EFFECTS OF SMALL DOSES OF L-THYROXINE AND TRIIODO-L-THYRONINE ON PITUITARY LH CONTENT OF THYROIDECTOMIZED RATS

Pituitary LH [luteinizing hormone] was studied by means of a specific radioimmunoassay (RIA) in male rats at different time intervals after thyroidectomy (.hivin.T), and in rats which were .hivin.T at least 30 days before and were then treated with different doses of L-thyroxine or triiodo-L-thyronine. A decrease in the pituitary LH of the .hivin.T animals, with respect to the intact age-paired controls, was demonstrable from 13 days after the operation, when total pituitary LH content was taken into consideration, or from 5 days after .hivin.T, when the LH concentration (.mu.g/mg pituitary) was considered. Doses of thyroid hormones lower than the daily maintenance dose for the rat produced very little effect on the pituitary LH levels of .hivin.T animals. A single dose of 1.75 .mu.g of T4 or 0.2 .mu.g of T3 (doses approximately equivalent to the T4 and T3 maintenance dose for .hivin.T rats) induced such a rapid and intense increase in the pituitary LH content that it no longer differed from that of the intact age-paired controls by 12 h. Surprisingly, 5.0 .mu.g of T4 and 1.0 .mu.g of T3 did not produce any increase in the pituitary LH content of .hivin.T rats.