The characterization of a spontaneous animal model equivalent to a human form of thyrotoxicosis would provide a useful resource for the investigation of the human disorder. Feline thyrotoxicosis is the only common form of hyperthyroidism found in domestic or laboratory animals, but its etiopathogenesis remains poorly defined. We have used the polymerase chain reaction (PCR) to amplify codons 480-640 of the previously uncharacterized feline thyrotropin receptor (TSHR) gene, and have determined the DNA sequence in this transmembrane domain region. We have analyzed single stranded conformational polymorphisms in thyroid DNA from 11 sporadic cases of feline thyrotoxicosis and leukocyte DNA from two cases of familial feline thyrotoxicosis. We have also determined the DNA sequence of this region of the TSHR in five of the cases of sporadic feline thyrotoxicosis and the two familial thyrotoxic cats. The normal feline TSHR sequence between codons 480-640 is highly homologous to that of other mammalian TSHRs, with 95%, 92%, and 90% amino acid identity between the feline receptor and canine, human, and bovine TSHRs, respectively. Thyroid gland DNA from 11 cats with sporadic thyrotoxicosis did not have mutations in this region of the TSHR gene. Leukocyte DNA from two littermates with familial feline thyrotoxicosis did not harbor mutations of this region of the TSHR gene. These studies suggest that TSHR gene mutations are not a common cause of feline thyrotoxicosis.