Term birth and levothyroxine dosage are significant factors associated with permanent congenital hypothyroidism: experience from a medical center in Taiwan.

BACKGROUND: Before the introduction of newborn screening, congenital hypothyroidism was the leading cause of intellectual disability in infants and children. Patients with permanent congenital hypothyroidism require lifelong levothyroxine supplementation to prevent intellectual disability and growth failure. With progressively lower thyrotropin (TSH) cutoffs in newborn screening programs, more transient congenital hypothyroidism cases-requiring only temporary treatment-have also been identified. To avoid unnecessary medication use and reduce the burden on healthcare systems, early differentiation is essential. METHODS: We retrospectively enrolled congenital hypothyroidism patients born between 2004 and 2018 and followed at MacKay Children's Hospital and classified them as permanent congenital hypothyroidism or transient congenital hypothyroidism based on levothyroxine dependence. Basic demographic data, including gender, gestational age, birth weight, newborn screening TSH levels, body height and weight, serum free T4, TSH levels, levothyroxine doses at every clinical visit, and age of TSH normalization were collected and compared between permanent and transient congenital hypothyroidism groups. RESULTS: A total of 152 infants were enrolled in this study, with 73 (48%) classified as permanent congenital hypothyroidism. Term births were more common in permanent than transient congenital hypothyroidism (80% vs. 48%, p < 0.01). TSH normalization took longer in permanent congenital hypothyroidism (75 vs. 45 days, p < 0.01). Serum TSH and levothyroxine doses remained higher in permanent congenital hypothyroidism at 6 months, and at 1, 2, and 3 years. The levothyroxine dose that provided the best discrimination between permanent and transient congenital hypothyroidism was 2.5 𝛍g/kg/d at age 2 years (sensitivity: 73%, specificity: 90%), and 1.8 𝛍g/kg/d at age 3 years (sensitivity: 82%, specificity: 91%). CONCLUSIONS: Nearly half of the patients required lifelong levothyroxine supplements under the current newborn screening program. Permanent congenital hypothyroidism patients were more likely term-born, showed delayed TSH normalization, had higher TSH levels, and required higher levothyroxine doses during follow-up. The best cut-off level to discriminate permanent from transient congenital hypothyroidism was 2.5 and 1.8 µg/kg/day at the age of 2 and 3 years, respectively.