Different effects of oral doses of triiodothyronine or thyroxine on the inhibition of thyrotrophin releasing hormone (TRH) mediated thyrotrophin (TSH) response in man.

Since contradicting results about the existence of T3 or T3 and T4 receptors in pituitary tissue have been reported, the influence of L-triiodothyronine (L-T3) or L-thyroxine (L-T4) on TRH stimulated TSH release was investigated. Oral administration of 50 mug L-T3 caused an increasing inhibition of TSH response to 400 mug TRH from 64% 2 h after L-T3 intake to 29% after 24 h, while serum T3 peaks up to 5.45 ng/ml occurred between 2 to 4 h after L-T3 ingestion and became normal after 8 to 10 h. This delay in the T3 action on TRH inhibition agrees with the postulate that T3 induces the synthesis of an inhibiting protein which is blocking TSH liberation. Oral administration of 1000 mug L-T4 induced increments of serum T4 up to 221 ng/ml between 6 to 24 h after intake; however, a TRH inhibition of 62% did not become evident before 48 h. At this time T3 levels had risen to the upper normal range. These results support the theory that T3 is responsible for the regulation of TSH secretion. An intra-pituitary conversion from T4 to T3 seems more likely the cause of the TRH inhibition rather than the peripheral T4-T3 conversion or a direct action by T4 binding sites in the pituitary.