Iodide and thyroxine metabolism in the brook trout, Salvelinus fontinalis (Mitchill), during sustained exercise.

Abstract

Iodide and thyroxine metabolism were followed in exercised and non-exercised starved trout.Radioiodide (125I) parameters suggested a slight increase in thyroid activity (% dose in thyroid, thyroid/serum ratio (T/S), and conversion ratio (CR)) and an increase in extrathyroidal 125I excretion resulting from exercise.The generally high and variable serum 127I levels increased up to 496 μg% during the experiments, being consistently higher in exercised fish. Individual variations in serum 127I were related in many instances to radioiodide parameters (I125I, T/S, CR) and to a limited extent to % thyroid. This demonstrates the uncertainty of radioiodide measurements when the specific activity of circulating 125I changes and the need for assessing 127I levels in radioiodide studies.Tissue uptake of radioactivity and serum loss of protein-bound radioactivity following radiothyroxine intraperitoneal injection, as well as serum stable thyronine levels, suggested both increased biliary loss of thyroid hormones and a higher thyroid hormone secretion rate as the result of exercise.