Resistance to thyroid hormone (RTH) is a rare dominantly inherited disorder caused by mutations in the thyroid hormone receptor beta gene which lead to impaired tissue responsiveness to thyroid hormone (TH). RTH is characterized by elevated free thyroid hormone and unsuppressed thyrotropin (TSH) levels. Two types of the disorder have been recognized: selective pituitary resistance and generalized resistance to TH (GRTH). Triiodothyroacetic acid has been used in patients showing hypermetabolism, and L-T4 treatment in high doses has been suggested in GRTH if patients have signs of clinical hypothyroidism such as growth retardation and developmental delay. The outcome of long-term treatment of GRTH with L-T4 has not yet been reported. We report a 5 year-old boy who presented with severe growth retardation, fatigue and speech delay. He had hyperactivity despite feeling tired easily. Elevated TH levels with unsuppressed TSH and delayed bone age were determined by laboratory evaluation and he was diagnosed as GRTH. There was no clinicical evidence of hypermetabolism. We could not demonstrate any mutation in thyroid receptor beta1, beta2 or alpha gene of this patient and his parents. L-T4 treatment was started at conventional doses (6 microg/kg), and after 3 months of treatment T4 and TSH levels were suppressed successfully. In 12 months of treatment, no side effects were detected and surprisingly clinical symptoms improved without requirement for high doses of L-T4.