Thyroid Hormone Resistance (RTH) is characterized by the diminished response of thyroid hormone-responsive tissues in varying degrees in association with elevated serum levels of total and free T4 and T3 and inappropriately normal or elevated TSH levels. In almost all cases it is due to different mutations in only one allele of the thyroid hormone receptor beta gene which blocks the action of normal allele thus producing dominantly inherited RTH. In RTH, varying degrees of target tissue responsiveness result in a heterogenous clinical presentation. Resistance in the thyrotrophs and the peripheral tissues is assessed by the evaluation of TSH secretion and changes in peripheral markers of thyroid hormone action after administration of L-T3, respectively. The treatment decision depends on the individual characteristics of each patient. Patients with hypothyroid and hyperthyroid symptoms may require treatment with thyroid hormone and with agents such as beta blockers, antithyroid drugs and thyroid hormone analogues.