A 35-year-old Caucasian system analyst presented to his GP with a 6-month history of intermittent weakness of both upper and lower limbs lasting 30–40 minutes and resolving spontaneously. He gave no history of any precipitating factors. There was no history of headaches, dizziness, loss of consciousness or sphincter disturbances. His vision, speech and swallowing were not affected during the episodes of limb weakness. All of the episodes occurred on getting out of bed, most on waking in the morning, except for one which occurred on waking in the middle of the night when he got out of bed to go to the toilet. His legs gave way but he did not lose consciousness. There were no symptoms and signs of thyrotoxicosis. He was referred for further assessment. Neurological and systemic examination did not reveal any abnormal findings. Investigations including magnetic resonance imaging scan of the spine, vitamin B12, red cell folate, urea and electrolytes, full blood count and erythrocyte sedimentation rate were within normal limits. However, his thyroid function revealed free thyroxine >50 μmol/litre and thyroid-stimulating hormone <0.02 pmol/litre suggesting thyrotoxicosis. He was positive for anti-microsomal thyroid antibodies. A technetium scan showed symmetric enlargement of both lobes of the thyroid gland with an avid uptake. On treatment with carbimazole the attacks became less frequent and over a period of 3 months regressed completely. He remains asymptomatic after treatment with carbimazole.