Hyperthyroidism in Graves' disease is caused by thyroid-stimulating autoantibodies to the thyrotropin receptor (TSHR) whereas hypothyroidism in Hashimoto's thyroiditis is associated with thyroid peroxidase and thyroglobulin autoantibodies. In some Graves' patients, thyroiditis becomes sufficiently extensive to "cure" the hyperthyroidism with resultant hypothyroidism. Factors determining the balance between these two diseases, the commonest organ-specific autoimmune diseases affecting humans, are unknown. Serendipitous findings in transgenic BALB/c mice, with the human TSHR A-subunit targeted to the thyroid, shed light on this relationship. Of three transgenic lines, two expressed high levels and one expressed low intrathyroidal A-subunit levels (Hi- and Lo-transgenics, respectively). Transgenics and wild-type littermates were depleted of T regulatory cells (Treg) using antibodies to CD25 (CD4+ T-cells) or CD122 (CD8+ T-cells) prior to TSHR-adenovirus immunization. Regardless of Treg depletion, Hi-expressor transgenics remained tolerant to A-sub-Ad immunization (no TSHR antibodies and no hyperthyroidism). Tolerance was broken in Lo-transgenics, although TSHR antibody levels were lower than in wild-type littermates and no mice became hyperthyroid. Treg depletion prior to immunization did not significantly alter the TSHR antibody response. However, Treg depletion (particularly CD25) induced thyroid lymphocytic infiltrates in Lo-transgenics with transient or permanent hypothyroidism (low T4, elevated TSH). Neither thyroid lymphocytic infiltration nor hypothyroidism developed in similarly treated wild-type littermates. Remarkably, lymphocytic infiltration was associated with inter-molecular spreading of the TSHR antibody response to other ‘self’ thyroid antigens, murine thyroid peroxidase and thyroglobulin. These data suggest a role for Treg in the natural progression of hyperthyroid Graves' disease to Hashimoto's thyroiditis and hypothyroidism in humans.