Thyroid hormone levels in late preterm, early term and term infants: a study with healthy neonates revealing reference values and factors affecting thyroid hormones

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BACKGROUND: Thyroid function tests in neonates have been challenging to interpret because their levels are affected by several neonatal and delivery-related factors. The aim of the study was to evaluate reference values of thyroxine (T4) and thyrotropin (TSH) levels in different gestational age groups and to demonstrate the affect of perinatal factors on thyroid hormones. METHODS: Medical records of 7616 neonates whose gestational age ranges between 34 and 42 weeks were analyzed retrospectively. Gender, mode of delivery, gestational age, postnatal age and birth weight were noted together with TSH and T4 levels. RESULTS: Gestational age (r=0.14, p<0.001) and birth weight (r=0.12, p<0.001) had positive correlation with T4 levels, whereas they had no effect on TSH levels. Males had higher TSH and lower T4 levels (p=0.001 for both) compared with females. T4 levels of babies born via vaginal delivery were lower than the ones born via cesarean section (p=0.01). Multivariable analysis yielded gestational age as the only factor affecting T4 levels (p<0.001). T4 and TSH levels based on 2.5-97.5 percentile cutoffs according to gestational age were presented. CONCLUSIONS: The thyroid hormone ranges given in this study can help pediatricians to interpret the thyroid hormone results with ease.