US EPA

9292662 

Journal Article 

Effects of sulfamethoxazole-trimethoprim on thyroid function in dogs 

Frank, LA; Hnilica, KA; May, ER; Sargent, SJ; Davis, JA; , 

2005 

Yes 

American Journal of Veterinary Research
ISSN: 0002-9645
EISSN: 1943-5681 

AMER VETERINARY MEDICAL ASSOC 

SCHAUMBURG 

256-259 

English 

15757124 

WOS:000226922600011 

To evaluate effects of trimethoprim-sulfamethoxazole (T/SMX) on thyroid function in dogs.6 healthy euthyroid dogs.Dogs were administered T/SMX (14.1 to 16 mg/kg, PO, q 12 h) for 3 weeks. Blood was collected weekly for 6 weeks for determination of total thyroxine (TT4), free thyroxine (fT4), and canine thyroid-stimulating hormone (cTSH) concentrations. Schirmer tear tests were performed weekly. Blood was collected for CBC prior to antimicrobial treatment and at 3 and 6 weeks.5 dogs had serum TT4 concentrations equal to or less than the lower reference limit, and 4 dogs had serum fT4 less than the lower reference limit after 3 weeks of T/SMX administration; cTSH concentrations were greater than the upper reference limit in 4 dogs. All dogs had TT4 and fT4 concentrations greater than the lower reference limit after T/SMX administration was discontinued for 1 week, and cTSH concentrations were less than reference range after T/SMX administration was discontinued for 2 weeks. Two dogs developed decreased tear production, which returned to normal after discontinuing administration.Results suggest that administration of T/SMX at a dosage of 14.1 to 16 mg/kg, PO, every 12 hours for 3 weeks caused decreased TT4 and fT4 concentrations and increased cTSH concentration, conditions that would be compatible with a diagnosis of hypothyroidism. Therefore, dogs should not have thyroid function evaluated while receiving this dosage of T/SMX for >2 weeks. These results are in contrast to those of a previous study of trimethoprim-sulfadiazine.