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

  • Fereidoun Azizi
    Address reprint requests to Fereidoun Azizi, M.D., St. Elizabeth's Hospital, 736 Cambridge Street, Boston, Mass. 02135.
    St. Elizabeth's Hospital, Boston, Mass., USA

    the Department of Medicine, Tufts Medical School, Boston, Mass., USA
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      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 (T4), and a decrease in serum total and free triiodothyronine (T3) following fasting. These changes were more striking in men then in women. The serum T3 declined during fasting even when the subjects were given oral L-T4, but not when given oral L-T3. After fasting, the serum reverse T3 (rT3) 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 T3, T4, rT3, and TSH to return to control values. In contrast, refeeding with protein (n = 6) did not cause an increase in serum T3 or in serum TSH of fasted subjects, while it did cause a decline in serum rT3 toward basal value.
      The present data suggest that: (1) dietary carbohydrate is an important factor in reversing the fall in serum T3 caused by fasting; (2) production of rT3 is not as dependent on carbohydrate as that of T3; (3) men show more significant changes in serum thyroid hormone concentrations during fasting than women do, and (4) absorption of T3 is not altered during fasting.
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