MON-238 - Thyroid Stimulating Hormone Normalization in Patients with Primary Congenital Hypothyroidism after Initiation of Thyroid Hormone Replacement

Background: Primary congenital hypothyroidism (CH) is commonly detected through newborn screening. Prompt initiation of levothyroxine (LT4) is essential to achieve biochemical control and prevent adverse neurodevelopmental outcomes. Guidelines recommend initiation of thyroid hormone replacement as soon as possible and within 2 weeks of birth. Variability exists in timing and dose while real-world data on factors influencing thyroid-stimulating hormone (TSH) normalization are limited.

Objective: To determine the median time to TSH normalization following LT4 initiation in infants with primary CH and to evaluate associations between timing of therapy, initial LT4 dose, and TSH normalization by 3 months of age.

Methods: This was a retrospective chart review of infants diagnosed with CH through newborn screening and managed at a tertiary children’s hospital. Demographics, newborn screening TSH values, age at LT4 initiation, starting LT4 dose (µg/kg/day), and serial thyroid function tests were collected. Infants were categorized into two groups: those who achieved TSH normalization by 3 months of age and those who did not. Median time to TSH normalization was calculated for each group, and Pearson’s Chi-squared test or Welch Two Sample T-test were used to compare factors associated with TSH normalization by 3 months.

Results: 67 infants were included. Median age at LT4 initiation was 26 days (IQR 17.5–59.75), and median starting dose was 9.5 µg/kg/day (IQR 6.37–11.50). Median time to TSH normalization was 7.5 weeks (IQR 4–14.75). Sex, race, and gestational age were not different between infants who did and did not achieve TSH normalization by 3 months. The group that achieved a normal TSH by 3 months had a first visit at a younger age (33 vs 21 days, p =0.006) and were younger at LT4 initiation (21 vs 40 days, p =0.05). While not statistically significant, there was a notable trend towards higher starting dose of LT4 in the group with TSH normalization within 3 months (10.4 vs 8.3 µg/kg/day, p= 0.10).

Conclusion: These real-world data show that LT4 is not initiated until nearly 4 weeks of age and that TSH normalization within 3 months was associated with early identification and treatment with LT4. Even a small difference in LT4 dosing may have a significant impact in TSH normalization by 3 months of age. Streamlining the referral process is important for achieving the best outcomes in these patients.

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