Chronically elevated thyroid‐stimulating hormone: resistance to thyroid hormone

A 49-year-old male farmer presented for management of hypercholesterolaemia. He was diagnosed as thyrotoxic in 1984: serum thyroxine (ST) was 234 μmol/L (normal range 58–155 μmol/L) and the free thyroxine index was 288 μmol/L (normal range 55–165 μmol/L). A technetium scan demonstrated a moderately enlarged goitre with increased thyroidal uptake and he commenced carbimazole (40 mg/day). In March 1985, ST was 144 μmol/L, thyroidstimulating hormone (TSH) was 45.6 mU/L (normal range < 4.5 mU/L) and the goitre had enlarged. Due to persistently elevated ST levels on carbimazole, a partial thyroidectomy was performed in April 1986. Thyroid function tests remained abnormal postoperatively (ST 202 μmol/L, TSH 5.4 mU/L) and the patient was treated with 131I in July 1986. Subsequently, normal ST was achieved with thyroxine (100–200 μg/day), during which time TSH levels ranged from 5 to 70 mU/L.