Thyroid hormones (T3, T4) exert multiple cellular effects through nuclear thyroid hormone receptors (TRα, TRβ). Thyroid hormone receptors are transcription factors that act by altering patterns of gene expression. Resistance to thyroid hormone (RTH) is a rare disorder caused by mutations in the TRβ gene. Biochemically, the syndrome is defined by elevated circulating levels of free thyroid hormones due to reduced target tissue responsiveness and normal, or elevated, levels of thyroid-stimulating hormone (TSH). This “inappropriate” TSH elevation contrasts with the situation in hyperthyroidism, where the pituitary secretion of TSH is suppressed. Patients with RTH usually present with goiter and an euthyroid or mildly hypothyroid metabolic state. Thus, pituitary resistance results in hypersecretion of TSH, which compensates, at least in part, for hormone resistance in peripheral tissues. Despite this compensation, clinical effects of RTH can include short stature, delayed bone maturation, hyperactivity, learning disabilities, and hearing defects, as well as variable features of hyper- and hypothyroidism.