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The small for gestational age newborn (SGA), which compared the adapted for gestational age newborn (AGA), has a higher risk of perinatal mortality and morbidity, also according to some authors of developing cardiovascular disease in adulthood. To establish if thyroid function of the newborn term and preterm SGA, in 2nd and 3rd day of life differs from that of the AGA infant of the same gestational age (GA). The levels of TSH and tT4 in 2nd and 3rd day of life were examined and obtained with the Screening for Congenital Hypothyroidism in a population of 1,958 infants, of which 1,671 resulted in AGA and 287 resulted in SGA. The overall population was divided into eight groups in relation to different GAs: 23-25, 26- 28, 29-30, 31-32, 33-34, 35-36, 37-38, 39-42 weeks (wks). For each age range or group (AGA and SGA) in which the population was divided, the median of the values obtained for TSH and tT4 was measured. TSH levels were higher in SGA infants at different GAs considered (significantly 31-32 wks: p=0.0428 and 37-38 wks: p=0.0149), with the exception of infants aged 26-28 wks, whose TSH levels were significantly lower: p=0.0591 compared with AGA infants of the same GA. Instead, in the levels of tT4 in the SGA, newborn findings were lower than those of AGA infants at all gestational ages considered, but significantly only at 26-28 wks: p=0.0001 and 39-42 wks: p=0.0190. tT4 levels show a linear and significant increase with the advancing of GA, reaching the highest values in term newborns at 39-42 wks, both in overall populations (AGA and SGA); conversely, TSH levels, though showing significant increases starting from lower classes of GA, increased in a less linear way, probably due to different stressful factors. Looking at the whole population examined, tT4 levels are significantly lower in SGA infants compared with AGA newborns (p< 0,00005), instead TSH levels are significantly higher in the SGA population compared with AGA infants (p=0,046). Overall, our data show significantly lower levels of tT4 and significantly higher levels of TSH in the SGA population compared with AGA infants of the same gestational age. These results suggest to monitor over time both the newborn term and the preterm SGA in order to assess the full normalization of the hypothalamic-pituitary-thyroid axis.
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