Serum triiodothyronine, total thyroxine, and free thyroxine concentrations in horses

The objectives of this experiment were to determine serum concentrations of triiodothyronine (T3), thyroxine (T4), and free thyroxine (fT4) at rest, following thyroid-stimulating hormone (TSH) administration, and following phenylbutazone administration in healthy horses. This was done to determine which available laboratory test can best be used for diagnosis of hypothyroid conditions in horses. Serum T3, T4, and fT4 concentrations in serum samples obtained before and after TSH stimulation and following phenylbutazone administration for 7 days were determined. Baseline values ranged from 0.21 to 0.80 ng of T3/ml, 6.2 to 25.1 ng of T4/ml, and 0.07 to 0.47 ng of fT3/dl. After 5 IU of TSH was administered IV, serum T3 values increased to 6 times baseline values in 2 hours. Thyroxine values increased to 3 times baseline values at 4 hours and remained high at 6 hours. Free T4 values increased to 4 times baseline values at 4 hours and remained high at 6 hours. Administration of 4.4 mg of phenylbutazone/kg, every 12 hours for 7 days significantly decreased T4 and fT4 values, but did not significantly affect serum T3 concentrations. It was concluded that a TSH stimulation test should be performed when hypothyroidism is suspected. Measurement of serum fT4 concentrations, by the single-stage radioimmunoassay, does not provide any additional information about thyroid gland function over that gained by measuring T4 concentrations. Phenylbutazone given at a dosage of 4.4 mg/kg every 24 hours, for 7 days did significantly decrease resting T4 and fT4 concentrations, but did not significantly affect T3 concentrations in horses.