Effects of short-term trimethoprim-sulfamethoxazole administration on thyroid function in dogs.
OBJECTIVE
To determine how rapidly trimethoprim-sulfamethoxazole 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.


METHODS
Prospective study.


METHODS
7 healthy euthyroid dogs.


METHODS
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 trimethoprim-sulfamethoxazole 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
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.
D000818:Animals; D000890:Anti-Infective Agents / Q000009:adverse effects* / Q000494:pharmacology; D004283:Dog Diseases / Q000097:blood / Q000139:chemically induced; D004285:Dogs / Q000097:blood / Q000502:physiology*; D005260:Female; D007700:Kinetics; D008297:Male; D009503:Neutropenia / Q000139:chemically induced / Q000662:veterinary; D011446:Prospective Studies; D013961:Thyroid Gland / Q000187:drug effects* / Q000502:physiology; D013972:Thyrotropin / Q000097:blood* / Q000187:drug effects; D013974:Thyroxine / Q000097:blood* / Q000187:drug effects; D015662:Trimethoprim, Sulfamethoxazole Drug Combination / Q000009:adverse effects* / Q000494:pharmacology
D000890:Anti-Infective Agents; D015662:Trimethoprim, Sulfamethoxazole Drug Combination; D013972:Thyrotropin; D013974:Thyroxine