We reported two unrelated Thai girls with resistance to thyroid hormone. The affected patients presented with goiter and no other stigmata of hyperthyroidism. Their serum T4, T3, free T4 and free T3 concentrations were high and they had normal levels of TSH. The affected girl in family 1 was treated with an antithyroid drug for 1-9/12 years. The affected girl in family 2 was only observed her thyroid function tests. TRH test showed normal TSH response in both girls. Analysis of the thyroid hormone receptor beta gene of both affected girls revealed the same missense mutation, changing the guanine in nucleotide 1234 to an adenine which results in the replacement of the normal alanine (GCT) with a threonine (ACT) at codon 317. Two proposita were heterozygous, and this mutation was not present in their parents compatible with a neo-mutation.