Comparative abilities of thyroxine and triiodothyronine as inhibitors of pituitary thyrotropin secretion in cattle

¹ Department of Dairy Husbandry, University of Missouri, Columbia, Missouri

A technique is described using radioactiveiodine (I¹³¹) and the goitrogen, Tapazole, in mature dairy cows, to compare the relative biological activity of l-thyroxine (T₄) and l-triiodothyronine (T₃) in individual animals to inhibit pituitary thyrotropin (TSH) release and, in turn, release of thyroidal-I¹³¹. The minimal amount of either hormone required is considered the equivalent T₄ or T₃ secretion rate. The individuals of the Holstein breed showed a mean molar relation of 1:2.5, which was significantly higher than the means of the other breeds. The mean of 22 animals showed a ratio of 1:2.14, indicating that T₃ is slightly over twice as effective as T₄ in blocking TSH and thyroidal-I¹³¹ release. While T₃ is more potent in blocking TSH discharge, it was observed that upon withdrawal of T₄ and T₃ the resumption of thyroidal-I¹³¹ release was more rapid after T₃ administration.