However, a recent study demonstrating the effectiveness and safety of the ketogenic diet for weight loss recorded no change in circulating T3 levels.(6)
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).
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.