99mTc pertechnetate for detection of ectopic dysgenesis of the thyroid in childhood hypothyroidism.99mTc pertechnetate scans of the neck and pharyngeal region were performed on 29 hypothyroid children. Eleven were found to have lingual or sublingual thyroid tissue. No child with persisting ectopic thyroid tissue was intellectually impaired, but 5 of 14 athyreotic children were of subnormal intelligence. Early thyroid scanning of hypothyroid infants can be of prognostic value. In addition, all children who had become hypothyroid between the ages of 6. and 11 years were found to have ectopic glands.