Effect of trimethoprim/sulfamethoxazole on endogenous thyroid stimulating hormone concentrations in dogs

The objectives of this study were to determine the effects of trimethoprim/sulfamethoxazole (T/SMX) on endogenous thyroid stimulating hormone (TSH) concentrations in dogs. The study was designed as a blinded crossover trial. Twelve clinically normal dogs with normal thyroid function (based on basal thyroid hormone values and TSH response tests) were randomly assigned to one of two groups of 6 dogs each. During each of two six-week treatment periods, dogs in one group received a placebo (sucrose). Serum samples were collected every 3 weeks for cTSH assay. Mean pretreatment serum cTSH concentrations was 45 mU/L (range 17-111). Following treatment with T/SMX, the mean serum cTSH increased to 125 mU/L (range 43-308) at 3 weeks and 278 mU/L (range 116-388) at 6 weeks. The normal reference range for the laboratory at the time of this study was 7-40 mU/L, with values greater that 60 mU/L considered suggestive of hypothyroidism. This study shows that dogs treated with T/SMX at 30 mg/kg q 12 hours have significant elevations in TSH concentrations. Veterinarians should consider the possibility of drug induced alterations in thyroid gland physiology when evaluating the results of any test of thyroid function.