Polymyositis-like hypothyroid myopathy: diagnostic challenges and therapeutic outcomes in a case series

Hypothyroid myopathy, affecting 30-80% of hypothyroid patients, often mimics polymyositis, posing diagnostic challenges. This retrospective case series analyzed 10 patients (70% female, mean age 46.1 ± 10.3 years) with polymyositis-like hypothyroid myopathy. All presented with proximal muscle weakness, myalgia, and/or arthralgia, accompanied by elevated muscle enzymes (CK, CK-MB, LDH), liver dysfunction (70%), hyperlipidemia (70%), and serous effusions (pericardial effusion: 70%). Notably, all patients tested negative for myositis antibodies but exhibited thyroid dysfunction (↓fT3/fT4, ↑TSH) and positive thyroglobulin antibodies (100%). Muscle biopsies revealed degeneration and atrophy without necrosis or inflammation. After 3 months of thyroxine replacement therapy (100-150 µg/day), symptoms significantly improved (P < 0.05), with reductions in ALT, AST, CHOL, CK, CK-MB, and LDH (all P < 0.01), and normalization of fT3/fT4 levels. Muscle enzyme levels correlated negatively with fT3/fT4 (r_s =  - 0.76 to - 0.78, P < 0.05) and positively with TSH (r_s = 0.70 to 0.72, P < 0.05). This study highlights the multisystem manifestations of hypothyroid myopathy (metabolic, hepatic, serous) and underscores the importance of thyroid screening in unexplained myopathy, even in the absence of classic hypothyroidism symptoms. Early hormone replacement therapy yields an excellent prognosis.