Conversion of atrial fibrillation after levothyroxine in a dog with hypothyroidism and arterial thromboembolism.

A six-year-old female spayed mixed-breed dog was referred following a 3-week history of lameness and progressive neurological deficits in both hindlimbs, and a 1-week history of a cardiac arrhythmia. The dog was diagnosed with hypothyroidism, atrial fibrillation, myocardial dysfunction and arterial thromboembolism. Cardioversion occurred after 2 weeks of levothyroxine supplementation, with improved systolic function over time. To the authors' knowledge, this is the first case reported in the veterinary or human literature documenting hypothyroidism with persistent atrial fibrillation being converted to sinus rhythm with levothyroxine as sole therapy.