Iatrogenic goitrous hypothyroidism induced by long-term trimethoprim-sulfamethoxazole therapy in a dog: A case report.

Background: Hypothyroidism is a common endocrine disorder in dogs; however, drug-induced hypothyroidism is uncommon in humans and veterinary medicine. Sulfonamides are among the few antibiotics known to interfere with thyroid function. However, reports in dogs are rare and often underrecognized. Case Description: A 12-year-old male Bichon Frise presented with alopecia and a history of slowness and weakness, weight gain despite unchanged food intake, and poor coat quality. A palpable bilateral symmetrical mass was observed in the neck area on examination. The patient had been receiving trimethoprim-sulfamethoxazole therapy for several months. Laboratory testing revealed reduced total thyroxine (TT4), consistent with iatrogenic hypothyroidism secondary to prolonged sulfonamide administration. Following drug discontinuation, TT4 values normalized within 7 days, and the dog showed marked clinical improvement, which confirmed the suspected diagnosis. Conclusion: This case highlights the potential of prolonged sulfonamide therapy to induce reversible hypothyroidism in dogs. Clinicians should be aware of this adverse effect because early recognition and drug withdrawal can lead to rapid clinical and biochemical recovery.