Thyroxine and Triiodothyronine in Milk of Cows,Goats,Sheep,and Guinea Pigs 1

Abstract

Milk was collected for the first 21 days of lactation twice daily from dairy cows and once daily from goats, sheep, and guinea pigs. Thyroxine (T₄) and triiodothyronine (T₃) were extracted from 100 μl of milk using acidified ethanol. T₄ and T₃ were reconstituted in 100 μl buffer and measured by radioimmunoassay. Concentrations (ng/ml) of T₄ and T₃ for milk of cows, goats, sheep, and guinea pigs, respectively, were: 0.97 and 0.94, 1.24 and 0.52, 0.99 and 0.79, and 1.41 and 0.53. T₄ concentration for guinea pig milk was significantly higher than for cow and sheep milk, but not for goat milk (P < 0.05). T₃ was found in higher concentration in milk of cows and sheep than in milk of goats and guinea pigs (P < 0.05). Species differences in conversion of T₄ to T₃ in mammary gland cells are suggested. Summations of T₄ and T₃ concentrations in milk indicated no differences among the four species. Regression analyses of changes in milk production, T₄ and T₃ concentrations, total T₄ and T₃ in milk per day, and ratios of T₄ to T₃ revealed variations in patterns. Concentrations of T₄ or T₃ tended to decrease as lactation progressed over 21 days. Total T₃ tended to increase, and the ratio of T₄ to T₃ tended to decrease. Amounts of T₄ and T₃ available to offspring from milk were calculated to be minor sources (4 to 7%) of total requirements for maintenance of metabolic functions.