The finding of increased thyroxine (T4) and tri-iodothyronine (T3) levels in a patient with normal or increased thyroid-stimulating hormone is unexpected and presents a differential diagnosis between a thyroid-stimulating hormone-secreting pituitary adenoma, generalized resistance to thyroid hormone (RTH) and laboratory artefact. Without careful clinical and biochemical evaluation, errors may occur in patient diagnosis and treatment. In the case of RTH, mutation of the thyroid hormone receptor beta gene results in generalized tissue resistance to thyroid hormone. As the pituitary gland shares in this tissue resistance, euthyroidism with a normal thyroid-stimulating hormone is usually maintained by increased thyroid hormones. To date, we have identified eight pedigrees in New Zealand with mutations in the thyroid hormone receptor beta gene, including two novel mutations. Mutational analysis of the thyroid hormone receptor beta gene allows definitive diagnosis of RTH, potentially avoiding the need for protracted and expensive pituitary function testing and imaging. Mutational analysis also enables family screening and may help to avoid potential misdiagnosis and inappropriate treatment.