A STUDY OF VARIOUS INFLUENCES ON THE IODIDE CONCENTRATING MECHANISM OF THE RAT THYROID1

Abstract

The thyroid: serum iodide concn. ratio (T/S) of normal rats was found to be approx. 25: 1. Hypophysectomy lowered this ratio to roughly one-fourth (6:1), and prolonged treatment with thyroxin to values approaching 1:1. The T/S of hypophysectomized rats could be further diminished by the admn. of thyroxin. Exogenous TSH elevated the T/S of hypophysectomized rats. The response of the T/S of hypophysectomized rats to the injn. of a single dose of TSH had a latency of around 5-6 hrs., reached a peak between 24-48 hrs. and had largely subsided by 72 hrs. When the 34-hr. response of the T/S of hypophysectomized rats to single doses of TSH was examined, a sigmoid log dose-response curve was obtained, with the rectilinear portion Lying in the range of .2 to 20 mg. TSH. Both the sensitivity and the slope of the response were enhanced when propylthiouracil (PTU) was injd. together with the TSH. Thyroxin, on the other hand, had no demonstrable effect on the T/S rise elicited either by a single dose or by more prolonged admn. of TSH. Extracts of transplanted mouse pituitary tumors (Furth) were found to have a high TSH content as judged by their ability to raise the T/S of hypophysectomized rats. No summation of the T/S response was found, when TSH was injd. repeatedly. Prolonged admn. of TSH to hypophysectomized rats in amts. which were not sufficient to restore the normal morph. appearance of the thyroids elevated the T/S values somewhat above normal. When the same dose of TSH was given together with PTU, there was little change in the histological response of the thyroid, but the T/S-raising effect of the hormone was greatly potentiated. PTU alone had a questionable influence on the T/S of hypophysectomized rats.