A young boy with a thyroid yo-yo.

We describe a boy with Hashimoto's thyroiditis and Addison's disease who was initially hypothyroid, developed hyperthyroidism and eventually became euthyroid. A rise and fall in thyroid binding inhibitory immunoglobulin and thyroid stimulating antibodies were demonstrated to have correlated with the appropriate changes in thyroid status. The fact that Hashimoto's thyroiditis can evolve in such a way emphasizes the importance of lifetime follow-up in these patients.