A Complex Case of Polymyositis Overlapping With Hypothyroid Myopathy Without Underlying Autoimmune Thyroid Disorder

A 78-year-old woman with a past medical history of hypothyroidism and Sjogren's syndrome presented with a two-month history of gradually progressive bilateral lower extremity weakness. Significant elevation in thyroid-stimulating hormone (TSH) and muscle enzyme, such as creatine kinase, was noticed on presentation. Due to concerns of hypothyroid myopathy, the patient was started on thyroxine and triiodothyronine supplementation. The patient reported no significant improvement in her weakness in the one-month follow-up. Laboratory workup revealed improving TSH levels but worsening creatine kinase levels. Electromyography study showed primarily myopathic features, such as abnormal insertional activity concerning for mild inflammatory myopathy. Muscle biopsy showed mild inflammatory exudate and features of myopathy with ongoing denervation. The patient was diagnosed with polymyositis and started on prednisone 0.5 mg/kg daily with a taper course and methotrexate. The patient reported significant improvement in her weakness when seen in six weeks with normalizing creatine kinase levels. The hallmark difference between hypothyroid myopathy (including polymyositis-like syndrome) and conventional polymyositis is the complete clinical recovery and resolution of laboratory abnormalities after treatment with thyroid hormone replacement in hypothyroid myopathy. There was no evidence of underlying autoimmune thyroid disorder which makes this case unique. This case highlights the complex case of polymyositis overlapping with hypothyroid myopathy with no underlying autoimmune disorder.