Unmasking dermatomyositis in a patient with hypothyroid myopathy

Dermatomyositis (DM) and hypothyroid myopathy are both treatable causes of muscle weakness and can present with overlapping clinical and laboratory features, posing a diagnostic challenge. We report the case of a young male who presented with proximal upper and lower limb muscle weakness, calf muscle hypertrophy and elevated serum creatine kinase (CK) and was found to have hypothyroidism. Muscle weakness and hyperCKemia persisted despite normalisation of thyroid function, prompting a muscle biopsy and extended autoantibody testing, which revealed anti-Ku-positive DM characterised by perivascular inflammation, perifascicular atrophy and membrane attack complex deposition. High-dose corticosteroids led to partial improvement and subsequent rituximab therapy achieved sustained clinical and biochemical remission. This case emphasises the importance of considering dual aetiology when appropriate treatment of the primary diagnosis does not yield expected results. Early biopsy and comprehensive serological evaluation could lead to excellent clinical outcomes.