A 25-year-old male with hyperthyroidism had in the first week of treatment signs of thyreotoxic periodic paralysis with very low potassium level (2.16 mmol/ml) and fivefold increase in serum myoglobin level (600 ng/ml) indicating damage to muscle fibres undetectable in EMG examination. The clinical symptoms regressed after intravenous administration of potassium.