Concurrent Thyrotoxicosis-Induced Atrial Fibrillation and Seropositive Rheumatoid Arthritis Masquerading as Urinary Sepsis: A Case Report

Autoimmune thyroid disease and rheumatoid arthritis (RA) share overlapping immunogenetic pathways, but their simultaneous, fulminant onset is rarely reported and can be mistaken for sepsis. A 68-year-old previously healthy woman presented with fever, chills, polyarthralgia, and an irregular pulse of 160 beats/min, initially diagnosed as urinary sepsis with hyponatremia. Work-up revealed suppressed thyroid-stimulating hormone (TSH) with elevated free T3/T4 and atrial fibrillation with fast ventricular response, alongside positive anti-cyclic citrullinated peptide antibody, rheumatoid factor, and elevated erythrocyte sedimentation rate, confirming synchronous thyrotoxicosis and seropositive RA. She improved with rate control, anticoagulation, carbimazole, and disease-modifying antirheumatic drugs (DMARDs). Autoimmune emergencies can closely mimic infection, a high index of suspicion and multidisciplinary management are essential to prevent cardiovascular and metabolic decompensation.