Basal Body Temperature and Your Thyroid: Why I Don’t Care and Neither Should You

One of my readers recently pointed out that people in the Facebook/thyroid world burn copious amounts of ATP obsessing over basal body temperature (BBT). I have certainly seen this temp-checking strategy promulgated across the alternative medicine world, but I’ve never done a deep dive into the genesis of this approach – until now.

A Bit of Background

Many alternative medicine practitioners believe that a sub-normal BBT, in the presence of normal thyroid function tests, indicates thyroid hormone deficiency. Like other fabricated alt med diseases (e.g. adrenal fatigue), the existence of this syndrome* has been universally disputed by reputable medical organizations.

This kind of situation makes me wonder: where does alt med get their ideas? In my experience, this genre of “hypothesis” typically germinates from a kernel of truth – somewhere – which is then extrapolated beyond all recognizability until becoming fully transmogrified into something new and demonstrably false.

In the case of BBT and hypothyroidism, many alt med practitioners credit Dr. Broda Barnes with laying the foundation for this approach. In his 1976 book Hypothyroidism: The Unsuspected Illness, he related his clinical experience using BBT to diagnose and guide the management of hypothyroidism with desiccated thyroid.

Per the typical modus operandi of modern-day alt med, they saw Dr. Barnes’ pig thyroid play and raised by adding pure T3 for the people who didn’t get better with pig thyroid alone. The rationale? As they saw it, there were two choices: admit that people who failed to improve with pig thyroid weren’t actually hypothyroid; or jack up the T3 to heroic levels, justifying that by claiming (without evidence) those people couldn’t convert T4 to T3. For the moment, let’s table the fact that pig thyroid already has supraphysiologic doses of T3 when given to humans.

So, alt med invents another nonexistent syndrome – big whoop, right? Unfortunately, like a tenacious guard dog, I just couldn’t release this from my jaws. I had to get a copy of the book that had such a profound influence on modern-day alt med and see where Dr. Broda Barnes got his ideas.

Hypothyroidism: The Unsuspected Illness

As much as it pained me to fork over $9 I could have blown at Chipotle (mmmm…barbacoa bowl) for a used copy of a book I suspect I will use exactly once in my life, I did it – for you guys. You’re welcome.

In retrospect, I’m actually glad I bought the book, as Dr. Broda Barnes’ origin story is a flaming example of confirmation bias. I’ll summarize it in the next few paragraphs, and you be sure to let me know if his overt bias doesn’t whack you in the face.

As a graduate student in 1930, he was thyroidectomizing baby rabbits. Not surprisingly, these rabbits looked like hell after surgery and died early, unless they were given thyroid hormone replacement. The chosen few who received treatment enjoyed a “miraculous return to health.”

Years later, in practice, Dr. Barnes noticed that he was seeing patients with a litany of nonspecific symptoms that didn’t fit neatly into any diagnosis. His patients’ troubles reminded him of the thyroidless rabbits, as the rabbits also had a diverse array of signs and symptoms. Therefore, it clearly stood to reason that Dr. Barnes should begin screening all his toughest patients for hypothyroidism. Because, well…who could possibly imagine any other chronic diseases that manifest with a litany of nonspecific symptoms?

Dr. Barnes’ “Research”

In an August 1942 issue of Journal of the American Medical Association, Dr. Barnes wrote about his clinical experience measuring basal body temperature in people and treating their low BBT with desiccated thyroid hormone. I realize the link is behind a paywall, so you can only see the first paragraph, but rest assured I did gain access and read his entire paper.

Before I jump into analyzing his paper, keep in mind that – until the 1940s when serum protein-bound iodine emerged as a diagnostic/therapeutic marker for thyroid disease – low basal metabolic rate (BMR) + hypothyroid symptoms was the only way to diagnose hypothyroidism. T4 assays didn’t come around until the ’60’s and TSH didn’t arrive on the scene until the ’70’s. Dr. Barnes recognized that BMR testing was hard to do right and prone to false elevations. He posited that BBT might be a good surrogate for BMR – maybe even better – and thus would be a great way to diagnose hypothyroidism.

On to the “study:” Dr. Barnes says that he collected data from 1000 college students who took an oral temp immediately upon awakening. His article is more a summary of his findings than a detailed accounting, so it is not clear how many subjects with “low” BBT had concerning symptoms and how many were asymptomatic. It’s also worth noting that his definition of normal BBT was arrived at using data from untreated patients without symptoms and treated patients whose symptoms had improved.

In his experience, desiccated thyroid hormone effectively raised BBT and improved symptoms. I can’t give you much more in the way of details, as Dr. Barnes did not provide them. But I did find it interesting that he stated:

The initial low temperature and the improvement seen when the temperature is elevated by thyroid therapy indicate that further work should be done in this field. No attempt will be made at present to decide whether or not these patients are hypothyroid.

That last sentence is critical, as it – perhaps unintentionally – alludes to something we now know to be true about giving thyroid hormone to people who aren’t hypothyroid: they sometimes feel better for awhile, before drifting back to their baseline. Giving supraphysiologic doses of any hormone – thyroid, prednisone, testosterone, etc – has the potential to act as a stimulant, even if that hormone is not needed as a replacement therapy. Think about it: injecting heroin probably feels pretty awesome, but it’s not correcting your heroin deficiency.

My point is: we have no idea if any of Dr. Barnes’ subjects truly had hypothyroidism, as their only objective abnormality was a “low” BBT that unsurprisingly increased with thyroid hormone treatment. While I concede that it may not have been as widely recognized in the 1940s that body temperature regulation involves more than just the thyroid, it was well-known by the 1960s. Regulation of Internal Body Temperature, a review article published in 1968, detailed just how complicated thermoregulation is over the duration of its 70 pages! Given that Dr. Barnes published Hypothyroidism: The Unsuspected Illness in 1976, he really had no excuse for drawing a straight line between low BBT and hypothyroidism, barely paying lip service to other possibilities in his book:

Although the basal temperature test is not 100% specific for thyroid function, the simple procedure is remarkably successful in uncovering hypothyroidism. Its results most often fit well with patients’ symptoms.

To summarize: Dr. Barnes believed that most people with nonspecific symptoms and low BBT would see a rise in BBT and a reduction in symptoms by giving them desiccated thyroid. He made no attempt to account for other factors involved in thermoregulation and no attempt to diagnose any condition other than hypothyroidism, in those with low BBT. Therefore, his a priori probability of diagnosing hypothyroidism in this subset of patients was 100%. If you believe with religious fervor that hypothyroidism is that prevalent (it’s not), then I’m not sure it’s even worth reading further. But if you’re starting to question whether alternative medicine knows more about the thyroid than your Endocrinologist, let’s bring it on home:

Why You Shouldn’t Measure BBT for Your Thyroid

1. The “normal” body temperature is wrong. In Dr. Barnes’ defense, Hypothyroidism: The Unsuspected Illness was published in 1976. Since that time, there have been numerous studies debunking the concept of what constitutes a “normal” body temperature – there’s actually a fairly wide range for normal when you look at the population as a whole. In Mackowiak et al‘s classic 1992 paper, some normal folks ran around a frigid 96°F (35.6°C) while others were a toasty 100°F (37.8°C).

If you take Dr. Barnes’ advice to use an axillary** (underarm) temp of 97.8 – 98.2°F (36.6 – 36.8°C) as the normal benchmark, you have a fair chance of finding a low BBT and giving yourself a diagnosis of hypothyroidism. Or, if you are already hypothyroid, you may convince yourself that your low BBT means your dose needs to be increased.

2. Only a small fraction of the variance in BBT can be attributed to hypothyroidism and other comorbidities. In a 2017 issue of The British Medical Journal, Obermeyer et al published an elegant, observational cohort study of more than 35,000 patients. What I found fascinating about this study is, although several comorbidities and other measured variables were linked to lower or higher temperatures, “measured factors collectively explained only 8.2% of individual temperature variation.” Y’all, that is a tiny fraction of any change you might hope to see on a thermometer. Does it really make sense to try to use BBT when there are other factors that contribute 91.8% to your temperature variation?

The other thing I liked about Obermeyer’s study was this (probably) unintentionally amusing sentence: “Several demographic factors were linked to individual level temperature, with older people the coolest…and African-American women the hottest (versus white men…).”

3. There is no evidence that low BBT is causative of the laundry list of symptoms attributed to hypothyroidism. Upon examining Dr. Barnes’ 1942 JAMA article and the relevant portions of his 1976 book, I couldn’t find any cited evidence that low BBT actually causes problems. Sure, he could raise someone’s BBT by giving her desiccated thyroid, but that doesn’t mean the low BBT was the problem – it just means that BBT increases in response to desiccated thyroid. I would argue that low BBT is more likely to be an associated marker than a cause of problems, given the number of normal, asymptomatic people walking around with “low” BBT.

4. Higher body temperature may kill you. OK, this statement is deliberately misleading and histrionic, but I wanted to draw your attention to another conclusion from the Obermeyer study described in #2, above. The authors found that “unexplained temperature variation was a significant predictor of subsequent mortality: controlling for all measured factors, an increase of 0.149°C (1 SD of individual temperature in the data) was linked to 8.4% higher one year mortality (P=0.014).” This is an observational study, so we absolutely cannot say that raising your body temperature with T3 or any other substance will increase your risk of dying, but the association is thought-provoking, at the very least. In addition, this finding dovetails nicely with the body of animal literature showing that a reduction in body temperature can increase longevity. Maybe it’s not that bad to run a little colder?

5. BBT varies over the course of the menstrual cycle. Premenopausal women attempting to use BBT to guide the diagnosis and treatment of hypothyroidism have an extra hurdle: BBT varies during the month. Note that the common practice of using BBT to predict ovulation for fertility purposes is outside the scope of this article.

6. TSH is an ultra-sensitive, accurate reflection of thyroid hormone status. Even Dr. Barnes admitted that low BBT is not 100% specific for hypothyroidism. Fortunately, we have access to the TSH blood test, which is highly sensitive and specific for thyroid disease. Given that TSH arrived on the scene in 1971 and Dr. Barnes published his book in 1976, I was surprised that he never mentioned TSH. I wasn’t practicing back then, so I don’t know if it just took awhile for TSH to catch on, and by then his book was pretty much done?

While I realize that alt med loves to hate TSH – probably because it doesn’t allow them to diagnose everybody with hypothyroidism – I think I crafted a solid defense of TSH in Is TSH the Best Test? Bottom line: medicine has evolved quite a bit since the early 1900s, so why would we use a test like BBT that is not specific to thyroid disease, is affected by myriad other factors, and has no single normal value that can apply to all individuals? Oh wait – I know the answer…it’s because it allows alt med to convince you they know what’s wrong with you, when the reality is they don’t.

Conclusion

The road to quackery is paved with good intentions. I have no doubt that most healthcare practitioners recommending BBT and other non-standard thyroid testing are desperate to help patients who have a constellation of nonspecific symptoms that have gone unresolved. Unfortunately, when the only tool you have is a hammer, everything looks like a nail. That is confirmation bias which, unfortunately, will often find its way into the care of individual patients. But when this type of bias taints a much broader approach to patient care, as it has in the case of trusting BBT over thyroid function tests like TSH, it needs to be branded as such and widely refuted.

*In accordance with this website’s policy, I do not identify people who have opinions with which I disagree, if they continue to make their living in the medical field. Therefore, any comments below that name individual practitioners advocating the use of BBT will be deleted.

**In his 1942 paper, Dr. Barnes tracked oral temp.  Over the next 30+ years, he found that axillary temp was similar to oral temp and therefore had his patients measure axillary temp.  Some modern sources say 0.5 – 1°F (0.3 – 0.6°C) must be added to an axillary temp to “convert” it to an oral temp.

By using this site and interacting with me in the Comments, you agree to abide by my Disclaimer. If you want to share personal information, that is your choice. Please understand that my policy is to not respond to requests for individualized medical advice. Note that using BBT for any reason other than diagnosing/managing thyroid dysfunction is outside the scope of this post, and I will not address comments asking about BBT and fertility, etc.

Image Credit: Photo by Kevin Goodrich on Unsplash

9 Replies to “Basal Body Temperature and Your Thyroid: Why I Don’t Care and Neither Should You”

  1. Regarding your point number four – hasn’t it been shown in humans that longevity is associated with a higher TSH? So perhaps we should all be aiming higher rather than a non-existent TSH level, as the alt med world does.
    Some part of me wonders whether so many people want to be on the hyper end of the scale for the purported weight loss that goes with thyrotoxicosis. “Unable to lose weight” seems to be one of the major complaints of sufferers (never mind that generally anybody who goes on a diet can’t maintain their weight loss). What do you think HD?

    1. Very interesting point, Emily. Yes, there are data linking higher TSH to longevity, but we haven’t quite disentangled all the underlying mechanisms that may contribute to this finding. At this point, we definitely can’t say that people treated for hypothyroidism will live longer with a higher TSH, as the data are not looking at this situation. While the longevity-TSH link is interesting, I’m not sure when/if we will really understand what to do with that information.

  2. Thank you very much for your article, which I enjoyed like always.. The thing with BBT is, that patients love easy answers to complex questions, i.e. hypothyroidism and all their symptoms. Alt meds often provide the patients with very easy, but kind of secret solutions patients haven`t heard of from their doctor- because there IS no easy solution. The patient concludes that the doctor is ignorant, slave of the pharmaceutical companies, not listening to the patient`s complaints etc.-well known. But- in my experience time runs for us: after being diagnosed hypothyroid by an alt med, patients start to feel much better ( due to nozebo and placebo effects and perhaps being hyperthyroid ) with armour thyroid etc., but after a while patient`s symptoms start to reappear or even get worse, because of overdoses of thyroid hormones and/or symptoms that have nothing to do with the thyroid. For reasonable people that is the time to return to their clinicians-others will be caught spending huge amounts of money in various expensive tests by an alt med therapist. Patients end up with a treatment plan that keeps you busy all day taking all kinds of vitamins, minerals and hormones…. and broke and frustrated. Usually I tell the patients: if your therapist really would be able to solve your problems with his regime, he would have won the Nobel Prize in Medicine years ago.

  3. When I was at university I took part in a study of various hormones over the reproductive cycle, which along with the frequent blood draws involved taking your temperature upon waking so I know that I used to run low – 36.5 C at my highest point in the cycle. But I also know that back then I was certainly not hypothyroid! I don’t know if being post-menopausal has changed my BBT or if it would, but I am now hypothroid which is well controlled by my GP testing my TSH and giving me the appropriate amount of levothyroxine, so much easier than regular BBT taking first thing in the morning.

  4. I’ve never tried looking into temperature differences, and maybe glad I didn’t. But something I have noticed is my heart rate seems to follow a pattern. When I’m low thyroid it’s pretty low. And when I’m in better shape it goes up. In fact, I noticed this in an interesting situation. I was splitting levothyroxine 100 and 88 to make 94 (the dose I was on since I was too much on 100 and too little on 88). I asked if I could get some 50s vice splitting the 100s. With the 50 pill and a split 88 to make 94, my heart rate started to drop (I track it when sleeping) and I was feeling bad. I asked to go back to splitting 100 pills and my heart rate went right back up and I felt better again. For whatever reason, the 100 Levothyroxine pill from CVS seems stronger than it probably should be. I switched to Synthroid about 1 year ago, and now I’m on 112 of Synthroid. So Synthroid seems a bit less strong for me at least. Or the CVS levo 100 is a bit high for some reason. That or I had other issues affecting me back then which caused for poor absorption or what have you. I’ve also noticed blood pressure changes, but it’s not something I track often.

  5. Great detective work HD. Thanks for pointing out a great example of how the interpretation of an old findings can runaway from the evidence. It is like the game of telephone we played as kids. By the time the message got to the end of the line, it often looked nothing like when it started. The same holds true for the interpretation of Dr Barnes’ work and what it actually showed. I am impressed that you forked over the $9. Just to be clear, however, you get the barbacoa bowl without rice, right? 🙂

    1. Without rice, yes, because I have that annoying voice in the back of my head that’s saying, “Do you want to eat your entire day’s worth of carbs in one sitting?!” Oh wait, that annoying voice is you, Dr. Scher!

  6. Thank you for sacrificing a barbacoa bowl for a junk book, and spending your time to read a junk article! I would’ve given up if I couldn’t find that book in my local public library. 100 years ago that book could’ve been used for fuel, but now it’s probably only good for recycle. Unfortunately junk articles sometimes do make it to reputable journals even Nature or Science… It just says how bad it could be when scientists don’t know they don’t know, or worse, pretend to know what they know they don’t know. However, as someone who’s constantly feeling cold, I’m more than willing to try things that can make me feel warmer, if they are not too expensive or likely to be harmful. OTC hormone supplements are out of the question, but I do wonder if sex hormones play a role, since I noticed that more women than men are cold intolerant.

    1. Outside of hypothyroidism, women feel colder base on physiology. So that is a normal thing. I however, have always been cold…but then again I do have hypothyroidism and am getting a bit better with better thyroid levels. When I was at my worst, many layers of clothes and under a winter blanket and still was freezing. Glad those days are behind me.

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