Physical injury due to injection or blood removal causes transitory elevations of plasma thyroxine in rainbow trout, Salmo gairdneri

Abstract

A transitory 100–200% increase in plasma thyroxine (T4) occurs within 2 h of either blood sampling from caudal vessels or saline injection into coelomic or cardiac regions of rainbow trout anaesthetized in methane tricaine sulphate (MS 222). Plasma 3,5,3′-triiodothyronine (T3) does not usually increase and may decrease slightly. MS 222 anaesthesia does not contribute to the response, which is believed to be caused by physical injury above a certain threshold.Physical injury does not modify the peripheral metabolism of labeled T4 as judged by its deiodination, enterohepatic excretion, or liver uptake, and it is concluded that the plasma T4 response is due to increased thyroidal discharge of T4. This view is supported by the marked elevation of plasma T4 but negligible change in plasma T3 following bovine TSH injection.It was also shown that plasma T4 and T3 levels of 1-year-old trout starved for 3 days do not exhibit significant diurnal cycling, do not vary between sexes, but are correlated with body weight.