The percentage of newborns with a neonatal whole blood thyroid-stimulating hormone (TSH) greater than 5 mIU/L has been used as an indicator of iodine deficiency at the population level. However, TSH levels in newborns may be influenced by many factors other than iodine status. The objective of this study was to identify neonatal, maternal, and pregnancy-related determinants of neonatal TSH levels in a retrospective cohort study. The study sample included 313 Belgian mothers and their 4- to 5-year-old children. The children had a neonatal TSH concentration between 0 and 15 mIU/L at neonatal screening, and blood samples were collected 3 to 5 days after birth. Children with suspected congenital hypothyroidism (neonatal TSH level >15 mIU/L), prematurely born (i.e., <37 weeks), or with a low birth weight (i.e., <2500 g) were excluded. Information about maternal and birth-related determinants was collected from the neonatal screening center via a self-administered questionnaire filled in by the mother together with the child's health booklet. Higher TSH levels were found in spring and winter compared to summer and autumn (P = .011). Higher TSH levels were associated with lifetime smoking behavior (up to child birth) in the mother (P = .005), lower weight gain during pregnancy (P = .014), and longer pregnancies (P = .003). This study showed that several neonatal, maternal, and pregnancy-related determinants are influencing neonatal TSH level.
