Resistance to thyroid hormone (RTH) is a rare inherited autosomal syndrome caused by mutations in the thyroid hormone receptor beta (TRb) gene. Although RTH is generally a familiar disease, 15% of sporadic cases have been also reported. So far, about 80 different mutations of TRb gene have been identified in patients affected by RTH. All these mutations localize to the binding domain and most of them cluster within two "hot spots" (codons 310-349 and codons 429-460). Here we describe in a patient with RTH, a new mutation in codon 426 (T426I) of the TRb gene leading to a threonine to isoleucine substitution. This is a "de nova" mutation which localizes in the so-called "cold" region, outside the two known "hot spots". The patient had the hallmark of RTH: elevated FT3 and FT4, normal TSH, and clinical features of both hypo and hyperthyroidism. Moreover, several dysmorphisms were present including triangular face appearance, synophris, low set ears, micrognathia with malocclusion, large upper incisors and apparent lack of lower cuspids which have not previously described in RTH patients.