We recently expressed the extracellular domain of the human TSHR (ETSHR) protein using a baculovirus expression system and purified it to homogeneity. The ETSHR specifically binds both TSH and antibodies to TSHR. In the present study, C57BL/6J, SJL/J, BALB/cJ and BlOBR.SgSnJ mice were immunized with the recombinant ETSHR or an equivalent amount of control antigen. All strains of mice produced high titers of antibody against the TSHR protein which were capable of blocking the binding of TSH to native TSHR. However, only BALB/cJ mice showed significantly elevated levels of thyroxine in their sera compared to the control mice. Similarly, BALB/cJ mice primed with ETSHR and then challenged with thyroid membranes showed significantly elevated levels of thyroxine. In addition, histopathological examination of thyroid glands from affected mice showed morphological changes characterized by hydropic and subnuclear vacuolar changes and focal scalloping, with no apparent inflammation or glandular destruction. Moreover, mice with elevated thyroxine levels showed increased in vivo thyroidal uptake ofl31Iodine. Together, these data suggest that BALB/cJ mice are susceptible to the induction of hyperthyroxinemia.