The combination of elevated serum levels of free thyroid hormones with non-suppressed thyroid-stimulating hormone suggests the differential diagnoses of resistance to thyroid hormone or thyroid-stimulating hormone–secreting pituitary tumour. Clinical differentiation of these two conditions can be difficult, because patients with thyroid hormone resistance may exhibit various combinations of hypermetabolic and hypometabolic features, and laboratory results have limited sensitivity and specificity. We report a case of resistance to thyroid hormone in a Chinese family that illustrates this difficulty. The diagnosis could only be confirmed by the identification of a known disease-causing mutation in the thyroid hormone receptor beta gene in peripheral leukocytes. Availability of genetic tests will identify more cases in the future and improve our understanding of this condition.