False elevation of serum thyroxine in myxoedema due to thyroxine-binding autoantibodies. A diagnostic pitfall.

A 29-year-old male Caucasian with no relevant family history presented with a history of constipation, increasing tiredness and a feeling of intense coldness. His performance at sport, notably rugby and cricket, had fallen off badly largely because of weakness and muscle stiffness. His hair was falling out and his skin had become very dry. The clinical diagnosis was of myxoedema. He had a past history of nocturnal convulsions related to heavy alcohol consumption for which he was taking phenytoin sodium 100mg at night, but alcohol intake was now modest. Initial tests of thyroid function were surprisingly high serum total T4 124nmol/1 (reference range 60-160), serum 'free' T4 168pmol/I (reference range 9-28). He was referred to St Luke's Hospital, Guildford, for further investigation.