CHANGES IN THYROID ACTIVITY DURING GROWTH IN THE SHEEP

Abstract

SUMMARY The rate constants for thyroid uptake (K1) and release (K4) of 131I, the serum protein-bound iodine (PBI) level and the rate of secretion of thyroid hormone have been determined for sheep from 5 weeks to 7 years old. The secretion rate of thyroid hormone has been shown to increase rapidly up to the age of 6–8 months, and then decline slightly with increasing age. The rate constant for 131I release from the thyroid (K4), showed an increase up to 1 year of age, and considerably lower values at 5 and 7 years old. When the thyroid hormone secretion rate is expressed per unit body weight, the highest rate was observed at approx. 6 months of age or 20–40 kg. body weight. The significance of this initial increase in thyroid hormone secretion rate per unit body weight is discussed in conjunction with the results of other workers on the energy metabolism of immature animals. Serum PBI levels decreased sharply during the 1st year of life and then remained relatively constant.