Hyperthyroidism following hypothyroidism. Data on six cases.

Hypothyroidism complicated by spontaneous hyperthyroidism is an interesting but rare occurrence in the spectrum of autoimmune thyroid disorders. We report data on six patients who had hyperthyroidism two to 20 years (mean, eight years) after the diagnosis of hypothyroidism. In three patients, triiodothyronine toxicosis accounted for the hyperthyroid symptoms. The presence of thyroid antibodies and lymphocytic infiltration in the thyroid gland points to an autoimmune mechanism. It is believed that this phenomenon occurs more often than is recognized.