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Abstract
Total thyroxine and total tri-iodothyronine concentrations were measured in the sera from 125 horses of mixed age, breed and sex, and varied clinical histories. While low serum thyroxine concentrations were detected in 35 horses, the majority of those horses had serum thyroxine values within the reference range when retested. Only one horse had a mildly decreased serum tri-iodothyronine concentration. Those horses in which the serum thyroxine concentration was low when retested had a normal thyrotropin releasing hormone stimulation test. Hypothyroidism was not diagnosed in any horses in this study. The low serum thyroxine concentrations measured in the present study were attributed to either normal fluctuations in serum concentrations in healthy horses, the effect of drugs, or to the effects of non-thyroidal illness. Because thyroid hormone concentrations are altered by many factors, hypothyroidism should not be diagnosed on the basis of a single low value and further testing, preferably including active stimulation of the thyroid gland, should be carried out.