Mutation in the thyroid hormone receptor (TR) beta gene (M313T) not previously reported in two unrelated families with resistance to thyroid hormone (RTH).

Two families expressing the RTH phenotype and harboring the same mutation in the TRbeta gene are described. Five and four affected members in each family were investigated as well as a total of 12 unaffected relatives. The mutation, a T to C transition of nucleotide 1223 in one allele of affected individuals, results in the replacement of the normal Met for a Thr.