A 16-year-old female with mild hirsutism was noted to have a small, smooth, non-tender goitre. A resting peripheral tremor, but no other symptoms or signs of thyroid dysfunction were present. Her only medication was the contraceptive pill. There was no family history of thyroid disease. Investigation showed elevated free thyroxine (28 pmol/l) and total triiodothyronine (3.4 nmol/l) with non-suppressed thyroid stimulating hormone (1.4 mU/l). Radioiodine uptake scan of the thyroid showed bilateral increased tracer uptake, suggestive of Graves' disease, however thyroid peroxidase and antithyroid stimulating hormone (TSH) receptor antibody testing was negative and sex hormone binding globulin concentration was normal. Laboratory analyses excluded assay artefact or abnormal circulating thyroid hormone binding proteins. Genetic analysis identified a thyroid hormone receptor gene mutation (T277I), making a diagnosis of resistance to thyroid hormone (RTH). RTH is a disorder characterised by elevated thyroid hormones, failure to suppress pituitary TSH secretion and variable refractoriness to hormone action in peripheral tissues.