Thyroid hormone homeostasis in states of relative caloric deprivation

Starvation is accompanied by significant changes in the hypothalamic-pituitary-thyroid axis and in peripheral thyroid hormone metabolism. Less well studied, however, are the effects on thyroid hormone economy produced by hypocaloric feeding. We explored these changes in obese patients fed 200, 400, or 600 cal/day of either carbohydrate or protein for 28 days. T 4, T 3, reverse T 3 and the TSH response to TRH were measured at frequent intervals. Each patient demonstrated a transient rise in reverse T 3 and a fall in T 3 that returned to near basal levels by the end of the study period. The TSH response to TRH on the other hand, declined to approximately 50% of control values and remained at that level throughout the course of study, regardless of the type of substrate or calorie level chosen. The results indicated that hypocaloric feeding is associated with changes in thyroid hormone economy similar to those in starvation and that peripheral (changes in T 3 and rT 3) and central (TRH response) events are controlled by separate mechanisms.