A Case of Definitive Therapy in Persistent Congenital Hyperthyroidism Secondary to an Activating Variant of the TSHR Gene

Background: Sporadic congenital non-autoimmune hyperthyroidism (SCNAH) causes permanent hyperthyroidism secondary to activating mutations in the TSHR gene. SCNAH usually presents during infancy and can be difficult to treat with antithyroid drugs or subtotal thyroidectomy.(1) When adequate therapy is delayed, irreversible sequela such as craniosynostosis, growth failure, and cognitive delays may develop. Definitive therapy for children with SCNAH is recommended with no consensus on timing or type of intervention. Our case highlights the importance of early total thyroidectomy to optimize clinical outcomes in patients with SCNAH. Clinical case: A now 3-year-old male was born small for gestational age (SGA) at 34 weeks gestation with biochemical and clinical signs of hyperthyroidism in the neonatal period. On day of life 6, his TSH was 0.02 mcU/mL (reference range 0.34-5.6 mcU/mL) and free thyroxine (T4) was 5.42 ng/dL (reference range 0.58-1.64 ng/dL). Methimazole (MMI) 0.3 mg/kg/day was started at that time, and the dose was titrated over 6 weeks of life to 0.8 mg/kg/day due to continued elevation of free T4 levels. Negative thyroid autoantibodies and normal maternal thyroid studies prompted genetic testing at 3 months of age. This testing revealed a heterozygous variant in the TSHR gene [c.842G->A (p.G116R)] that was determined to be pathologic. Despite frequent MMI dose changes and confirmation of medication adherence, high-dose MMI was not effective in consistently normalizing his TSH and free T4 levels in the first two years of life. Concerns for poor growth and hyperactive behavior were also present. A “block and replace” strategy was not feasible since the patient remained hyperthyroid even at MMI doses up to 0.8 mg/kg/day. Despite normal insulin-like growth factor 1 and insulin-like growth factor binding protein 3 levels, poor linear growth persisted with a height Z-score of -3.61 by age 3 years. While he was SGA at birth, his recurrent hyperthyroidism impeded catch-up growth potential, so treatment with growth hormone (rhGH) was initiated. His height Z-score improved to -2.39 after 9 months on rhGH dosed at 0.18 mg/kg/week. Due to the difficulty in consistently managing him medically with MMI, he underwent total thyroidectomy at age three. Conclusion: There is currently no consensus on the timing of definitive therapy for children with SCNAH. This case highlights the importance of considering total thyroidectomy at an early age to avoid long-term sequela of persistent hyperthyroidism. 1. Gozu HI, Lublinghoff J, Bircan R, Paschke R. Genetics and phenomics of inherited and sporadic non-autoimmune hyperthyroidism. Mol Cell Endocrinol. 2010;322:125–34.