Effects of high doses of levothyroxine sodium on serum concentrations of triiodothyronine and thyroxine in horses.

OBJECTIVE: To investigate the effect of high doses of orally administered levothyroxine sodium (LT4 ) on serum concentrations of triiodothyronine (T3 ) and thyroxine (T4 ) in euthyroid horses.

ANIMALS: 12 healthy adult horses.

PROCEDURES: 10 horses initially received water (vehicle) or 240 mg (5X treatment) or 480 mg (10× treatment) of LT4 , and blood samples were collected at baseline (0 hours) and 0.5, 1, 2, 4, 6, 8, 10, 12, 18, 24, 48, 72, 96, and 120 hours after treatment to measure serum T3 and T4 concentrations. Three horses then received 480 mg of LT4 for 14 days, and T4 concentration was measured on days 0, 14, 21, 28, and 35. Changes in T3 and T4 concentrations were compared over time and among treatments.

RESULTS: One-time administration of LT4 resulted in variable but significant increases in both T3 and T4 concentrations for up to 120 hours; however, T3 and T4 concentrations rarely exceeded reference intervals with either treatment. Prolonged administration of 480 mg of LT4 resulted in a 15-fold increase in T4 concentration after 14 days, but concentration returned to day 0 values within 21 days after LT4 administration was discontinued.

CONCLUSIONS AND CLINICAL RELEVANCE: In euthyroid horses, administration of a high dose of LT4 resulted in mild increases in thyroid hormone concentrations; however, prolonged administration of high doses of LT4 resulted in markedly increased thyroid hormone concentrations that returned to pretreatment values within 3 weeks after discontinuation of LT4 administration. These results indicated complex kinetics of LT4 and suggested a possible saturation of T4 excretion in euthyroid horses.