Effect of dietary composition on fasting-induced changes in serum thyroid hormones and thyrotropin

Abstract

To assess the effect of starvation and refeeding on serum thyroid hormones and thyrotropin (TSH) concentrations, 45 obese subjects were studied after 4 days of fasting and after refeeding with diets of varying composition. All subjects showed an increase in both serum total and free thyroxine (T 4), and a decrease in serum total and free triiodothyronine (T 3) following fasting. These changes were more striking in men then in women. The serum T 3 declined during fasting even when the subjects were given oral L-T 4, but not when given oral L-T 3. After fasting, the serum reverse T 3 (rT 3) rose, the serum TSH declined, and the TSH response to thyrotropin-releasing hormone (TRH) was blunted. Refeeding with either a mixed diet ( n = 22) or a carbohydrate diet ( n = 8) caused the fasting-induced changes in serum T 3, T 4, rT 3, and TSH to return to control values. In contrast, refeeding with protein ( n = 6) did not cause an increase in serum T 3 or in serum TSH of fasted subjects, while it did cause a decline in serum rT 3 toward basal value. The present data suggest that: (1) dietary carbohydrate is an important factor in reversing the fall in serum T 3 caused by fasting; (2) production of rT 3 is not as dependent on carbohydrate as that of T 3; (3) men show more significant changes in serum thyroid hormone concentrations during fasting than women do, and (4) absorption of T 3 is not altered during fasting.