Shoulder pain is a common complaint encountered in primary care and is most frequently attributed to benign musculoskeletal conditions. However, persistent symptoms that fail to respond to conventional treatment may occasionally represent the first manifestation of a serious underlying disease. Bone metastases are a common complication of advanced lung cancer, yet they rarely present as the initial clinical feature in the absence of respiratory symptoms. We report the case of a 58-yearold man who presented with progressive atraumatic shoulder pain initially diagnosed as rotator cuff tendinopathy in a family medicine setting. Despite conservative management, his symptoms worsened and became associated with nocturnal pain, fatigue, and unintentional weight loss. Further investigation revealed anemia, elevated inflammatory markers, and an osteolytic lesion of the proximal humerus. Advanced imaging identified a previously undiagnosed pulmonary mass, and histopathological examination confirmed metastatic lung adenocarcinoma. The patient underwent prophylactic orthopaedic fixation of the humerus followed by systemic oncological treatment. This case highlights the importance of recognizing redflag symptoms in patients with persistent musculoskeletal pain and emphasizes the value of interdisciplinary collaboration between family medicine, internal medicine, radiology, pathology, orthopaedic oncology, pulmonology, and medical oncology.