We have demonstrated previously an association between a polymorphism in the human thyrotropin receptor gene and an increased prevalence of autoimmune thyroid disease in individuals bearing this polymorphic allele. The polymorphism involves the nucleotide base substitution of a cytosine for the wild-type adenine at the first position of codon 52 and is found generally in the heterozygotic state. Such a change results in the substitution of a threonine for the wild-type proline at this position in the receptor protein sequence. The resulting protein would lack a β turn (at position 52) in a potential loop conformation, and thus would have a significantly altered three-dimensional conformation. The biologic consequences of this conformational change in the receptor are unknown, but may involve altered function or immunogenicity. We report here two individuals with normal thyroid function who are homozygous for the thyrotropin receptor polymorphism, suggesting that the altered receptor is able to respond normally to thyrotropin with respect to the maintenance of the euthyroid state.