Effects of experimental hypo- and hyperthyroidism on iodothyronine deiodinases in Nile tilapia, Oreochromis niloticus

In the present study, we examined the effects of experimentally-induced inc reases or decreases in plasma concentrations of thyroid hormones on iodothy ronine deiodinases in tilapia, Oreochromis niloticus. To obtain hyperthyroi d tilapia, fish were injected with porcine follicle stimulating hormone (pF SH) 36 hours before sampling or fed on demand for 11 days with tilapia pell ets containing 12 ppm T-3. Tilapias were made hypothyroid by providing them food containing 0.2% methimazole for 11 days. Plasma T-4 and T-3 and the i n vitro deiodinase activity in liver, kidney, brain and gill were measured at the end of the treatment period. Injection with pFSH caused an increase in plasma T-4 but had no influence on plasma T-3 levels. A small increase i n plasma T-3 was observed in T-3-fed fish. Plasma levels of both T-4 and T- 3 were decreased by methimazole treatment. We observed no changes in kidney type I deiodinase (D1), whereas liver type II deiodinase (D2) was increase d during hypothyroidism and decreased during hyperthyroidism. Hypothyroidis m resulted in a significant decrease in brain, gill and liver type III deio dinase (D3). An pFSH-induced increase in T-4 stimulated brain and gill D3 b ut not liver D3, whereas the opposite was true in T-3-fed fish. We conclude that the regulation of D1 and D3 in tilapia is probably different compared to mammals.