Hyperthyroxinemia at birth: a cause of idiopathic neonatal hyperbilirubinemia?

Background: Some neonates develop idiopathic hyperbilirubinemia (INHB) requiring phototherapy, yet with no identifiable causes. We searched for an association between abnormal thyroid levels after birth and INHB. Methods: Of 5188 neonates, 1681 (32.4%) were excluded due to one or more risk factors for hyperbilirubinemia. Total thyroxine (TT4) and thyroid stimulating hormone values were sampled routinely at 40–48 hours of age and measured in the National Newborn Screening Program. Results: Of the 3507 neonates without known causes for hyperbilirubinemia, 61 (1.7%) developed INHB and received phototherapy. Univariate analyses found no significant association between mode of delivery and INHB (vacuum-delivered neonates were a priori excluded). Nonetheless, in cesarean-delivered (CD) neonates, two variables had significant association with INHB: TT4 ≥ 13 µg/dL and birth at 38–38.6 weeks. In vaginally delivered (VD) born neonates, INHB was associated with weight loss > 7.5% up to 48 hours of age. Multivariate logistic regression analysis showed a strong effect of mode of delivery on possible significant association with INHB. In CD neonates, such variables included: TT4 ≥ 13 µg/dL [P = 0.025, odds ratio (OR) 5.49, 95% confidence interval (CI) 1.23–24.4] and birth at 38–38.6 weeks (P = 0.023, OR 3.44, 95% CI 1.19–9.97). In VD neonates, weight loss > 7.5% (P = 0.019, OR 2.1, 95% CI 1.13–3.83) and 1-min Apgar score < 9 (P < 0.001, OR 3.8, 95% CI 1.83–7.9), but not TT4, showed such an association. Conclusions: INHB was significantly associated with birth on 38–38.6 week and TT4 (≥ 13 µg/dL) in CD neonates, and with a weight loss > 7.5% in VD neonates. We herein highlight some acknowledged risk factors for neonatal hyperbilirubinemia, and thus minimize the rate of INHB.