The acute effects of food and glucose challenge on plasma thyroxine and triiodothyronine levels in previously starved rainbow trout (Oncorhynchus mykiss).

The acute effects of a single meal on plasma L-thyroxine (T4) and 3,5,3'-triiodo-L-thyronine (T3) levels were examined in rainbow trout starved for 3 days. Plasma T4 increased within 2 hr of food intake and remained elevated to 8 hr. Plasma T3 was not altered consistently. Feeding-induced elevations in plasma T4 were present only in trout weighing less than 250 g and if they consumed a ration exceeding 0.38% of body weight. Postprandial elevations in plasma glucose paralleled those in plasma T4, suggesting a possible relationship between glucose intake and food-induced alterations in plasma T4. In trout intraperitoneally (ip) injected 4 hr earlier with 0.7% NaCl containing 0.2 or 2.0 g/kg D-glucose, plasma T4 increased relative to that in saline-injected controls. In starved trout cannulated in the dorsal aorta to permit serial blood removal, ip injection of glucose (0.85 g/kg) increased plasma glucose at 1 hr and plasma T4 at 2 hr, but did not alter plasma T3. It is concluded that enhanced glucose availability associated with feeding starved trout contributes to the postprandial elevation in plasma T4.