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.