The body has two types of thyroid hormones (42). The primary active thyroid hormone is
T3, called triiodothyronine. T3 is responsible most of the metabolic effects in the body. The other
thyroid hormone is T4, called thyroxine. Thyroxine is approximately one-fifth as metabolically
active as T3 and is considered to be a storage form of T3 in that it can be converted to T3 in the
liver.
T3 levels in the body are primarily related to the carbohydrate content of the diet (44-46)
although calories also play a role (47-49). When calories are above 800 per day, the
carbohydrate content of the diet is the critical factor in regulating T3 levels and a minimum of 50
grams per day of carbohydrate is necessary to prevent the drop in T3 (44,48,49). To the
contrary, one study found that a 1500 calorie diet of 50% carbohydrate and 50% fat still caused a
drop in T3, suggesting that fat intake may also affect thyroid hormone metabolism (50).
Below 800 calories per day, even if 100% of those calories come from carbohydrate, T3
levels drop (47). Within days of starting a ketogenic diet, T3 drops quickly. This is part of the
adaptation to prevent protein losses and the addition of synthetic T3 increases nitrogen losses
during a ketogenic diet (1). In fact the ability to rapidly decrease T3 levels may be one
determinant of how much protein is spared while dieting (51).
Hypothyroidism and euthyroid stress syndrome (ESS)
There are two common syndromes associated with low levels of T3 which need to be
differentiated from one another. Hypothyroidism is a disease characterized by higher than
normal thyroid stimulating hormone (TSH) and lower levels of T3 and T4. The symptoms of this
disease include fatigue and a low metabolic rate.
The decrease in T3 due to hypothyroidism must be contrasted to the decrease seen during
dieting or carbohydrate restriction. Low levels of T3 with normal levels of T4 and TSH (as seen in
ketogenic dieting) is known clinically as euthyroid stress syndrome (ESS) and is not associated
with the metabolic derangements seen in hypothyroidism (1). The drop in T3 does not appear to
be linked to a drop in metabolic rate during a ketogenic diet (17,52).
As with other hormones in the body (for example insulin), the decrease in circulating T3
levels may be compensated for by an increase in receptor activity and/or number (1). This has
been shown to occur in mononuclear blood cells but has not been studied in human muscle or fat
cells (53). So while T3 does go down on a ketogenic diet, this does not appear to be the reason for a
decrease in metabolic rate.
Summary
The primary adaptation to ketosis (as it occurs during total starvation) is a gradual
decrease in the body’s glucose requirements with a concomitant increase in the use of free fatty
acids and ketones. The main adaptation which occurs is in the brain which shifts from deriving
100% of its fuel from glucose to deriving as much as 75% of its total energy requirements from
ketones. Thus the commonly stated idea that the brain can only use glucose is incorrect.