Lingual Thyroid on I-123 Scan

C. Lingual thyroid — ectopic thyroid tissue at the base of the tongue representing the only functional thyroid tissue

Lingual thyroid is the most common form of ectopic thyroid, resulting from failure of the thyroid gland to descend from its embryologic origin at the foramen cecum (base of tongue) to its normal position in the anterior neck during fetal development. On thyroid scan, uptake appears only at the base of the tongue with absent activity in the normal thyroid bed — indicating the lingual thyroid is the ONLY functional thyroid tissue. This is clinically important: surgical removal would render the patient permanently hypothyroid. I-123 scanning is essential before any surgical intervention on such masses.