Effects of temperature on plasma thyroxine and lodide kinetics in rainbow trout, Salmo gairdneri

Plasma levels of thyroxine (T4) and iodide and their plasma kinetics were studied in fed immature rainbow trout maintained at 11° and then held for up to 14 d (days) at 5, 11, or 19°. Elevation (5°) or depression (19°) of plasma T4 occurred during the first 3 d following abrupt transfer from 11°, but by 7 and 14 d plasma T4 did not differ markedly between the three temperatures. In contrast, the T4 plasma degradation rate increased fourfold from 5 to 19°, due largely to an increased plasma T4 distribution space and plasma T4 pool. The proportion of plasma T4 undergoing deiodination and the T4 to T3 conversion rate also increased with temperature. The plasma iodide level and plasma iodide pool were greater at 5° than at 11 or 19°, but both the fractional rate of plasma iodide loss and plasma iodide clearance rate increased with temperature, resulting in a greater loss of plasma iodide at 19° than at 5 or 11°. Thyroid radioiodide uptake was greater at 11 and 19° than at 5°. In conclusion, maintenance of trout at higher temperatures increased the plasma T4 degradation rate and rate of T4 to T3 conversion, implying elevated thyroidal secretion of T4 into the blood and elevated thyroidal status at higher temperatures. This trend was reflected neither in thyroid radioiodide uptake nor in plasma levels of T4 or iodide.