Ectopic thyroid glands have frequently been surgically removed with the mistaken diagnosis of thyroglossal duct cyst. In three cases removal of thyroid tissue caused profound hypothyroidism before the condition was recognized and treated. Analysis of earlier growth measurements revealed that hypothyroidism was present prior to surgery and was the stimulus for compensatory enlargement of the ectopic glands. A fourth patient with ectopic thyroid tissue showed no compensatory enlargement despite the early evidence of hypothyroidism. These cases illustrate the spectrum of thyroid dysgenesis and the importance of the small foci of functioning thyroid tissue during the critical period of central nervous system development.