Thyroid Function and Thyroid Hormone Requirements of Very Preterm Infants

Abstract

After completing this article, readers should be able to: 1. Describe the thyroid state of very preterm infants. 2. List the factors that influence preterm thyroid function. 3. Understand possible implications of preterm thyroid function. 4. Cite the evidence available regarding treatment with thyroxine in very preterm infants. Understanding fetal thyroid function and thyroid hormone requirements is important for comprehending pre-term thyroid function because pre-term thyroid function shares both fetal and neonatal properties. Before the onset of fetal thyroid function, the presence of thyroid hormone has been demonstrated in embryonic cavities and tissues. Nuclear thyroid hormone receptors bound to bioactive triiodothyronine (T3) have been found in human brain and lung tissue during the ninth week of fetal life. It is generally believed that the presence of thyroid hormone during early fetal life results from maternal-fetal transfer of thyroxine (T4). By 10 to 12 weeks’ gestational age, the fetal thyroid gland acquires its capacity to concentrate iodide and synthesize iodothyronines. By that same time, the pituitary can produce and secrete thyrotropin, and hypothalamic neurons can synthesize thyrotropin-releasing hormone. Thyroxine binding-globulin also can be demonstrated in fetal serum by 12 weeks. Significant thyroid hormone production, however, does not occur before the 20th week of gestation. Fetal thyroid function and the hypothalamic-pituitary-thyroid axis continue to mature throughout pregnancy; serum levels of T4, free T4, thyroglobulin, and thyrotropin increase until the end of pregnancy. Thyroid hormone metabolism is immature during fetal life. There is high activity of iodothyronine mono-deiodinase type III, which converts T4 to bioinactive reverse-triiodothyronine (rT3) and T3 to diiodothyronine. The activity of iodothyronine monodeiodinase type I, which converts T4 to T3 for the plasma T3 pool, is believed to be low. As a consequence, the secreted T4 is preferably converted to rT3, which is present in high concentrations during pregnancy and only decreases during the …