Influence of short‐term oestradiol treatment on plasma thyroid hormone kinetics in rainbow trout, Salmo gairdneri

Abstract

The effects of 17β‐oestradiol (E2) on plasma kinetics of thyroid hormones (T4, l‐thyroxine; T3, 3,5,3′‐triiodo‐l‐thyronine) were studied in immature rainbow trout. E2‐3‐benzoate (0.5 mg/100 g) was injected intraperitoneally on days 0 and 3, and on the morning of day 4 each trout received an intracardiac injection of either [125I]T4 and Na 131I or [I25I]T3. Groups of trout were bled and killed from 5 min to 4 days post‐injection of tracer. E2 did not alter the plasma T4 level but depressed the T4 plasma clearance rate, plasma‐to‐total tissue flux of T4 and thyroidal T4 secretion rate. Monodeiodination of T4 to T3 was also depressed, as judged from plasma [I25I]T3 and I25I − levels in [125I]T4‐injected trout. E2 had no major effect on T3 plasma clearance rate but depressed the plasma T3 level, plasma‐to‐total tissue flux of T3 and the T3 plasma appearance rate. E2 had no influence on biliary transport of [I25I]T4 or [125I]T3. The above results suggest that E2, at the dose range employed, depresses extrathyroidal T4 to T3 conversion, which may in turn decrease plasma T4 clearance and thyroidal T4 secretion.