A 42-year-old woman reports fatigue, cold intolerance and proximal muscle weakness. Examination shows dry skin and bradycardia. Serum creatine kinase is 1400 U/L with a normal CK-MB fraction, and ECG shows sinus bradycardia without ST changes. The most likely cause of the creatine kinase elevation is:

• A Silent myocardial infarction
• B Hypothyroid myopathy causing sarcolemmal leak from skeletal muscle ✓
• C Polymyositis requiring muscle biopsy before treatment
• D Occult statin-induced rhabdomyolysis

Explanation

Hypothyroidism commonly raises total creatine kinase, sometimes markedly, through impaired clearance and increased sarcolemmal permeability in skeletal muscle. The normal CK-MB fraction excludes significant myocardial injury, and the clinical picture of cold intolerance and bradycardia points to thyroid failure. Polymyositis would show inflammatory features and typically a higher CK with weakness out of proportion, and there is no history of statin use to suggest drug-induced rhabdomyolysis.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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Written and medically reviewed by the StethoPrep medical team.