Effects of Thyrotrophin, L-Thyroxine, and L-Triiodothyronine on Levels of Plasma Protein-Bound Iodine in Dairy Bull Calves

Intramuscular injections of thyrotrophin and intravenous injections of sodium L-thyroxine pentahydrate and sodium L-triiodothyronine were made in dairy bull calves (body wt. from 166 to 264 kg.), and plasma levels of protein-bound iodine (PBI) were used as a response criterion. The injection of thyrotrophin (Armour) in amounts of 10, 20, 40, and 80 U.S.P. units caused maximum PBI increases above initial levels ranging from 3.05 to 8.02/~g. % at 15 to 31 hr. postinjection. The mean biological half-life of the endogenously produced PBI, during its return to normal levels, was 22.1 hr. L-thyroxine (2.5 and 5.0 mg.) and L-triiodothyronine (5.58 and 11.16 rag.) were injected in each of three trials. Two first-order exponential regressions characterized the decline of PBI to normal in each instance. The first represented the rate of movement of the materials into their respective volumes of distribution (mean biological half-time of 1.93 hr. for L-thyroxine and 0.31 hr. for L-triiodothyronine). The second regression measured the rate of peripheral utilization or disposal of the materials (mean biological half-life of 18.0 hr. for L-thyroxine and 5.10 hr. for L-tri~odothyronine). The mean volume of distribution for L-thyroxine was 9.07% of body wt. ; whereas, the corresponding value for L-triiodothyronine was 37.5%. The constants developed in the L-thyroxine infusion trials were used to estimate the rate of thyroxine secretion or degradation (mean of 0.231 rag. per day per 100 lb. body wt.).