Changes in plasma thyrotrophin, thyroxine, and triiodothyronine after acute or chronic administration of iodide to iodine-deficient rats.

The effect of chronic oral or acute iv administration of small graded doses of iodide was studied in severely iodine-deficient rats. Drinking water supplemented with 0.1, 0.2, or 0.4 mug 127I-/ml was given for up to 42 days. Plasma T3, which was initially in the normal range, increased 2--3-fold on days 1 and 2 after initiation of iodide supplementation, then dropped to approximately the intial concentration at day 3. Plasma T4, initially undetectable, was calculated to increase approximately 20-fold within one day after beginning iodide supplementation, but was still climbing into the normal range at day 3. In all groups, the initially high plasma TSH fell significantly by day 1 and was in approximately the normal range for iodine-sufficient rats by day 3. At 42 days, thyroid weight and 131I uptake were significantly elevated in all groups and inversely correlated with the dose of iodide, even though the thyroidal labeled T3/T4 ratio and plasma TSH, T4, and T3 were within the normal range. Intravenous injection of 0.1 or 0.5 mug 125I-/100 g caused an approximately 2-fold increase in plasma T3 within 24 h, which was not statistically different between the 2 groups. Plasma T4 rose to the normal range 24 h after 0.5 mug and was slightly, but significantly, elevated after 0.1 mug; it then declined in both groups. Twenty-four hours after injection of 2.5 mug 125I-/100 g, there was a significant fall in plasma T3, no change in plasma TSH, and no detectable plasma T4. Thereafter, there was a dramatic increase in plasma T4 and a fall in plasma TSH. The duration and degree of suppression of plasma TSH was related to the dose of iodide injected. We conclude that, under the conditions of our experiments, plasma T3 derives primarily from thyroidal secretion rather than from extra-thyroidal conversion of T4 to T3. Severely iodine-deficient rats secrete maximum quantities of TSH which are not increased by a further transient drop in plasma thyroid hormone concentration.