RESUMEN
OBJECTIVE: To determine among infants born very preterm (VPT) or with very low birth weight (VLBW) the incidence of alterations in thyroid function and associated comorbidities; the incidence of atypical congenital hypothyroidism (CH) requiring thyroxine therapy; and reference ranges for rescreening at 1 month of age. STUDY DESIGN: A retrospective review of infants born VPT or with VLBW and admitted to UC Irvine Medical Center between January 1, 2012, and December 31, 2020. Repeat thyroid screening was obtained at 1 month of life (+10 days). Infants with thyroid-stimulating hormone (TSH) >5 µIU/mL or free thyroxine <0.8 ng/dL underwent follow-up testing and endocrinology consultation. Initial newborn screening (NBS) and repeat thyroid screening data were collected via chart review. Demographic data and short-term outcomes were abstracted from the California Perinatal Quality Care Collaborative database. RESULTS: In total, 430 patients were included; 64 of 429 patients (14.9%) had TSH >5 µIU/mL and 20 of 421 patients (4.8%) had free thyroxine <0.8 ng/dL. Logistic regression analysis identified small for gestational age (P = .044), patent ductus arteriosus (P = .013), and late-onset sepsis (P = .026) as risk factors associated with delayed TSH rise. Atypical CH requiring treatment through neonatal intensive care unit discharge was diagnosed in 6 patients (incidence of 1.4%); none were identified by NBS. The 90th percentile TSH for infants with extremely low birth weight (<1000 g) was 7.2 µIU/mL, and the 95th percentile for those with birth weight of 1000-1500 g was 6.1 µIU/mL; using these cutoff values identified all infants diagnosed with atypical CH with 100% sensitivity and 90%-95% specificity. CONCLUSIONS: Abnormal thyroid function is common in infants born preterm. Those infants, including some with atypical CH, are missed by NBS. We recommend repeat thyroid screening with TSH at 1 month of age in infants born VPT or infants with VLBW to identify CH that may require therapy.