ALWAYS COLD? WHAT UNDER-FUELLING DOES TO YOUR THYROID
Walk into any gym in October and you will see it. People halfway through a session, working hard, still in their jackets. Hands that do not warm up. Someone saying they cannot seem to get warm this year.
The usual explanation is the weather, and often that is exactly what it is.
But there is a second possibility, and it has nothing to do with the temperature of the room.
First, a term worth knowing
Energy availability is a simple idea but it's worth diving in a little deeper to the term. It refers to the food you eat, minus the energy your training burns. Whatever is left is what your body has to run everything else on. That includes your heart, brain, immune system, hormones, and all the recovery and repair work that most of us never think about. It is not the same as calories in and calories out, and this difference matters. You can eat what looks like a sensible amount and still leave your body very little to work with, because your training took most of it before anything else got paid.
The study that separated two things everyone confuses
In 1993, Loucks and Callister published a study in the American Journal of Physiology that did something really cool. They took 46 healthy young women and split them three ways on exercise. One group did none at all, one did 1,300 calories worth a day at a moderate effort, and the last group did the same 1,300 calories, but at high intensity.
Then, separately from all of that, they controlled energy availability. It's that separation that is the really cool part. Most research cannot tell you whether an effect comes from training or from the shortfall that comes with it, because the two happen together. This design pulled them apart. The study was over four days and with bloods taken throughout. What they saw was that when energy availability was low, the active thyroid hormone, T3, fell. So did free T3, the portion actually available to your tissues. And the inactive version, reverse T3, rose. The body had not stopped making thyroid hormone. It had started making the version that does not work, on purpose.
So which was it, the training or the shortfall?
The exercise made no difference at all. Not the amount, not the intensity. Women who did no exercise showed the same picture, provided they were short on energy. Women who trained hard were completely fine, provided they ate enough fuel to cover it. What I love is what the researchers tested next - they looked at what happened when it was done in reverse. They gave the exercising women enough food to cover their training, and the effect did not appear. Fuel up sufficiently and you prevent it. That's what I feel sets this paper at a higher bar than most nutrition research.
It was never the training. It was the shortfall.
What your body does when you are under-fuelled
Most people assume being short on energy shows up somewhere obvious. You feel a little flat. Your session is not as sharp as last week. A heavy few days, a bad night’s sleep, nothing to worry about. That is not what happens. Your body does not spread the shortage evenly.
Think about what your phone does when the battery drops. It does not simply run slower. The icon turns yellow and the phone starts deciding what counts as a priority. The screen dims, background apps stop, things switch off so the essentials keep running. Your body does something similar. It funds what it must and turns down what it can live without, and thyroid hormone is one of the levers it uses. Turning down the rate at which you burn energy is a sensible response to a shortage rather than a malfunction. The difference is that your body does not ask you first, and it does not tell you it has done it.
So does that explain the cold?
The study measured hormones, it did not measure how anybody felt - nobody was specifically asked if they were cold. Temperature was not a variable.
What we have is a mechanism that is well demonstrated, and a long-standing clinical association between low thyroid function and feeling the cold.
Putting those together gives you a reasonable hypothesis, not a proven chain.
Where the research has moved on to now
When I started digging, I assumed a 1993 study would have been comprehensively superseded. The honest answer is more interesting than that. It has been partly replicated and with more sophisticated tools.
Last year, Shekhar and colleagues published a study in the European Journal of Endocrinology. Nineteen young women, each doing two five-day blocks. One on a diet matched exactly to what their body needed ad one cut by 55 percent with each women doing both. They ran both blocks at the same point in the menstrual cycle on purpose, because where a woman is in her cycle changes her thyroid readings on its own. It is the sort of design detail that should be standard and still is not.
Total T3 fell and reverse T3 rose. The same direction as 1993, measured using mass spectrometry rather than the antibody-based methods used in 1993, which matters because T3 and reverse T3 are so similar in structure that older tests struggle to tell them apart. Those women were not training at all. Which is precisely what Loucks found 32 years earlier.
The exercise was never the variable
In July last year, Hackney, Moore and Smith-Ryan proposed something more specific. That it is not only energy availability that matters, but carbohydrate intake alongside it, with a stronger cortisol response suppressing both the production of T3 and the conversion of T4 into it. It was an idea that needed testing, not a finding.
Then, in 2026, it was actually tested. Nusser and colleagues, with Hackney among the authors, ran sixteen recreationally active adults through two four-day periods of low energy availability, one on low carbohydrate and one on high. High carbohydrate significantly protected IGF-1, a hormone central to growth and repair. Insulin, leptin and testosterone all fell further on low carbohydrate, though those differences did not reach statistical significance. Body mass reduced in both groups.
So: partial support. Carbohydrate appears to matter beyond the calories alone, for some things, and the picture is not yet complete.
I like this sequence because it is science working properly, somebody proposed an idea, said plainly that it was untested, and then helped test it within a year - brilliant work!
The study that goes against the narrative
Earlier this year, a Czech group published a study of female endurance athletes, runners and cross-country skiers, screened for low energy availability. The athletes identified as at risk had lower oestradiol, lower progesterone, lower leptin, lower white blood cell counts and lower phosphorus. This is exactly what the low energy availability picture predicts. Their energy availability was severely low, at 24.3 kcal per kilogram of fat free mass per day. For context, around 45 kcal per kilogram of fat free mass is usually taken as the level at which everything runs normally, and below 30 is the figure most often used to define low. These athletes were at 24.3, so not marginally short. But their free T3 was higher, not lower.
Every other hormone behaved as expected. It is a small sample size, 23 athletes against 21 controls, and it is a snapshot rather than an experiment, so it cannot establish cause either way. But I am including this one because it did the opposite and does not fit the current narrative. Science works by being willing to be wrong. You start with an idea and then go looking for whether it holds, rather than going looking for evidence that it does. As practitioners we have to stay open to findings that cut against the mainstream narrative, and more importantly to findings that cut against our own.
Honesty time
The 1993 study was four days, 46 women, sedentary rather than trained, young and regularly cycling. Whether it applies to a trained 45 year old, or to men, is a question it does not answer. The 2025 study was 19 women over five days. Small and short, like most controlled feeding work, because studies like this are expensive and demanding to run. And none of these studies measured symptoms. They measured hormones, a blood result moving is not the same as somebody noticing a difference.
The carbohydrate question is still open. One hormone, IGF-1, was clearly protected. Several others pointed the same way without reaching significance.
One more thing. Reverse T3 is not a test I would recommend you go and buy. The mechanism is sound and the marker is not. The assay is poorly standardised, and routine reverse T3 testing is not recommended by mainstream endocrinology. As with everything I write here, this is information to help you make your own decision.
If this sounds like you
You are training consistently. You are eating carefully, probably more carefully than most people you know. And you are cold, flat, slow to recover, and nothing you try seems to shift it. The useful question is not how hard you are working; it is whether you are leaving your body anything to work with.
Doing more is unlikely to be the answer. Eating less almost certainly is not. Three things worth looking at before anything else:
What you eat before you train, rather than after. Most of the shortfall sits in the hours before a session, not the hour following it.
Where your carbohydrate sits, given it now looks as though it may matter beyond the total.
What your day genuinely costs you. Not the hour in the gym. The standing, the walking, the stress, the sleep you did not get, and everything else you have never counted.
If you would rather know than guess
Everything above comes down to one thing. You cannot tell from the inside whether you are short, by how much, or why. That is the work I do. Looking at where somebody actually is, reading it properly, and then working out what is worth changing and what is not. Book a free 30 minute call to find out more.
Resources:
Loucks AB, Callister R. Induction and prevention of low-T3 syndrome in exercising women. American Journal of Physiology. 1993;264(5 Pt 2):R924-930.
Shekhar S, Tonleu JT, Okigbo CC, et al. Thyroid axis adaptations to moderate short-term energy restriction in healthy, young women. European Journal of Endocrinology. 2025;192(5):568-576.
Hackney AC, Moore SR, Smith-Ryan AE. Low Energy Availability, Carbohydrate Intake, and Relative Energy Deficiency in Sport: The Low Triiodothyronine Hypothesis. International Journal of Sports Physiology and Performance. 2025.
Nusser V, Teubner J, Buerger SA, et al. Increased carbohydrate availability partially attenuates endocrine suppression in response to low-energy availability. Journal of Applied Physiology. 2026;141(2):538-549.
Woronyczová J, Nováková M, Gerych D, et al. Relative energy deficiency in sport: a cross-sectional study of nutritional, biochemical and hormonal profiles in Czech female endurance athletes at risk of low energy availability. Frontiers in Sports and Active Living. 2026.
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