Comparison of Methods for Estimating Thyroid Secretion Rate in Chickens

THE classical goiter-prevention assay of Dempsey and Astwood (1943), first applied to chickens by Mixner et al. (1944), has been used often and to good advantage in studying thyroid secretion rate in poultry. In recent years, availability of radioactive iodine (I131) and development of modern detectors and counting equipment have allowed a new approach in secretion rate studies. Both the goitrogen and the radioiodine techniques are based on the assumption that the daily exogenous dose of thyroid hormone (usually thyroxine) which will block secretion of pituitary thyrotrophic hormone (TSH) is equivalent to the normal secretion rate. In the goiter-prevention assay, the blocking dose of thyroxine is estimated from thyroid weight data in goitrogen-fed animals; while in the I131 assay, the blocking dose is estimated from in vivo measurement of changes in thyroid radioactivity.1 Underlying both methods is the classical concept that the thyroid and hypophysis constitute a “feedback” mechanism in …