Overlap syndrome involving rheumatoid arthritis and systemic sclerosis complicated by Hashimoto's thyroiditis and interstitial lung disease detected in primary care: a case report.

Background: Overlap syndrome (OS) refers to a condition where a patient meets the classification criteria for at least two connective tissue diseases (CTDs), either simultaneously or at different points in times. The spectrum of OS includes mixed CTDs such as rheumatoid arthritis (RA) and systemic sclerosis (SSc). This syndrome may occur alongside other medical conditions, such as Hashimoto's thyroiditis and interstitial lung disease (ILD), complicating its diagnosis and management. Case Description: This report describes the case of a 48-year-old Malay woman who visited a primary care clinic, presenting with a year-long history of intermittent joint pain, swelling, morning stiffness, and skin thickening involving her hands and arms. On physical examination, swelling was observed in the proximal interphalangeal joints, with sclerodactyly extending to the upper arms. Initially, in primary care, she was diagnosed with RA and suspected SSc before being referred to a rheumatologist, where the diagnosis of OS was ultimately confirmed. Treatment with disease-modifying anti-rheumatic drugs and analgesics was initiated, leading to symptom improvement. However, she returned to primary care with symptoms of fatigue, dry skin, constipation, and dry cough which already persisted for 6 months. Thyroid function tests and anti-thyroid peroxidase antibodies confirmed the diagnosis of Hashimoto's thyroiditis. Spirometry showed a restrictive lung pattern, and high-resolution computed tomography indicated early ILD changes. She was managed by multidisciplinary teams, including primary care, rheumatology, endocrinology, respiratory medicine and radiology, and her condition improved. Conclusions: Managing OS in primary care is challenging due to multiple related conditions that complicate diagnosis and management. Primary care physicians play a vital role in early detection, timely referral, and coordinating multidisciplinary care for these complex conditions.