Peer-reviewed veterinary case report
Dog developed hypothyroidism and neck swelling after long antibiotic
By Antea Ljubez et al.·Published in Open Veterinary Journal·2026·Zmaja od Bosne 90, 71000 Sarajevo, LY·View original on DOAJ →
PetCaseFinder translated the abstract of this peer-reviewed paper into plain English so pet owners can read it. We do not publish original research — every detail traces back to the citation above. How we work →
Original publication title: Iatrogenic goitrous hypothyroidism induced by long-term trimethoprim–sulfamethoxazole therapy in a dog: A case report
- Species:
- dog
Plain-English summary
A 12-year-old male Bichon Frise was brought to the vet with symptoms like hair loss, weakness, weight gain despite eating normally, and a noticeable swelling in the neck. After some tests, it was found that the dog had low thyroid hormone levels due to long-term use of an antibiotic called trimethoprim-sulfamethoxazole. Once the medication was stopped, the dog's thyroid levels returned to normal within a week, and he showed significant improvement in his overall health. This case serves as a reminder that some medications can affect thyroid function in dogs, but stopping the drug can lead to a quick recovery.
People also search for: dog weight gain thyroid problem · Bichon Frise hair loss treatment · trimethoprim-sulfamethoxazole side effects in dogs
Abstract
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. [Open Vet. J. 2026; 16(1.000): 758-762]
Find similar cases for your pet
PetCaseFinder finds other peer-reviewed reports of pets with the same symptoms, plus a plain-English summary of what was tried across them.
Search related cases →Original publication on DOAJ: https://doi.org/10.5455/OVJ.2026.v16.i1.73