Acute effects of thyroxine, triiodothyronine, and iodide on thyrotropin secretion

Rats fed a Purina or low-iodine diet (LID) for varying periods were serially sampled before and after a single iv injection of T4, T3, iodide or saline. Suboptimal replacement doses of T4 and T3 were given to rats fed LID for 2 months or 1 year (basal TSH, approximately 1000 and 2000 muU/ml, respectively). Both 1 mug T4 and 0.25 mug T3/100 g BW dropped plasma TSH to 70% of the initial level at 15 min and to 10--20% at 4 h. By 12 h TSH had begun to rise in 2-month LID rats, followed by a secondary decline 3--5 days after injection. Statistical comparison of the slopes of the initial TSH decline indicated there was no significant difference between the effect of T4 and T3. There effect of graded doses of T3 (0.01--0.3 mug/100 g) was also examined. There was a highly significant correlation of the magnitude of TSH suppression with the dose of T3 administered. Saline had no effect but 0.65 mug iodide/100 g BW (equal to that in 1 mug T4) had a delayed effect, depressing TSH to a minimum of 25% of the initial value at 48 h in 2-month LID rats. There was no difference in the effect of these doses of T4, T3, or saline in purina-fed rats (basal TSH, 170 muU/ml). T4 or T3 in physiologically equivalent doses thus produces an identical prompt rate of decrease in plasma TSH, indicating that both hormones possess intrinsic hormonal activity. The delayed effect of iodide is presumably because it must first be incorporated into T4 and T3 and secreted by the thyroid gland. The similarity of depression of plasma TSH by thyroid hormones or saline injection in Purina-fed rats is believed due to a nonspecific stress effect in these animals with a low basal rate of TSH secretion. The non-specific inhibition of TSH secretion is minimal in the iodine-deficient rats with a much higher basal rate of TSH secretion, presumably because of relative vectorial influences.