Effects of Thiouracil Level and Pen Position on Thyroxine Secretion Rate Determined by I131 Assay

Abstract

THE recent use of radioactive iodine (I131) in thyroid secretion rate (TSR) studies has constituted a significant advance in the field of thyroid physiology. Potential applications in poultry research are numerous, and some investigators have already started to exploit these (Premachandra et al., 1958; Mueller and Amezcua, 1959; Mellen and Wentworth, 1960).Pipes et al. (1958) have called attention to the complicating effect of “recycled” I131 on apparent release rate of thyroidal I131, pointing out that some of the inorganic I131 available from metabolized thyroid hormone again reaches the thyroids and can be re-incorporated into thyroglobulin. In chickens, this re-utilization of I131 occurs to such an extent that TSR estimates (based on calculation of the exogenous thyroxine dose which blocks further release of thyroidal I131) are difficult or impossible unless an antithyroid drug such as thiouracil is administered. As Pipes et al. (1958) state: “It is not necessary to completely …