To determine the endocrine, neuropsychological and genetic features of a child with resistance to thyroid hormone (RTH), and his response to long-term triiodothyroacetic acid (TRIAC) therapy. Growth, thyroid function, and neuropsychology were assessed at baseline and during 12-month TRIAC therapy. Genetic analysis was performed by PCR and denaturing high performance liquid chromatography. The main clinical finding was the attention deficit-hyperactivity disorder (ADHD). A novel mutation in exon 10 (phenylalanine to isoleucine in codon 455) was found. Long-term TRIAC therapy was effective in the management of the endocrine and neuropsychological manifestations of the syndrome. ADHD was the only phenotypic manifestation of this novel mutation of thyroid hormone (TH) receptor. TRIAC is an effective and safe drug in the long-term treatment of children with RTH.