The effects of TRH on plasma thyroid hormone levels of rainbow trout (Salmo gairdneri) and arctic charr (Salvelinus alpinus).

Plasma levels of L-thyroxine (T4) and 3,5,3'-triiodo-L-thyronine (T3) were measured in 2- to 4-year-old rainbow trout (Salmo gairdneri) and arctic charr (Salvelinus alpinus) intraperitoneally injected with 0.7% NaCl alone (controls) or 0.7% NaCl containing synthetic thyrotropin-releasing hormone (TRH). Blood was obtained from terminally sampled fish or from serially sampled fish cannulated in the dorsal aorta. In trout, TRH (1 microgram/g body wt) significantly raised plasma T4 to maximal values at 2 hr. A dose of 0.1 microgram/g was ineffective in trout but doses below this level elevated plasma T4 in arctic charr. In charr starved for 2 or 3 weeks the plasma T4 response to TRH appeared blunted. There was no consistent influence of TRH on plasma T3 under any condition for either species. It is concluded that TRH influences the salmonid thyroid system at some level to elevate plasma T4, possibly through thyrotroph stimulation.