was anything interesting in there. 🙂

    1. It is really interesting to hear about thyroid function test changes in someone doing low-carb/fasting, who is reliant on thyroid hormone replacement (no intact thyroid gland). Of course, there are many reasons why the TSH can change (different absorption of thyroid hormone, etc) and different reasons for the appearance of “hypothyroid” symptoms, but it is still provocative nonetheless. Thanks for sharing.

      1. Hi – I stumbled on this fabulous blog in search for answers to my exact same issue as this commentator “Emily”. I too have had a total thyroidectomy, been doing IF for over 2 years – found it to have amazing amazing benefits in the first year such as reducing inflammation and getting rid of all my body aches (starting was so so difficult though) only to have the weight slowly creep back on and the RT3 increase and T3 decrease.
        My dr increased my T4 thyroxine from 100 to 150 I’ve been dropped back down again to 100 and trying to loose the weight I regained – still doing IF but I noticed something strange and I wonder if it has something to do with what your saying about those different types of diets. About 3 months ago I started experimenting with carbs – I started eating Pasta again and I noticed the day after I ate a bowl of pasta – I lost aprox 0.5 – 1kgs ???
        I was wondering if perhaps the key is not to completely cut out carbs????
        I do a OMAD fast I found it was the only one that worked for me and eat predominantly veges, red meat every now and then (once a month) but mostly fish 2-3 nights and then having pasta once a fortnight.

        Am I wrong in doing this? What is this?? Am I imagining it?

        I also noticed that if I eat tooo much of the pasta on that night I can feel bloated the next day so there’s a fine line between how much carbs I can eat.

        And since doing the IF I also noticed that if I eat predominantly protein such as chicken or red meat – I stack on the weight. The veges with alternate days of pasta seemed to make me loose????



        1. I can’t really address your situation, but I can tell people that our weight often fluctuates by at least a couple of pounds from day to day, partly just due to fluid shifts. I’d be hesitant to draw any conclusions from a data point like the one you describe after a night of pasta.

  9. Hi there.

    Thanks for the post. In case you’re interested, I’ve also written about the keto/thyroid/T3 issue, with similar conclusions to Phinney, but my article focuses more on the function of lowered T3:

    This year, I presented at the Boulder Carnivore Conference on the topic of RDAs and how some might be skewed, because they assume Extreme, grain-based diets. I delved into thyroid a bit more in relation to potentially reduced iodine needs on low carb. That’s here:


    1. Amber, thanks for the link to your article about keto and T3. I think much of what you wrote makes a lot of sense. It would be interesting to see T3 data from two cohorts of people: those losing weight on keto vs those losing weight on a mixed, calorie-restricted diet. If the folks eating plenty of carbs still have normal T3, as I suspect they might based on data I cited in the post, then that would argue against the low T3 in keto having a whole lot to do with the muscle-sparing effect during weight loss that you discussed.

      Also, just wanted to clarify: in one of your green boxes, you offered the opinion that it doesn’t make sense to give thyroid hormone to a certain group of people; I assume you meant people with non-thyroidal illness syndrome, not people with primary hypothyroidism? People with primary (or secondary) hypothyroidism really do need thyroid hormone; their situation is not analogous to type 2 diabetes.

  10. Thanks for a very thoughtful post with measured conclusions & crystal clarity about when you were speculating.

    You wrote: I don’t believe that “calories don’t matter.”
    I believe the insulin hypothesis isn’t that calories don’t matter but that calories out is a DEPENDENT variable, and since body fat can’t be preserved without insulin (T1 diabetics wasted away before synthetic insulin regardless of consumption), calories taken in without an increase in the basal insulin level won’t be retained. Whether the body runs hotter, RMR is raised, NEAT increases, dumping in the stool increases, or all of these, the body will find a way to discard calories that the body is unable to retain.

    Surely, I believe any sensible person should have no quarrel with the first law of thermodynamics, but a scientific discussion also needs to include the biology of open systems & metabolism. Given the overfeeding experiments by Sam Feltham and others, it seems pretty clear that consuming excess fats without gaining weight is a real phenomenon. I did my own overfeeding experiment in May and gained no weight adding 42,000 calories to my normal level of eating over a 3-week period, but in a scientific manner designed to minimize insulin levels.

    This is a repeatable N=1 experiment for anyone who doubts it. Add loads of animal fat trimmings and/or heavy cream and/or butter to your diet and see if you gain weight. [While it should also work drinking oils, the health risk of such an approach is too scary for me to contemplate]. Science advances by experimentation.

    1. Totally agree with your points. I will say that Dr. Scher had a rather prominent voice in the LCHF community on his podcast a while back, and that person clearly stated, “Calories don’t matter.” I have seen other bloggers/commenters say similar things, so I did want to call out their hyperbole.

      Also, I think there are many people who will try your experiment and gain weight, suggesting that there are other mechanisms at play in chronically overweight individuals trying to lose weight that thwart these attempts to lose.

      1. Oh, I have no doubt that many supporters of the insulin hypothesis say “calories don’t matter.” I was just noting that this isn’t a reasonable summary of the hypothesis & you and I clearly agree about that.

        Similarly, if someone just read my last paragraph & assumed that adding heavy cream to carb-heavy meals wouldn’t cause weight gain, they are missing the context (or I am guilty in not providing it). I mentioned Sam Feltham because he did THREE overfeeding experiments, creating low, high & medium insulin states, respectively, & got 3 radically different results while consuming the same number of calories in each & not changing intentional exercise. I did say that the manner in which I added those 42,000 calories was scientific (I followed the principles of food order, frequency & speed discussed by Gabor Erdosi in his outstanding 2018 Prague speech (

        You’re speculating that other people upping fat consumption would gain body fat rather than pointing to people who have actually done so. Perhaps, & it would be ridiculous for me to deny that there might be exceptional cases. But I think the insulin hypothesis, by and large, holds up to actual testing. The heartbreaking attempts of pre-1921 T1 diabetics to avoid wasting away show that retaining body fat without insulin is pretty much hopeless. And I’m not the only guy to try getting fat with fat & not succeeding. Sumo wrestlers do it with rice & beer, not fat trimmings & cream, for a reason.

        1. Thanks for the links. I will have to check out Sam and Gabor’s stuff, as I am not familiar with them.

          As for type 1 diabetics, I am not sure we can point to that as a good example of your position, since untreated T1 DM patients have extremely high blood sugar that they can’t utilize for energy. They just urinate out all those calories. So of course they have to break down every other kind of tissue they can find to use for their metabolic needs.

          This situation is very different from talking about people who have normal BG, who are eating fat to attempt weight control.

          But overall, I do not disagree that the insulin hypothesis has plenty of merit.

          1. Your point on T1 is well taken: not enough by itself, though suggestive And to be clear, I’m not suggesting overeating fat lowers weight, except if it is reducing consumption of insulin-spiking foods. LCHF is clearly a strategy showing enormous promise but we should never stop looking for contrary evidence to our hypotheses.

  11. I can’t recall where I heard it but I thought T3 was needed for carb metabolism. Fle that under vaguely useful, (or not).

    I’m suspicious of a diet that 1) is taught by a “guru” 2) has an abundance of manufactured technical terms such as “reverse dieting”, 3 ) requires belief in several counter-intuitive mechanisms and 4) needs an “alt. med.” practitioner to identify the problems said diet will fix.

    If I was so spun around I might be PULLING my hair out.

    I’m just a regular guy, but I think Clarice might benefit from a simpler diet/lifestyle based on low carb, seasonal whole foods she knows and likes. At least then there would be fewer variables to consider regarding the hair loss.

    Finally, regarding calories, I have always wondered where and when exactly the body is supposed to be counting them, how it determines when a surplus or deficit exists, and how it might turn a systemic surplus into localized fat (or turn fat into calories!) Rather, and forgive my bluntness, it seems to me when one speaks of calories they are not speaking of biology or biochemistry, which as far as I can tell means they are not saying much about metabolism at all.

    Thank-you for your thoughtful blog post.

    1. Thanks for your thoughtful comment. Yes, T3 appears to play a significant role in carb metabolism. It is not clear to me if it plays an equally important role in fat metabolism, though I suspect maybe not based on things I cite in the post.

  12. Hi. I am very glad to found your website. It is the first time I see and endo explaining why fT3 and rT3 are not useful and writing about what alt med says. I think you are doing great about the question alt med x mainstream med like you said it is necessary.
    I’ve read a lot of stuff about fT3 and rT3 in alt med blogs and it seems to make sense to me and after reading your posts I finally got it why endo generaly do not look at these markers.

    My experience is LCHF for 4.5 years. I’m lean, lift weight and do HITT one/wk. Right now i’m really impressed how easy it is to maintain weight eating LCHF. I usually have to eat more than what satisfies me to not lose weight.

    But I can’t say i don’t have questions about LCHF. Right now i am reading a lot trying to find answers:
    1) My fast insulin is 1.8 (altough I eat sweet potatos daily) and I wonder if that low insulin could be bad for thyroid. Since i am not having issues with weight gaining i usually think it is ok (and good) and your text seems to agree with it, but last month I get TSH 2.5 from 1.5 (1.3 – 1.7) that I got it from old tests. I know TSH 2.5 is ok, but I wonder if for each person individually an elevation in TSH from the usual values could mean something. I mean: if you always have TSH around some value and then you got a higher value that could be something to look deeper even if TSH is still in normal range?
    I’m having dificult recovery from lifting and some digestive issues, but these are not specific symptoms.

    2) Reading alt meds blogs you find Wilson’s syndrome. So, “let’s check your temperature as soon as you wake up in the morning”, they say, “forget TSH, fT4”, exactly what you wrote in other text about alt med. It should be at least 97,6 F. Mine is 95,4 F (I measured for 5 days). I am lost in that question. That temperature measure means something? Wilson’s syndrome is real or is not science? If you could write about it would be great.

    3) Technically speaking my LDL is a bit high, 150. Years ago i had low LDL, then it wnt up and when LCHF it went up a bit more. Everything else is great. I read about hypo and high cholesterol and I’m trying to figure out that LDL, read about LDL receptor, but I never read anything good about this hypo high cholesterol link. Does this alter HDL too? Don’t know.

    Well, I brought many questions… hoping someday you write about these.

    1. Although I can’t speak to your situation directly, I can say a few things, in general, about your numbered points above:

      1. I don’t think that anyone needs to worry about “low” insulin levels being bad for the thyroid. Also, TSH varies to some degree by time of day, and it may also exhibit some variability from day to day, even at the same time. I discuss some of the subtleties of how to interpret TSH here.

      2. Wilson’s syndrome is not real. In addition, the “normal” body temperature is very different for different people, and there is a pretty wide range for what’s normal. It is an exercise in futility to monitor body temperature for the purpose of trying to diagnose a thyroid problem – with the possible exception of someone who is clearly hypothermic in a comatose state induced by severe, untreated hypothyroidism (this does not apply to anyone who is reading this right now).

      3. Untreated hypothyroidism is associated with high cholesterol, but most high cholesterol has nothing to do with the thyroid, especially in someone eating LCHF.

      1. Thanks for your attention!

        I was guessing that Wilson’s syndrome is not real, but only a way of alt med overtreat patients.

        About higher LDL, I think that it is now the major critic to KD or LCHF since there is no evidence showing is harmless like a lot of guys believe and it seems that increase is much more common than LCHF community says it is. It is still a bet. I’m trying to figure out, stepping back on some fats and adding some healthy carbs and fiber, think that low insulin and exercise offers me good space to add carbs.

        I read your article about selenium. Never worried about it since I always eat some fish and brazil nuts. Other mineral that is now maybe overhyped is iodine in alt meds sites. “Japanese eat a lot of iodine cause they eat a lot algaes etc and they have very low thyroid issues, iodine in salt is very low etc”. I think a article about iodine like the one you wrote about selenium could clear a lot of stuff. Just a humble idea of course.

        1. Regarding the LDL issue on KD, I agree that we need to not be cavalier about dismissing its importance. There is nowhere near enough evidence to do that. On the other hand, let’s say a KD enthusiast has an LDL that jumps from 100 to 150. But hsCRP, ESR, triglycerides, and insulin are all low and HDL is high. And let’s say the coronary calcium score is 0 at baseline and stays at 0 over time. Then it might be reasonable for that person to subscribe to the argument that “LDL is necessary but not sufficient” in the pathophysiology of obstructive coronary disease. But for a whole bunch of people with high LDL and no personal longitudinal data to say, “Forget it, I’m not gonna worry about it…” that doesn’t seem wise to me.

          As for the iodine issue in thyroid disease, I did have a section about this in You Can’t Eat for Your Thyroid.

  13. Is it possible to reverse late stage Hashimoto’s disease? Can the thyroid repair itself after 50? Is 175 mcg too much for 140 pound woman? Ive been talking it for years and began KD this year.

    1. I can’t speak to your situation, but I can say that there is no known treatment to reverse Hashimoto’s. Although the typical formula for estimation of one’s levothyroxine requirement is 1.6 mcg/kg, there are plenty of people who will need more or less than what their calculated estimate says.

  14. I’ve come across this article and am in need of an answer. I’ve been on the keto diet for about a half year now, I’ve lost 22 pounds putting me at 146. I’ve been having blood work done for sometime for RA. Recently my doctor did blood work on my thyroid and it was very low and said I have hypothyroidism. He prescribed me 75mg of synthyroid ever since I’ve started the medication I’ve gained 2 pounds and can’t seem to loose anything I’ve done everything. I have yet to follow up with blood work with my doctor to make sure the dosage is correct. I’m not sure if I had hypothyroidism before keto or the diet has just caused my T3 to drop. But is there any advice you could give me!? Should I take synthyroid or not .. should I do keto or a calorie deflect

  15. absolutely desperate to increase body temperature I hate feeling cold all the time it’s all I think about all day for the past decade and completely ruining my life.

  16. I’ve been hypo for 40 years. I’m a 63 year old woman. My endo has me on Synthroid(T4) only. My T3 is on the lowest end of the normal scale and dropping, same with my TSH and my RT3 is higher then the highest range for”normal.” I was following KD and lost a few pounds after 6 months(8 pounds). My blood tests suggested I was malnutritioned even though I ate 1500-1800 calories daily on KD. I’m 5’1″ tall, 170 pounds. So, what I’m getting from all this info you graciously supplied to us, is that T3 and RT3 do not matter on a KD diet. However, I didn’t get results like many have with the weighloss. T4 binds with T3 to rev metabolism so I’m still confused what to do to lose weight. Isn’t RT3 stressful on one’s body too? Also, AC1 was 100 and endo says I’m not diabetic. I have read so much on all this for years and I still feel lost. Can you help please? I so want to lose weight, desperate.

    1. The A1C range is from less than 5.7 being normal and above 6.5 being diabetic, it does not go up to 100.

      I believe you mean your Blood Sugar level was 100, which is a normal fasting blood sugar, the normal blood sugar range goes up to 125.

      Calories are not an indication of nutritional status, you can eat lots of calories and be malnourished because you are eating processed calories with no nutritional value.

      In addition, being female our hormones make it harder to lose weight than losing weight is for men. If our fat level gets too low, it lowers fertility and the ability to maintain a pregnancy, so the female body wants to hold onto fat like it is precious gold.

      Also, the human body uses fat to make our various hormones, so again it wants to maintain body fat to have a supply of fat to make hormones as needed.

      I have found a good diet of protein (chicken, turkey, beef, bison), green vegetables (no spinach which is too high in oxalates), lower carb veggies under a glycemic index of 50, and about 1/2 tablespoon of good fats (virgin coconut oil, and extra virgin olive oil) at each meal works for me. I eat no grain other than some rice twice a week, plus gluten-free pasta made with red lentils or chickpeas three times a week (I have Celiac Disease, but am also allergic to corn and react to oats also, so I avoid any grain higher in proline. This leaves only rice for me to eat.)

      I hate fish and do not eat it, but all fish has mercury in it and the government recommends eating fish no more than twice a week, only once every two weeks if it is a higher mercury fish like swordfish, and once a week for white tuna. But if you like fish, once a week is fairly safe.

      I also found weighing my food helps me lose weight. When I was eyeballing portions thinking they were servings, by weighing the food I found I was actually eating double or even triple servings. So weighing food is very important.

      Americans are so used to huge restaurant portions, we are actually overeating enough food for two other people in most cases.

      I bought a simple electronic scale that will measure either ounces/pounds or grams/kilograms with a flick of a switch.

      The scale is the item that helped me finally start losing weight, if I place a dish on the scale before measuring my food, the scale does not add the dish weight, the scale starts at zero.

      Hope this helps, good luck and have patience.

  17. I have Hashimoto’s & I tried the KD religiously for 21 days & didn’t drop a lb or inches. I’m convinced(for me anyway) it’s not the way to go, so I think I’ll concentrate on eating whole foods, exercising, & taking my supplements.

  18. I am Hypothyroid. I take 225mcg daily. I am severely obese. I have decided to do the Keto Diet. Should I be careful to watch my levels and may have to have my medications adjusted? I read through your article but find it overwhelming.

    1. If you are severely obese, say 300plus pounds, you need to talk to your physician about the diet and any testing needed.

      Safe mild exercise like walking is helpful, so is a accurate scale to weigh food so you know you are eating a serving of food. Eyeballing or estimating food servings in studies, shows people usually are eating two servings when they think it is a proper single serving.

      But when you are so overweight and/ or have a medical condition, then consulting with your own physician is the first step to take.

  19. Since 2010 i been experimenting with intermittent fasting, i also did keto low carb etc…. the first thing i notice was huge hair loss.
    That started in 2013 after my ex left me i lost 40 pounds doing fasting and some keto, i never though it was the diet genetics but no one in my family sheds there hair…… i was also 41 a bit late to start shedding.

    I got tested a few months later testosterone, thyroid, ferritine etc… and all my levels where low i was borderline anemic…..

    This year better late then never i dropped everything, even the fat burners, caffeine over dosing etc to lose weight, because like you even with the fasting fat burner and low calorie i was gaining instead of losing.

    I am done with this, i am eating a nutritional plan now dairies are back in milk 2 cups after training, creatine 5g per day and collagen 10g multi vitamin all i take has supplements.

    I eat red meat i eat more protein and i train strong lifts power lifting style. At 47 my values are back up my body temperature is back to a normal 37 degree (98.5), i was down to around 35 degree Celsius at some point. My testosterone is back to 780 i always had high t level and i started losing weight slowly but surely.

    I eat around 2400 calories fruits smoothies potatoes carbs are back in…. no more fad diets.

  20. Hi! To begin, appreciate the information. It really is challenging for me to stick to diet programs… but I’m trying to do better and begin Intermittent Fasting.

    I have had my problems with pounds for a long time, but I believe I’m actually ready to fix that. I’m going to do intermittent fasting, and I’m going to do it RIGHT – beginning with proper research.

    Have you checked out the Eat Stop eat intermittent fasting e-book yet? I learned about it on another website (Google “PushPedalCrank Eat Stop Eat” if you’re curious) and it sounded GREAT in her review, but I wanted a second opinion before I buy it.

    Have you seen that/checked it out? If so, How did you like it?It sounds like a excellent book to buy from the review, but I’m still on the fence.

    I just require a great, top quality researched guide on the methodology of Intermittent Fasting – I think doing that kind of research would help me stick with it when elements start to get challenging, or when sweets start to get tempting.

    Thanks yet again for the information! Hope your health and goals are going great.

    Xoxo J.J.

  21. Thank very much for your amazing blog. I’ve been fasting and lowcarbing for 3 years. In this period, I lost 50 lbs, lowered insulin from 16 to 6, lowered HOMA-IR from 2.7 to 1.5, Hba1c from 6.1% to 5.5% and got rid of hypo- so that my Endo stopped me from T3, but I’m still obese (no Hashi). However, my benign thyroid nodules are getting a bit bigger year by year. I was told that thyroid function has nothing to do with thyroid nodules. Is this true? There are no mentions to nodules and fasting or KD, why?

    1. First, let me just congratulate you on your progress – well done! Second, generally speaking, it is true that thyroid function is a separate issue from thyroid nodules. While there are some caveats to that (like so-called “hot” nodules), the caveats are irrelevant if we’re focusing on people who have nodules and a history of an underactive thyroid. Thyroid nodules have a fair chance of growing, and there haven’t been any interventions reliably shown to impede that growth.

      As for intermittent fasting and low-carb eating, I wouldn’t expect either of those interventions to have any effect on the growth of thyroid nodules. To the best of my knowledge, there are no data exploring this, and I don’t expect there ever will be, unless someone can postulate a plausible mechanism for why IF or keto would affect nodule growth.

  22. Dear HD,

    If a patient has good levels of ft4 and a low normal TSH, is it possible that his TSH will rise when the patient tries to lose weight with calorie-restrictive dieting? Wouldn’t this patient possibly get a prescription for too high of a levothyroxine dose by a doctor because his TSH indicates leeway to rise the dose?

    Maybe it’d be necessary to tell a doc if you’re fasting/dieting to avoid being put on too high of a dose?

    1. According to this PubMed article peripheral conversion of T4 into T3 is impaired by moderate weight loss. ………
      “Conclusions: T3 concentration closely correlates with individual nutritional status, and moderate weight loss results in a decrease in T3 with minimal changes in other thyroid hormone homeostasis parameters. The data suggest that a decrease in peripheral conversion of the prohormone T4 into its hormonally active metabolite T3 is at least in part responsible for the observed changes in thyroid hormone homeostasis.”
      Since T3 is the active form of thyroid loss of T3 formation is significant, since T3 is involved in hundreds of metabolic processes.

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