The function of the thyroid gland in the pre-term neonate reflects the immaturity of the hypothalamic-pituitary-thyroid axis which corresponds to the week of gestation of the pre-term neonate. There is a gradual increase in the concentration of TSH, TBG, T3 and T4 during gestation [42, 43]. After parturition there is an increase in T4 and TSH just as in full-term neonates, but the increase is much smaller in pre-term neonates than what it is in the full-term neonates and there is a dramatic decrease in the concentration of T4 during the following 1-2 weeks . This decrease in T4 is more important in low birthweight and significantly premature neonates (<1.5 kg and <30 weeks of gestation) where the level of T4 may not be detectable [44, 45]. In most cases though total T4 is influenced and not FT4 as much since TBG is low in pre-term neonates due to immaturity of the liver. Another reason for the fall in T4 in pre-term neonates is the reduced storage of iodine which exists due to the prematurity . Preterm neonates have greater difficulty in maintaining a positive iodine balance than full-term neonates because pre-term neonates lose large quantities of iodine in the urine and because their iodine uptake system is immature [45-47]. Also because the requirement for thyroid hormones is considerably enhanced within the first few months of life it is normal that the turnover rate of thyroidal iodide increases. Even in the presence of TG with normal hormone content, the renewal rate of the intrathyroidal pool of T4 has to be very rapid to provide the premature infant with a normal hormone supply. This could be an important factor for increased risk of neonatal hypothyroxinemia in very premature infants .
Due to the immaturity of the thyroid gland preterm neonates have a reduced ability of adjusting to excessive amounts of iodine which are found in skin antiseptics which contain iodine and are frequently used in preterm neonatal units. That is why it is recommended that these should not be used.
Additionally, rT3 remains at higher levels and T3 remains at low levels for a longer time in pre-term than in full-term neonates because type I deiodinase is immature .
Was this article helpful?