J Pediatr Endocrinol Metab 2017; 30(11): 1191–1196 Ozge Nur Aktas, Tugba Gursoy*, Elif Soysal, Ecem Esencan and Secil Ercin Thyroid hormone levels in late preterm, early term and term infants: a study with healthy neonates revealing reference values and factors affecting thyroid hormones https://doi.org/10.1515/jpem-2017-0215 Received April 21, 2017; accepted May 28, 2017; previously published online October 9, 2017 Abstract 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 gesta- tional 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 hor- mone results with ease. Keywords: gender; gestational age; mode of delivery; preterm; term; thyroid hormones. Introduction Thyroid function in postnatal period has been challenging to interpret for pediatricians because it is subject to varia- tions in gestational age [1–3], postnatal age at which blood samples are collected [4, 5], birth weight [2], mode of deliv- ery [5, 6] and gender [7]. Thyrotropin (TSH) levels rise and peak at first 30 min and decline rapidly during the first 24 h of life followed by slower decrease over the next 48 h in full-term infants [8, 9]. The peak observed in TSH levels is partially explained by cold exposure and stress after birth [9]. Thyroid hormone levels of term infants are well deter- mined, but there is still lack of knowledge about those of late preterm and early term infants in the literature. Late preterm infants (34–36 6 weeks) and early term infants (37–38 6 weeks) have higher morbidity and mortality rates compared with term infants [10, 11]. In fetal life, the hypo- thalamus-hypophysis-thyroid axis continues to develop, so the problems seen in late preterm and early term babies may stem from this interruption of development in the axis. There has been no study comparing thyroid hormone levels in late preterm, early term and term infant in the lit- erature. The objective of this study is to compare thyroid hormone levels in late preterm, early term and term neo- nates, to provide reference ranges of TSH and thyroxine (T4) values according to gestational age and to help pedia- tricians to interpret thyroid hormone results accurately. We also aim to identify the impact of gestational age, birth weight, gender and mode of delivery on thyroid function in late preterm and early term neonates. Subjects and methods Subjects In this retrospective study, newborn data were collected from the electronic archive system of American Hospital, Istanbul, Turkey, between 1 January 2008 and 31 December 2013. Gender, mode of delivery, birth weight, postnatal age at which blood samples were collected and gestational age were noted together with TSH and T4 levels. The total number of neonates born in this period was 8164. After excluding the newborns with a gestational age below 34 weeks *Corresponding author: Tugba Gursoy, MD, Professor of Pediatrics, Neonatology Unit, Department of Pediatrics, Koc University Hospital, 34010, Istanbul, Turkey, Phone: +90 533 5445944, E-mail: tgursoy@ku.edu.tr Ozge Nur Aktas, Elif Soysal and Ecem Esencan: Neonatology Unit, Department of Pediatrics, Koc University School of Medicine, Istanbul, Turkey Secil Ercin: Neonatology Unit, Vehbi Koc Foundation American Hospital, Istanbul, Turkey Brought to you by | Koc University Authenticated Download Date | 5/14/18 2:14 PM