The hyt/hyt hypothyroid mouse has an autosomal recessive, fetal-onset, severe hypothyroidism related to TSH hyporesponsiveness and associated with elevated TSH. Our previous work has suggested that the hypothyroidism and TSH hyporesponsiveness may result from a mutation in the hyt/hyt TSH receptor (TSHr) of the thyroid gland. Based on DNA sequencing of the entire coding region of the TSHr gene from the wild-type BALB/cBY +/+ mouse, the +/+ TSHr is 92% and 94% identical at the nucleotide and amino acid residue levels, respectively, compared to the rat TSHr gene. The coding region of the hyt/hyt TSHr, compared to that of the +/+ TSHr, has a single base change, CCG to CTG, at nucleotide position 1666, which leads to the replacement of a highly conserved proline at amino acid position 556 with a leucine in transmembrane domain IV. This mutation was introduced by site-directed mutagenesis into the wild-type human TSHr and transiently expressed in COS-7 cells. Although the size and abundance of the mutant TSHr mRNA suggested that there was no effect on the nature of the mRNA, TSH binding and the response to TSH in transfected cells were abolished. Further studies are necessary to clarify how the Pro to Leu replacement interferes with receptor expression on the cell surface or influences TSH binding. These functional consequences of the mutation appear to account for the observed TSH hyporesponsiveness and hypothyroidism in the hyt/hyt mouse.