Thyrotropic and Peripheral Activities of Thyrotrophin and Thyrotrophin-Releasing Hormone in the Chick Embryo and Adult Chicken

Summary

The influence of an intravenous injection of thyrotrophin-releasing hormone (TRH) and bovine thyrotrophin (TSH) on circulating levels of thyroid hormones and the liver 5′-monodeiodination (5′-D) activity is studied in the chick embryo and the adult chicken. In the 18-day-old chick embryo, an injection of 1 μg TRH and 0.01 I.U. TSH increase plasma concentrations of triiodothyronine (T₃) and of thyroxine (T₄). TRH, however, preferentially raises plasma levels of T₃, resulting in an increased T₃ to T₄ ratio, whereas TSH preferentially increases T₄, resulting in a decreased T₃ to T₄ ratio. The 5′-D-activity is also stimulated following TRH but not following TSH administration. The increase of reverse T₃ (rT₃) is much more pronounced following the administration of TSH. In adult chicken an injection of up to 20 μg of TRH never increased plasma concentrations of T₄, but increases T₃ at every dose used together with 5′-D at the 20 μg dose. TSH on the other hand never increased T₃ or 5′-D, but elevates T₄ consistently. It is concluded that TSH is mainly thyrotropic in the chick embryo or adult chicken whereas TRH is responsible for the peripheral conversion of T₄ into T₃ by stimulating the 5′-D-activity. The involvement of a TRH induced GH release in this peripheral activity is discussed.

Key-Words

Chicken - T₄ monodeiodination - TRH - TSH