Effects of small doses of L-thyroxine and triiodo-L-thyronine on pituitary LH content of thyroidectomized rats

Pituitary LH was studied by means of a specific radioimmunoassay (RIA) in male rats at different time intervals after thyroidectomy (T), and in rats which were 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 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 T, when the LH concentration (mug/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 T animals. However, a single dose of 1.75 mug of T4 or 0.2 mug of T3 (doses approximately equivalent to the T4 and T3 maintenance dose for 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 hours. Surprisingly, 5.0 mug of T4 and 1.0 mug of T3 did not produce any increase in the pituitary LH content of T rats.