Derangements of Thyroid Hormones and Hypothalmo-Pitutary-Thyroid (HPT) Axis in Preterm Neonates

Introduction: Preterm neonates have deranged thyroid hormones levels often characterized by low T 3 /FT 3 and often low to normal T 4 /FT 4 and TSH levels. Thus, it may be essential to find the pattern of thyroid hormones levels in preterm in comparison to term neonates, the status of HPT axis in preterms and occurrence of Transient Hypothyroxinemia of Prematurity (THOP) in this part of globe, since serum FT 3 and FT 4 levels are correlated with TSH levels after establishment of HPT axis. We have focussed on these hormones in our study. Methods: This prospective observational study was conducted at a tertiary care centre in Northern India including 100 neonates in the study group (33 preterm and 67 term neonates as per gestational age). FT 3 , FT 4 and TSH were estimated by electrochemiluminescence immunoassay (ECLIA) using diagnostic kits from Roche Diagnostics. The history of maternal illness, gestational age and birth weight of each neonate was recorded. Results: All the three hormones i.e. FT 3 , FT 4 and TSH were significantly lower in preterm in comparison to term neonates. In term neonates FT 3 was significantly correlated to both FT 4 (r=0.453; p=0.00) and TSH (r=0.299; p=0.014) while no such correlation was found in preterm neonates. Discussion: Both sick euthyroid (low T 3 syndrome) and transient hypothyroxinemia of premature infants (THOP) are noticed in pereterm neonates. The causes of low T 3 syndrome may include hypoxemia, acidosis, infections, hypoglycemia, hypocalcemia, malnutrition, transient secondary/tertiary hypothyroidism, transient primary hypothyroidism and permanent primary hypothyroidism while THOP may be caused by iodine deficiency, maternal thyrotropin receptor blocking antibodies, maternal intake of antithyroid drugs, maternal or neonatal iodine exposure, loss of function mutations and hepatic haemangioma(20), maternal hyperthyroidism, prematurity and drugs. Since it may lead to undesirable neurodevelopmental outcome, thyroid hormones supplementation in preterms should be cautiously studied in larger sample size. Also, these factors must be considered while screening for congenital hypothyroidism.