In vivo uptake of radiothyroxine by the tissues of Atlantic salmon (Salmo salar L.) parr, presmolt, and smolt

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.