Residual Thyroid Tissue After Thyroidectomy in a Patient With TSH Receptor-Activating Mutation Presenting as a Neck Mass

Background: Activating mutations of the TSH receptor (TSHR) are rare, with few reported cases of long-term follow-up. Case: We present a follow-up report on a patient with neonatal thyrotoxicosis known to have a rare activating mutation of the TSHR, a heterozygous substitution in exon 10 (p.Ile568Thr). Initial treatment included total thyroidectomy at age 2 ½ years, resulting in iatrogenic hypothyroidism and hypoparathyroidism. The patient was treated with levothyroxine replacement to maintain TSH levels within normal range, as well as calcitriol and calcium carbonate to treat postsurgical hypoparathyroidism. However, 4 years later, while euthyroid, he developed a palpable 1-cm midline neck mass. Methods and Results: Functional imaging with 123-I thyroid scan demonstrated active thyroid tissue within the thyroglossal duct remnant and in the tracheoesophageal groove. Surgical removal of the neck mass revealed cytologically bland thyroid follicular cells. Conclusion: These findings suggest that even after t...