Effect of Varying Levels of Tapazole on Thyroidal-I131 Uptake in New Hampshire Pullets

IN THE routine determination of thyroxine secretion rate (TSR) in fowls, 0.05% tapazole (methimazole) has been added to the ration to prevent recycling of I131 resulting from the metabolism of thyroxine-I131. This has been done to obtain the true release rate of thyroidal-I131. In chicks under 4–6 weeks of age, the release rate is rapid and goitrogens are not necessary but in older birds the release rate generally slows down and prevention of recycling of I131 is necessary to indicate the end-point of TSR determination. At the level of 0.05% tapazole used up to this time, it has been noted that tapazole even with thyroxine administration tended to reduce egg production and feed consumption and birds sometimes began to molt. It has been suggested, also, that goitrogens may influence thyroxine secretion. Sell and Balloun (1960) reported an increased occurrence of capillary rupture in the thyroids of cockerels treated…