EFFECT OF TRI-IODOTHYRONINE REPLACEMENT ON THE METABOLIC AND PITUITARY RESPONSES TO STARVATION

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 (P<0.01), but remained slightly above base-line values during the T3 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 (P<0.01) after the T3 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) DECREASED extrathyroidal conversion of thyroxine (T4) to 3, 5,3′-tri-iodothyronine (T3) occurs in patients with many different acute and chronic illnesses.1,2 This decrease is so common that it must be considered part of the normal host adaptation to illness, perhaps serving to reduce basal energy requirements at that time. The prototype of these illnesses is starvation, and the changes in extrathyroidal iodothyronine metabolism and pituitary-thyroid function that occur during starvation are typical of the changes in patients with many non-thyroidal illnesses.3 4 5 6 These changes consist of decreased serum total and free T3 concentrations, increased serum 3,3′,5′-tri-iodothyronine (rT. © 1979, Massachusetts Medical Society. All rights reserved.