Regulation of plasma T3 levels in T4-challenged rainbow trout, Salmo gairdneri.

Levels and kinetics of plasma T3 (3,5,3'-triiodo-L-thyronine) and T4 (L-thyroxine) were studied in rainbow trout injected intraperitoneally with carrier T4. Plasma T4 levels were increased almost fivefold by a T4 dose of 9.4 ng/g body wt given 24 hr earlier, and almost 50-fold by a dose of 69 ng/g. However, neither treatment altered plasma T3. T4 injection did not alter significantly the metabolic clearance rates of plasma T4 and T3 or [125I]T4 uptake by liver, intestine, and gall bladder, but reduced markedly the generation of plasma [125I]T3 and 125I- from [125I]T4. It is concluded that during T4 challenge the constancy in plasma T3 is achieved in large measure by a reduction in the proportion of circulating T4 undergoing 5'-monodeiodination to T3.