Abstract

Tissue uptake of radiothyroxine in Atlantic salmon parr, presmolts, and smolts held at 10.5 to 13.5 °C was assessed by measuring T/S ratios (tissue radioactivity/serum radioactivity) from 1 hour to 10 days after intraperitoneal injection of L-thyroxine–125I.T/S ratios were highest for gall bladder, liver, and gut, indicating a biliary thyroxine excretion route. Somewhat less uptake occurred in spleen, kidney, and gill. T/S ratios were lower still for skin and muscle but the pattern of T/S change with time suggested some radiothyroxine uptake by these tissues. Little uptake occurred in brain where low T/S ratios were consistently found.For most tissues there was no difference in thyroxine uptake between parr, smolts, and presmolts. However, for presmolts the means of gill T/S ratios were much higher than those for parr or smolts while the biliary thyroxine excretion route was most prominent in parr.The biological half-life of serum radioactivity after intraperitoneal injection of L-thyroxine–125I was similar for parr, presmolt, and smolt. Most of the serum radioactivity was not protein-bound and was probably free radiothyroxine. The injected radiothyroxine may have so increased the serum free thyroxine pool that serum protein thyroxine-binding sites were saturated.

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