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Effects of sulfamethoxazole-trimethoprim on thyroid function in dogs

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  • 1 Department of Small Animal Clinical Sciences, Veterinary Teaching Hospital, College of Veterinary Medicine, University of Tennessee, Knoxville, TN 37996-4544.
  • | 2 Department of Small Animal Clinical Sciences, Veterinary Teaching Hospital, College of Veterinary Medicine, University of Tennessee, Knoxville, TN 37996-4544.
  • | 3 Department of Small Animal Clinical Sciences, Veterinary Teaching Hospital, College of Veterinary Medicine, University of Tennessee, Knoxville, TN 37996-4544.
  • | 4 Department of Small Animal Clinical Sciences, Veterinary Teaching Hospital, College of Veterinary Medicine, University of Tennessee, Knoxville, TN 37996-4544.
  • | 5 Department of Small Animal Clinical Sciences, Veterinary Teaching Hospital, College of Veterinary Medicine, University of Tennessee, Knoxville, TN 37996-4544.

Abstract

Objective—To evaluate effects of trimethoprim-sulfamethoxazole (T/SMX) on thyroid function in dogs.

Animals—6 healthy euthyroid dogs.

Procedure—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 and Clinical Relevance—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. (Am J Vet Res 2005;66:256–259)

Abstract

Objective—To evaluate effects of trimethoprim-sulfamethoxazole (T/SMX) on thyroid function in dogs.

Animals—6 healthy euthyroid dogs.

Procedure—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 and Clinical Relevance—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. (Am J Vet Res 2005;66:256–259)