The effect of handling and blood removal on plasma levels and hepatic deiodination of thyroid hormones in adult male and female rainbow trout, Oncorhynchus mykiss

We studied the effects of handling and blood removal (0.4% of body mass) on the thyroid system of fasted ~1000-g adult male and female rainbow trout, Oncorhynchus mykiss, over a 3-day period. Relative to undisturbed controls, bleeding lowered the mean hematocrit (HCT) by 19% and on day 1 increased the hepatic inner-ring deiodination of 3,5,3'-triiodothyronine (T3) (T3IRD), but did not alter thyroxine (T4) outer-ring deiodination (T4ORD) or inner-ring deiodination (T4IRD) activity or plasma T4 or T3 levels. Regardless of treatment, hepatic T4ORD activity and HCT were lower in females than in males, and over the 3 days following bleeding, the plasma T3 level increased and the plasma T4 level decreased progressively for both sexes. We conclude that handling and bleeding induce a temporary increase in hepatic T3 conversion to 3,3'-diiodothyronine with no change in T4 conversion to reverse T3. This is consistent with independent control of T3IRD and T4IRD activities. Furthermore, HCT and hepatic T4ORD activity are lower in adult females, and 10 days after transfer of trout to smaller tanks, plasma T4 and T3 levels may still be adjusting.