Advertisement

Effects of short-term trimethoprim-sulfamethoxazole administration on thyroid function in dogs

Nicola L. WilliamsonDepartment of Small Animal Clinical Sciences, College of Veterinary Medicine, University of Tennessee, Knoxville, TN 37901.

Search for other papers by Nicola L. Williamson in
Current site
Google Scholar
PubMed
Close
 DVM
,
Linda A. FrankDepartment of Small Animal Clinical Sciences, College of Veterinary Medicine, University of Tennessee, Knoxville, TN 37901.

Search for other papers by Linda A. Frank in
Current site
Google Scholar
PubMed
Close
 MS, DVM, DACVD
, and
Keith A. HnilicaDepartment of Small Animal Clinical Sciences, College of Veterinary Medicine, University of Tennessee, Knoxville, TN 37901.

Search for other papers by Keith A. Hnilica in
Current site
Google Scholar
PubMed
Close
 MS, DVM, DACVD

Abstract

Objective—To determine how rapidly trimethoprimsulfamethoxazole affects serum total thyroxine (T4) and thyroid-stimulating hormone (TSH) concentrations in euthyroid dogs and how quickly hormone concentrations return to reference values following discontinuation of administration.

Design—Prospective study.

Animals—7 healthy euthyroid dogs.

Procedure—Dogs were given trimethoprim-sulfamethoxazole (26.5 to 31.3 mg/kg [12 to 14.2 mg/lb], PO, q 12 h) for a maximum of 6 weeks. A CBC and Schirmer tear test were performed and serum total T4 and TSH concentrations were measured weekly. Administration of trimethoprim-sulfamethoxazole was discontinued if total T4 concentration was less than the lower reference limit and TSH concentration was greater than the upper reference limit or if persistent neutropenia developed.

Results—Six dogs had total T4 concentrations less than the lower reference limit within 3 weeks; T4 concentration was decreased after 1 week in 3 of these 6 dogs. In these 6 dogs, TSH concentration was greater than the upper reference limit within 4 weeks. In 1 dog, T4 and TSH concentrations were not affected, despite administration of trimethoprimsulfamethoxazole for 6 weeks. Neutropenia developed in 4 dogs. In 1 dog, the neutropenia resolved while trimethoprim-sulfamethoxazole was still being administered. In the other 3, neutrophil counts returned to reference values 1 week after drug administration was discontinued.

Conclusions and Clinical Relevance—Results suggest that administration of trimethoprim-sulfamethoxazole at a dosage of 26.5 to 31.3 mg/kg, PO, every 12 hours can substantially alter serum total T4 and TSH concentrations and neutrophil counts in dogs within as short a time as a few weeks. (J Am Vet Med Assoc 2002;221:802–806)

Abstract

Objective—To determine how rapidly trimethoprimsulfamethoxazole affects serum total thyroxine (T4) and thyroid-stimulating hormone (TSH) concentrations in euthyroid dogs and how quickly hormone concentrations return to reference values following discontinuation of administration.

Design—Prospective study.

Animals—7 healthy euthyroid dogs.

Procedure—Dogs were given trimethoprim-sulfamethoxazole (26.5 to 31.3 mg/kg [12 to 14.2 mg/lb], PO, q 12 h) for a maximum of 6 weeks. A CBC and Schirmer tear test were performed and serum total T4 and TSH concentrations were measured weekly. Administration of trimethoprim-sulfamethoxazole was discontinued if total T4 concentration was less than the lower reference limit and TSH concentration was greater than the upper reference limit or if persistent neutropenia developed.

Results—Six dogs had total T4 concentrations less than the lower reference limit within 3 weeks; T4 concentration was decreased after 1 week in 3 of these 6 dogs. In these 6 dogs, TSH concentration was greater than the upper reference limit within 4 weeks. In 1 dog, T4 and TSH concentrations were not affected, despite administration of trimethoprimsulfamethoxazole for 6 weeks. Neutropenia developed in 4 dogs. In 1 dog, the neutropenia resolved while trimethoprim-sulfamethoxazole was still being administered. In the other 3, neutrophil counts returned to reference values 1 week after drug administration was discontinued.

Conclusions and Clinical Relevance—Results suggest that administration of trimethoprim-sulfamethoxazole at a dosage of 26.5 to 31.3 mg/kg, PO, every 12 hours can substantially alter serum total T4 and TSH concentrations and neutrophil counts in dogs within as short a time as a few weeks. (J Am Vet Med Assoc 2002;221:802–806)