Effects of starvation and refeeding on plasma T4 and T3 levels and T4 deiodination in rainbow trout, Salmo gairdneri

Plasma T4 (L-thyroxine) and T3 (3,5,3′-triiodo-L-thyronine) levels were decreased by starvation in both terminally sampled and serially sampled rainbow trout held at 11 °C, significant changes in T3 occurring by day 6. Diel variation in plasma T4 was confirmed for fed trout and could explain the previous inability to detect differences in plasma thyroid hormone levels between starved and fed states. Significant elevations in plasma T4 and T3 could be obtained 4 h after feeding previously starved trout. Based on studies with [125I]T4 it is concluded that the food-induced elevation in plasma T3 is due, in part at least, to increased extrathyroidal T4 deiodination to T3.