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Hyperthyroidism occurred following primary hypothyroidism in a 46-year-old man with serologically and histologically proved chronic lymphocytic thyroiditis. Repeated thyroid biopsy specimens revealed that histologic features compatible with chronic lymphocytic thyroiditis seen at the initial hypothyroid state subsequently underwent a remarkable resolution, with titers of circulating antithyroid antibodies being reduced in correspondence with the histologic improvement. This case shows that primary hypothyroidism in adults can spontaneously evolve into a hyperthyroid state, as has been suggested previously, and that the histologic abnormalities in chronic lymphocytic thyroiditis may not necessarily be irreversible.