Estimation of Thyroid Secretion Rates in Dairy Goats and Measurement of I131 Uptake and Release with Regard to Age, Pregnancy, Lactation, and Season of the Year1

Abstract

The substitution technique for measuring thyroid secretion rate in individual intact animals was adapted to the dairy goat. Normal goats were found to have a sufficiently rapid thyroidal I¹³¹ output rate to permit application of the method without thiouracil administration. Thiouracil-treated goats had a more rapid I¹³¹ output rate, but their thyroid secretion rate as measured by the thyroxine substitution method was not significantly different from values found concurrently in normal goats. In both young (2-year-old) and aged (4-to-6-year-old) goats, the thyroid secretion rate was high in the fall, spring, and winter, but declined to about one-half the winter level during July. Comparisons between 8-month-old, 2-year-old, and 4- to 6-year-old animals showed a progressive decline in thyroid secretion rate with increasing age. The percent uptake of injected I¹³¹ by the thyroid showed no consistent correlation with thyroid secretion rate. However, there was a definite increase in uptake during the summer. Thyroidal I¹³¹ output half-time (t ½) values were smaller in both young and aged groups during February and May than in July, August, and October. Again there was no consistent correlation with thyroid secretion rate. On the basis of these comparisons, it was concluded that neither thyroidal I¹³¹ uptake nor output rate are reliable indices of thyroid function in goats exposed to normal environmental variations. Both aged and young lactating goats had a lower thyroid secretion rate than nonlactating controls of similar age during each of the three months in which comparisons were made. Thyroidal uptake of I¹³¹ was only about one-half as great in lactating as in nonlactating animals. Within lactating groups, there was a negative correlation between milk yield and thyroid secretion rate and also between milk yield and I¹³¹ uptake. Determinations of I¹³¹ in milk and blood serum from five animals showed an average maximal concentration of I¹³¹ in milk that was six times as high as that in serum. During the first 50 hours after I¹³¹ administration, an average of 26.7% of the dose was secreted in milk and 9.6% retained in the thyroid. These data were interpreted to indicate that through the udder-kidney-thyroid competition for available iodine, a functional iodine deficiency was created, and this resulted in a reduced thyroid secretion rate. Copyright © . .