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Effects of Induced Mild Hyperthyroidism on Serum Protein-Bound Iodine, Thyroxine Distribution Volume, and Biological Half-Life of Thyroxine-¹³¹I in Dairy Cattle^{1, 2,}

Department of Dairy Husbandry, University of Missouri, Columbia

ABSTRACT

Lactating and dry dairy cattle of several breeds were injected with exogenous L-thyroxine at levels 25 and 50% in excess of the normal thyroid hormone secretion rate. The biological half-life of L-thyroxine ¹³¹I became shorter as the degree of induced hyperthyroidism increased. Serum protein-bound iodine levels increased markedly above control values as the level of injected L-thyroxine increased. Thyroxine distribution volume showed an apparent increase over control values at 125 and 150% of normal thyroid secretion rate, but values at the 150% level declined below those at the 125% level. Thyroid secretion rates of control animals as determined by replacement technique and isotope dilution technique were in fair agreement with each other. The calculated utilization rates of L-thyroxine at the 125 and 150% of normal thyroid secretion rate level were two to three times higher than the known amounts of L-thyroxine injected daily. Excess thyroxine, up to 50% above normal secretion rate, seemed to have been eliminated from the body by rapid removal from the blood after the thyroxine-binding proteins were saturated.

¹ Contribution from the Missouri Agricultural Experiment Station, Journal Series no. 2808. Approved by the Director.

² Supported in part by grants from the U.S. Atomic Energy Commission. Contract AT(11-1)COO-301-102 and AT (11-1)COO-1758-7.