The influence of short-term 17 beta-estradiol treatment on plasma T3 levels and in vitro hepatic T4 5'-monodeiodinase activity in immature rainbow trout, Salmo gairdneri.

To determine the effects of 17 beta-estradiol (E2) on aspects of thyroid function, immature rainbow trout were intraperitoneally injected with estradiol benzoate (0.5 mg/100 g) on Days 0 and 3 and sampled on Days 7 and 12. This protocol created plasma E2 concentrations during the first 7 days comparable to those during naturally induced vitellogenesis. Control trout received peanut oil alone. Plasma levels of 3,5,3'-triiodothyronine (T3) were significantly depressed on Day 7 but returned to levels by Day 12. Plasma thyroxine (T4) levels were not altered consistently by E2 treatment. Hepatic microsomal T4 5'-monodeiodinase (5'D) activity responsible for conversion of T4 to T3 was significantly depressed on Day 7 but returned to control levels by Day 12. Lineweaver-Burke plots showed that the lower hepatic 5'D resulted from a 10-fold decrease in Vmax, indicating a lower enzyme concentration. A slight reduction in Km was also observed. These results confirm that high E2 levels, comparable to those in vitellogenesis, depress plasma T3 levels in trout and show that, at least in part, this effect is the result of a decrease in the amount of functional hepatic 5'D.