EXTRA-THYROIDAL EFFECT OF THYROTROPHIN ON SERUM THYROXINE

Abstract

ABSTRACT Injections of bovine thyrotrophin (TSH) into thyroidectomized guinea pigs which were receiving maintenance doses of thyroxine (T4) reduced the PBI levels. The phenomenon was demonstrated only with relatively large quantities (0.8-6 U per day) of TSH, but the response was dosedependent. The quality to affect the PBI remained associated with the thyroid-stimulating activity of TSH when partial purification of the trophic hormone was achieved. Multiple effects of TSH on T4 economy were found in these animals. Diminished binding affinity for T4 by serum proteins and enhanced deiodination of T4 by hepatic tissue could be demonstrated. The depression of the PBI by TSH appeared to be related to a redistribution of T4 which may, in part, have resulted from changes in the serum binding proteins. A relationship of increased hepatic deiodination of T4 to the decrement in PBI could not be established with certainty. In doses that could be reasonably administered, bovine TSH had no effect on the thyroid hormone economy in an athyreotic man.