Resistance to thyroid hormone (RTH) syndrome is a rare disorder usually inherited as an autosomal dominant trait. The combination of elevated serum levels of free thyroid hormones with elevated thyroid-stimulating hormone (TSH) suggest differential diagnoses of RTH, thyroxine-binding globulin abnormalities, familial dysalbuminemic hyperthyroxinernia and TSH-secreting pituitary tumors. We report a patient with RTH in a Turkish family. The diagnosis was confirmed by the identification of a known disease-causing mutation in the thyroid hormone receptor-beta (THR beta) gene, but is the first published in the Turkish population. Genetic analysis of the mother and the patient yielded a mutation in the THRP gene, A317T, due to a base pair substitution of an adenine for a guanine.