Commentary: Do preterm infants need thyroxine replacement?

摘要

The interpretation of thyroid function tests in babies, especially in premature babies, is difficult. In adults the thyroid gland secretes thyroxine which can be deiodinated peripherally to generate iodothyronines that are more (triiodothyronine) or less (reverse triiodo-thyronine) potent than the parent compound. Triiodothyronine has three to four times the metabolic potency of thyroxine, and 80% is derived from peripheral conversion, the remainder coming from the thyroid gland. In the fetus serum triiodothyronine concentrations are low throughout gestation because the fetus preferentially converts the thyroxine its own thyroid gland produces and that which it acquires from the mother to reverse triiodothyronine, which is inactive. Clearly there has to be a switch at birth but, as is so often the case, the preterm infant is somewhere between the fetus and the newborn infant.