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Effects of Thyrotrophin, L-Thyroxine, and L-Triiodothyronine on Levels of Plasma Protein-Bound Iodine in Dairy Bull Calves^{1, 2,}

New Jersey Agricultural Experiment Station, Sussex

ABSTRACT

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 µg. % 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 mg.) 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-triiodothyronine). 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 mg. per day per 100 lb. body wt.).

¹ Paper of the Journal Series, New Jersey Agricultural Experiment Station, Rutgers University, the State University of New Jersey, Department of Dairy Science, New Brunswick.

² This study was part of a Northeast Regional Project (NE-30, The Relation of Certain Physiological Factors to the Productive Capacity of Dairy Cattle), a cooperative study involving agricultural experiment stations in the Northeastern Region and supported in part by regional funds of the United States Department of Agriculture.