Carbohydrate-restricted diets have come under fire recently over
concerns that their association with reduced levels of the
active thyroid hormone (T3) might cause the symptoms of
hypothyroidism. I will argue that those concerns are unfounded.
There’s no shortage of evidence that, when calories are held
constant, blood serum levels of T3 fall in concert with
decreases in dietary carbohydrate, and zero-carb diets result in
levels of T3 similar to fasting. There is, on the other hand, no
evidence that low-carb or ketogenic diets actually cause any
symptom of inadequate thyroid hormone. Indeed, the evidence we
do have shows that the primary function of T3, regulating basal
metabolism, is unaffected by the amount of carbohydrate in a
diet, and suggests that lower levels of T3 associated with low-
carb diets are the result of a reduced need to metabolise
glucose. A (Very) Brief Introduction to Thyroid Hormones Thyroid
hormone blood serum levels are controlled by the hypothalamic-
pituitary-thyroid (HPT) axis. The hy