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.