Development of Graves' hyperthyroidism from primary hypothyroidism in a case of thyroid hemiagenesis.

We report a 42-year-old female with right thyroid hemiagenesis who initially presented with hypothyroidism and then developed hyperthyroidism. The serum titer of thyroid-stimulating antibody was weakly positive in the initial hypothyroid state, and then markedly increased along with the development of hyperthyroidism, while thyroid stimulation-blocking antibody was continuously negative throughout the observation period. Thyroid histology of biopsied specimens during the hypothyroid state demonstrated diffuse thyroiditis with mononuclear cell infiltrations; however, the histology during the hyperthyroid state showed hyperplasia in follicular epithelial cells accompanied by partial lymphocyte infiltration. This is the first case of thyroid hemiagenesis associated with a conversion from primary hypothyroidism due to Hashimoto's thyroiditis to hyperthyroidism due to Graves' disease.