Utility of Repeat Testing for Congenital Hypothyroidism in Very Low Birthweight Infants.

Objective To assess for possible missed hypothyroidism in very low birthweight (VLBW) infants whose initial newborn screening (NBS) was within normal reference range. Study design We analyzed serum TSH obtained at 36 weeks corrected gestational age or at hospital discharge if earlier (Retest TSH) in VLBW infants in the Neonatal Intensive Care Unit in order to determine the prevalence and factors associated with Retest TSH > 5 mU/L, a concentration requiring close follow-up for hypothyroidism. Utility of alternative cut-offs for NBS TSH was also assessed. Results A total of 398 infants, median gestational age 29 (range 22-36) weeks, birthweight 1138 (470-1498) g, were included in this study. Retest TSH was obtained at 49.5 (12-137) days after birth. Median Retest TSH was 3.1 (0.5-27.9) mU/L. Seventy-three (18.3%) of the cohort had Retest TSH > 5 mU/L. Adjusting NBS cut-off to > 15 or > 10 mU/L identified > 5 mU/L, resulting in 6% false positives and >70% false negatives. Multiple regression modeling indicated that 35% of variance in Retest TSH value was explained by NBS TSH concentration, birthweight, and gestational age, all P Conclusions Retesting for hypothyroidism at 36 weeks CGA in VLBW infants with normal NBS could identify infants who require ongoing surveillance until thyroid function has been definitively ascertained. Adjusting NBS TSH cutoffs is not a valid option for identifying potential hypothyroidism in VLBW infants because of lack of sensitivity and, unacceptable false positive and false negative rates.