[Periodic paralysis as the first manifestation of hyperthyroidism]

A 19-year-old man had mild diarrhoea at the time that suddenly one night he was unable to turn in his bed and the following morning could not move his arms and legs for 4 hours. Neither he nor any family members had previously had any paralysis. Physical examination was unremarkable except for mild tachycardia and first-degree goitre.

A provocation test with glucose (3 g/kg) and insulin (0.1 IU/kg) caused renewed paralysis for several hours, serum potassium falling from 4.3 to 3.4 mmol/l. The paralysis was reversed on oral potassium (40 mmol) Thyroid function tests revealed hyperthyroidism with an increased concentration of free thyroxine (25.5 pg/ml) and free triiodothyronine (9.7 pg/ml), while thyroid-stimulating hormone was decreased (0.07 mU/I), supporting the diagnosis of autoimmune thyroiditis.

Thyrostatic treatment was started with thiamazole (10 mg every other day). There was no further periodic paralysis and another provocation test was negative.

Fleeting paralysis is often misdiagnosed as being psychogenic. Potassium abnormalities are the most common cause but are only rarely associated with hyperthyroidism. This case of thyrotoxic hypokalaemic paralysis was probably based on a genetic defect of muscle fibre membrane manifesting itself only in the presence of hyperthyroidism.