How Thyroid Health Impacts Weight

When looking at the thyroid, health practitioners tend to have a wide array of perspectives. One might say that TSH (thyroid stimulating hormone) is the only test needed, and another might say that TSH is not the only answer. When experts disagree, how is a patient supposed to make sense of things? If a patient is experiencing typical symptoms of hypothyroidism, yet their TSH is normal, are they to be brushed aside and told “it’s all in your head,” or is it crucial for practitioners to dig a little deeper into other ways to get to the root of the patient’s symptoms?

A growing body of research and patient reports suggest that it is entirely possible to have a normal TSH level while still experiencing hypothyroidism. There are a number of situations in which TSH may be normal, but total and/or free T3 or total and/or free T4 are low. One example is during or after weight loss achieved through drastic caloric restriction, especially if combined with strenuous exercise. This has been called “famine response hypothyroidism,” and has been documented among overweight patients who have experienced weight loss, but then reach a plateau where further weight loss is not achieved despite continuing to follow the formerly effective diet and exercise recommendations. 

Symptoms of hypothyroidism include hair loss, constipation, depression, high cholesterol, brain fog, feeling cold, and struggling to lose weight despite adhering to a diet and exercise program that was previously working. Telling this to their doctor, patients might be dismissed and told to eat even less and exercise even more. And if their TSH is normal, they’ll be told there’s nothing wrong with their thyroid, despite their symptoms. It might be assumed that their self-reporting of food intake and physical activity is untruthful. This can be extremely discouraging to someone working hard to lose additional weight, whose own body appears to be sabotaging them.

A reduction in thyroid function, or changes in hormone levels, may be expected in such a situation: as a survival mechanism to keep the body from burning out while over-exercising and undereating, production of T3 is decreased and that of reverse T3 (rT3) is upregulated. Reverse T3 binds to the thyroid hormone receptor, but does not have the same metabolic effects. Reverse T3 is typically elevated during acute illness or severe physical stress—the kind the body experiences while over exercising and undereating, particularly over an extended period of time. TSH, however, often remains normal, as does T4.

This well-documented effect led researchers to admit, “…it is our job as clinicians to educate our patients that their challenges with weight are not lack of will power but a highly evolved system which can be treated. To continue to view our patients’ failures as a weakness of will power is to ignore the reality of this condition and do a major disservice to our patients and society at large.”

With the increased popularity of ketogenic diets for weight loss, it would be important to address the effect on thyroid function. It has been documented that after some length of time on a keto diet, T3 levels decrease in some people. This is not solely an artifact of weight loss, though, as many people adopt ketogenic diets for reasons unrelated to body weight. It appears to be something some people—but not all—experience on a keto diet even if there’s no caloric restriction. This doesn’t necessarily mean KDs are harmful for thyroid health or hormone regulation. World renowned KD researcher Stephen Phinney, MD, PhD, has speculated that it’s possible the state of ketosis makes the body more sensitive to thyroid hormone, such that a lower level of T3 is needed to induce the same metabolic effects. To be clear, this is speculation; we don’t know for sure what the implications of decreased T3 on a KD are, but as long as the individual is asymptomatic and feels well, that’s the critical issue. If there are no signs or symptoms of hypothyroidism, then decreased T3 could be, at least for some people, a natural feature of being in nutritional ketosis. 

The many different thyroid-related tests, pitfalls in how to interpret these, and uncertainty regarding what should be considered the “normal” reference ranges all point toward assessing each patient individually, and considering the clinical context and especially symptomology when interpreting results.