A study of thyroid activity in dwarf and non-dwarf female chicks during key physiological states of growth and reproduction.

Thyroid function, as indicated by the maximum percent uptake of iodine-131, was measured in non-dwarf and recessive, sex-linked dwarf White Leghorn female chickens at different ages. The rate of uptake and release of 131-I by the thyroid gland was closely monitored over a period of eight days after the injection of the isotope. No significant differences were observed in thyroid uptake value of dwarf chicks in one-week-old group (approximately equal to 11.5 per cent for each group). At the age of four weeks the maximum uptake was only 5.58 per cent in dwarfs compared to 10.11 per cent in non-dwarf chicks. Potassium thiocyanate treatment caused a significant reduction in the uptake of iodine-131 by the thyroid glands of both 4-week-old dwarf and non-dwarf chicks although there was no difference in the response pattern of either group to the treatment. Dwarf laying-hens had a significantly lower maximum thyroid uptake (1.4 per cent) when compared to non-dwarf laying hens (2.48 per cent). A consistantly slower rate of uptake by the thyroid glands of dwarf chickens was observed for all age groups than equivalent non-dwarfs but there were no apparent differences in the release rates of 131-I by dwarfs when compared to non-dwarfs. No significant differences were observed between the 131-I uptake by the ovaries or eggs of both dwarf and non-dwarf laying hens. A gradual decrease in thyroid activity was observed in both dwarf and non-dwarf chickens with increased age. From this study it was evident that the dwarf bird had significantly lower thyroid activity but with normal organification. The defect in thyroid function in the dwarf bird apparently was not at the level of synthesis but at the level of uptake of iodine.