Respective roles of circulating T4 and T3 in control of TSH secretion in severely iodide-deficient rats.

After a 6-month iodide deficiency, Wistar male rats were submitted to a normal iodine diet (20 and 50 micrograms of 127I daily). Plasma T3, T4 and TSH were determined by RIA from 0 to 140 days of iodide refeeding. A highly significant correlation was found between plasma TSH and T4 concentrations, but not between plasma TSH and T3 levels. These data suggest that an increase in plasma T3 alone, up to the normal value, is not able to inhibit TSH secretion. It is only when a certain plasma T4 concentration is also reached, resulting in further T3 formation through deiodination, that TSH secretion is inhibited.