Effect of Tri-Iodothyronine Replacement on the Metabolic and Pituitary Responses to Starvation

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Abstract
To determine the implication of decreased T3 production during fasting, seven normal men were fasted for 80 hours on two occasions; they received 5 μg of T3 every three hours during the second fast. The mean serum T3 concentration declined during the control fast from 120 to 73 ng per deciliter (P3 fast. Mean serum T4 concentrations did not change, and mean serum rT3 concentrations increased, during both fasts. The peak serum TSH increment after TRH was 11.1 μU per milliliter before fasting, 8.9 (not significant) after the control fast and 2.2 (P3 fast. Urea excretion was 9.1 per cent higher during the T3 fast; there were no differences in the changes in blood glucose, plasma fatty acids or other substrates during the two fasts. Pretreatment with potassium iodide lowered serum T4 concentrations and increased the serum TSH response to TRH after fasting. We conclude that the decrease in serum T3 concentrations during fasting spares muscle protein. Fasting is accompanied by a lower set point of TSH secretion, which remains sensitive to changes in serum thyroid hormone concentrations. (N Engl J Med 300:579–584, 1979)