The impact of gestational age at birth on thyroid function; does it influence thescreening protocol for congenital hypothyroidism?

摘要

Aim:to demonstrate the impact of gestational age at birth on thyroid function and also to determine the validity of a repeat thyroid function test in screening for congenital hypothyroidism (CH) for preterm neonates. Methods: A total of 280 appropriate for GA neonates(premature or sick full term)between 28-41 weeks gestation admitted to Al Galaa teaching hospital were enrolled in the study). The study was prospective and had been carried out in the period between Junes 2011and December 2012. TSH and T4 were measured in blood between 6-7 days (test 1) and once again 4-5 weeks after birth (test 2).In test 2 samples were only drawn from 209 neonates as 3 neonates died in the period between the first and second test while the remaining were missed during follow up. The studied population were divided into 3 gestational age groups; group A( 28-31 weeks), group B (32-36 weeks) and group C; control( ≥ 37) weeks. The mean gestational age, birth weight and other clinical data were determined for each group. Results:In test (1), our results showed significantly lower plasma concentrations of T4 and TSH in premature than in term babies while in test (2), values were within the normal range in all neonates included in the study.This study showed that 48% of preterm infants had transient hypothyroxinemia on day 6-7 (test 1) which was corrected spontaneously by day 28-35 (test 2). Gestational age and birth weight were significantly lower in preterm infants with hypothyroxinemia than in preterm infants without hypothyroxinemia (p<0.01). Hypothyroidism was diagnosed in 10 infants, about (6%). Preterm infants with hypothyroidism also had significantly lower gestational age and birth weight than control (p<0.01).In test (2) TSH and T4 attain normal values. There were a significant difference in serum T4 values and serum TSH when comparing group A (28-31ws) with other groups (B&C), but this difference was found to be nonsignificant when group B&C are compared with each other (test1). The maximum effect of gestational age and birth weight on thyroid function was on group A. TSH levels had a tendency to increase in test 2 in all groups. No cases with delayed TSH rise were detected in this study. Conclusion: Premature infants especially those born before 31 week gestations, had a high incidence of transient thyroid dysfunction (hypothyroxinemia and less commonly hypothyroidism).Up till now the optimum screening protocol of congenital hypothyroidism for preterm neonates is not settled. In this study all infants with low T4 and TSH attains normal values after 4-5 weeks (test 2), also there were no cases with delayed TSH rise diagnosed. So we believe that routine second screening test may not be needed due to increased costs with a very low yield of cases and to limit repeat screening to selected newborn populations (including preterm especially those with gestational age below 31weeks and sick newborn) in whom the incidence of hypothyroxinemia and hypothyroidism are high.