title: Effects of sulfamethoxazole-trimethoprim on thyroid function in dogs.
abstract: OBJECTIVE
  To evaluate effects of trimethoprim-sulfamethoxazole (T/SMX) on thyroid function in dogs.
  
  
  METHODS
  6 healthy euthyroid dogs.
  
  
  METHODS
  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.
  
  
  RESULTS
  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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Department of Small Animal Clinical Sciences, Veterinary Teaching Hospital, College of Veterinary Medicine, University of Tennessee, Knoxville, TN 37996-4544, USA.
authors.forename: Linda A
authors.initials: LA
authors.lastname: Frank
authors.forename: Keith A
authors.initials: KA
authors.lastname: Hnilica
authors.forename: Elizabeth R
authors.initials: ER
authors.lastname: May
authors.forename: Sandra J
authors.initials: SJ
authors.lastname: Sargent
authors.forename: Jacqueline A
authors.initials: JA
authors.lastname: Davis
mesh_terms: D000818:Animals; D000890:Anti-Infective Agents / Q000494:pharmacology*; D004285:Dogs; D005260:Female; D015592:Immunoradiometric Assay / Q000662:veterinary; D008297:Male; D011863:Radioimmunoassay / Q000662:veterinary; D013961:Thyroid Gland / Q000187:drug effects* / Q000378:metabolism; D013972:Thyrotropin / Q000097:blood; D013974:Thyroxine / Q000097:blood; D015662:Trimethoprim, Sulfamethoxazole Drug Combination / Q000494:pharmacology*
chemical_list: D000890:Anti-Infective Agents; D015662:Trimethoprim, Sulfamethoxazole Drug Combination; D013972:Thyrotropin; D013974:Thyroxine