Influence of injection route on radiothyroxine kinetics in brook trout, Salvelinus fontinalis (Mitchill)

Abstract

Groups of starved yearling brook trout at 12 °C were injected with ¹²⁵I-labelled thyroxine (*T₄) into the heart, coelom, or dorsal musculature. Disappearance of plasma-protein-bound radioactivity was described by fast and slow exponents. The fast exponent representing *T₄ loss was used to calculate T₄ metabolic clearance rate (MCR). MCR values were 2.79 (heart), 2.96 (coelom), and 4.92 ml/h per 100 g (muscle). The slower turnover of *T₄ in muscle-injected fish was accompanied by slower deiodination and enterohepatic excretion of *T₄. Heart injection is recommended for routine *T₄ kinetic studies in fish.