THYROID STIMULATING HORMONE AND THYROXINE SCREENING IN THE PRETERM NEWBORN: REFERENCE VALUES

The screening test for Congenital Hypothyroidism, performed in the 2nd - 3rd day of life, highlighted in the premature infant significant lower levels of thyroid stimulating hormone (TSH) and thyroxine (tT4) than those found in the full term newborn. Studies carried out after birth in the preterm infant, showed a persistence of low levels of thyroid hormones (TH) in the first two weeks of life, with a tendency toward spontaneous resolution at the end of the first month. This hormone deficiency has an incidence which is inversely related to gestational age (GA). However, there are no works which have evaluated the levels of TSH and tT4 in an appropriate large population of extremely low gestational age newborns (ELGANs) in the 2nd - 3rd day of life divided by different GAs. To assess the levels of TSH and tT4 in the 2nd - 3rd day of life, from the lowest GA up to the end, in order to indicate the reference values which can be used to establish whether a replacement therapy with thyroid hormones in the premature infant is useful or not. In the present study we examined a total population of 1,671 adapted for gestational age newborns (AGA) of which 1,159 resulted in full term and 512 resulted in preterm. The population was divided into eight groups of GAs (23-25, 26-28, 29-30, 31-32, 33-34, 35-36, 37-38, 39-42 wks) and for each age group the median values of TSH and tT4 was determined. The TSH and tT4 were obtained in the 2nd - 3rd day of life at the screening for Congenital Hypothyroidism. Looking at the whole population examined, the preterm infant presents sig- nificant lower levels of TSH (p=0,00036) and tT4 (p<0,00001) compared to the full term newborn in the 2nd - 3rd day of life. TSH and tT4 levels increase gradually with the advancing of GA, reflecting the progressive maturation of the hypothalamic-pituitary-thyroid axis. A linear and significant correlation (p=0,001) was found between the levels of tT4 and GA. Instead, the TSH values which show an upward trend with the advancing of the GA, confirm a trend which is less clear. In preterm newborns, significant increases of tT4 levels are found for minimal increases of GA, suggesting that even in extremely pre- mature infants (23-25 wks) there is already an appropriate function of hypothalamic-pituitary-thyroid axis, which matures rapidly and progressively. It is believed that even if the preterm newborn tT4 levels are lower than those found in a full term neonate, the metabolic needs of an ELGAN are sufficient. Our reference values in various GAs in preterm infants may be used to determine whether premature newborns may or not be required to make a substitution therapy, which in our opinion should be implemented when the tT4 levels are <2 SD for that gestational age.