SAT491 Development of Thyroidal Nodule in Congenital Hypothyroid Girl With Thyroid Peroxidase Gene Mutation

Disclosure: J. Lee: None. A. Oh: None. H. Han: None. Background: Reduced thyroxine synthesis in dyshormonogenesis induces increased thyrotropin (TSH) secretion through reduced negative feedback of thyroxine. Consequently, the affected patients are born with goiter or develop postnatal goiter, when L-thyroxine (LT4) supplementation is delayed. Cases of thyroid cancer have been reported in congenital hypothyroidism (CH) due to dyshormonogenesis, usually of follicular histology. Clinical Case: A 22-year lady has been taking LT4 since 8-years, under the diagnosis of goitrous CH. Her mother recalled neck swelling at newborn period. However, normal newborn screening test prevented neonatal goiter from further investigation. Initial test at 8-year of age revealed normal thyroid function without thyroid auto-antibodies, diffuse enlarged thyroid gland with increased uptake on (99m)Tc scan, and diffuse thyroid enlargement with heterogeneous echogenicity on ultrasonography (US). The goiter size waxed and waned according to the drug compliance. The highest TSH level (normal 0.17-4.05 μIU/mL) was 8.68 μIU/mL even without medication for 8 months by her own judgement. At 11.5-years, US found isoechoic solid nodule of 1.8 x 1.7 x 1.6 cm with peripheral halo in the inferior aspect of right thyroid, which was demonstrated to be benign follicular lesion favoring nodular hyperplasia on fine needle aspiration biopsy. With good drug compliance, the nodule size decreased to 1.1 x 0.9 cm at 18-years, and peripheral calcification appeared around the isoechoic solid nodule previously reported without change in size at 22-years. NGS panel for CH reported compound heterozygous variants in TPO gene. One pathogenic variant from asymptomatic father is c.2757del;p.(Met921Trpfs*53), the other likely pathogenic variant from asymptomatic mother is c.1580G>T;p.(Trp527Leu). Conclusion: This case is demonstrating development of thyroid nodule even with mild increased TSH level during LT4 medication in dyshormonogenic CH caused by TPO mutation. Regular follow-up and compliance to LT4 might be important to avoid thyroid tissue stimulation and possibly nodule formation by elevated TSH. Presentation Date: Saturday, June 17, 2023