Effects of Thiouracil on Turnover of Thyroidal I 131 and Thyroxine Secretion Rate Measurement

Summary

The effects of thiouracil on turnover of I¹³¹ by the thyroid in immature and mature chickens were studied. Rate of uptake of I¹³¹ by the thyroid was significantly reduced and release rate of I¹³¹ from the thyroid was significantly augmented in thiouracil-fed birds. Rate of recycling of I¹³¹ from the metabolized thyroid hormone measured by accumulation of I¹³¹ by the thyroid from I¹³¹-labelled thyroxine injected was always a little less than rate of uptake of I¹³¹ by the thyroid. Both uptake and recycling of thyroidal I¹³¹ decreased with length of the period of thiouracil feeding. Rate of release of I¹³¹ from the thyroid increased with the lengthened time of thiouracil feeding. The thyroxine secretion rate expressed either per 100 g body weight or per bird measured by the thyroxine substitution significantly increased in thiouracil-fed birds compared with normal diet-fed birds. It also increased with length of thiouracil feeding. Use of thiouracil is not adequate for precise determination of thyroxine secretion rate in the chicken.