Iodine Deficiency in Pregnancy: The Effect on Neurodevelopment in the Child

Abstract

Iodine is an integral part of the thyroid hormones, thyroxine (T₄) and tri-iodothyronine (T₃), necessary for normal growth and development. An adequate supply of cerebral T₃, generated in the fetal brain from maternal free T₄ (fT₄), is needed by the fetus for thyroid hormone dependent neurodevelopment, which begins in the second half of the first trimester of pregnancy. Around the beginning of the second trimester the fetal thyroid also begins to produce hormones but the reserves of the fetal gland are low, thus maternal thyroid hormones contribute to total fetal thyroid hormone concentrations until birth. In order for pregnant women to produce enough thyroid hormones to meet both her own and her baby’s requirements, a 50% increase in iodine intake is recommended. A lack of iodine in the diet may result in the mother becoming iodine deficient, and subsequently the fetus. In iodine deficiency, hypothyroxinemia (i.e., low maternal fT₄) results in damage to the developing brain, which is further aggravated by hypothyroidism in the fetus. The most serious consequence of iodine deficiency is cretinism, characterised by profound mental retardation. There is unequivocal evidence that severe iodine deficiency in pregnancy impairs brain development in the child. However, only two intervention trials have assessed neurodevelopment in children of moderately iodine deficient mothers finding improved neurodevelopment in children of mothers supplemented earlier rather than later in pregnancy; both studies were not randomised and were uncontrolled. Thus, there is a need for well-designed trials to determine the effect of iodine supplementation in moderate to mildly iodine deficient pregnant women on neurodevelopment in the child.