Effects of Induced Mild Hyperthyroidism on Serum Protein-Bound Iodine, Thyroxine Distribution Volume, and Biological Half-Life of Thyroxine-131I in Dairy Cattle1,2

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.