Growth hormone stimulates hepatic thyroxine 5'-monodeiodinase activity and 3,5,3'-triiodothyronine levels in rainbow trout (Salmo gairdneri).

Intraperitoneal injection of rainbow trout (Salmo gairdneri) with 0.4 microgram/g of human growth hormone (hGH) increased plasma levels of 3,5,3'-triiodo-L-thyronine (T3) and Vmax of the hepatic microsomal 5'-monodeiodinase enzyme (5'D) that converts thyroxine (T4) to T3, with no effect on Km or plasma T4 levels. A dose of 0.4 microgram hGH/g increased both plasma T3 and Vmax as early as 8 hr postinjection (pi). Maximal levels for both parameters occurred at 24 hr pi and significant stimulation was sustained to 48 hr pi. In trout injected with hGH over a dose range of 0.1-1.0 microgram/g and sampled at 24 hr pi, both Vmax and plasma T3 increased in a dose-dependent manner. It is concluded that the increase in the level of functional hepatic 5'D may contribute to the rapid hGH-induced elevation in plasma T3.