The relative potencies of L-T4 and L-T3 on the pituitary-thyroid axis in the Japanese quail.

We have studied indirectly the relative contribution of the two thyroid hormones T4 and T3 to the feedback regulation of thyrotropin (TSH) secretion in quails, by means of the TRH-test. Adult euthyroid quails received a single injection of thyrotropin-releasing hormone (TRH) (1 or 10 micrograms/100 g body wt) and were killed different times after. Peak level of T4 occurred at 40 min and further served as the TRH-test response. The administration of two different doses of T4 or T3 (0.5 or 2 micrograms/100 g body wt) increased plasma T4 or T3 levels by 3- to 12-fold, respectively, according to the dose. The dose of 0.5 micrograms T4 slightly impaired the TRH-test response, while 2 micrograms rapidly blunted this response. Changes in plasma T3 levels had no deep repercussion on the TRH-test response. It is concluded that the pituitary of the quail is more sensitive to changes in plasma T4 levels than to changes in plasma T3.