In the first trimester (3 mos) of pregnancy, the fetus is completely dependent upon the mother for thyroid hormone. During the second and final trimesters of pregnancy, most fetuses are able to provide some thyroid hormone but also continue to rely upon their mothers for some hormone. Problems of brain development occur when either the mother is unable to provide for the fetus' needs in the first trimester, or neither the mother nor the fetus can provide for fetal needs during the remainder of pregnancy. Another important issue is the availability of iodine to the mother since iodine is an important requirement to provide for proper thyroid production by both the mother and the developing fetus.
Should all pregnant women routinely be evaluated for thyroid function early in pregnancy? The thyroid status of a large group of pregnant women and their offsprings were evaluated by the authors.1 In those women with low levels of thyroid hormone, the authors show that hypothyroidism (low thyroid function) adversely affects the child's performance on tests of intelligence, language, reading ability and motor performance. This can be true even when the mother is presenting no symptoms. As a result of their study, the authors suggest that an evaluation of the mother's thyroid function should become routine early in pregnancy for all women.
In an editorial in the same journal (pages 601-602), Dr. Robert D. Utiger raises questions about the cost/benefit effects of routine examination of thyroid function in all pregnant women. He suggests instead that since iodine is essential for thyroid hormone production, an adequate supply of iodine needs to be made available to all pregnant women. This is particularly true in that a common source of iodine in the U.S. diet is iodized table salt. With efforts to decrease the use of table salt in order to prevent high blood pressure, the consumption of iodine may no longer be adequate --- particularly for pregnant women. He suggests iodine needs to be added to other foods and be included in all vitamin products as the first step to prevent developmental brain damage due to maternal hypothyroidism. Thus, he proposes that before a national program of routine thyroid function in pregnant women is initiated, an increased supply of iodine be added to vitamin products used by pregnant women.
Comment: It has been recognized for a very long time that an adequate supply of thyroid hormone is essential for normal brain development. This is true during the entire period of pregnancy but it is particularly true in the last half of pregnancy and in the early years of infant life. Modest thyroid hormone insufficiency leads to cognitive inadequacies (e.g. learning disabilities); a large insufficiency leads also to neurological deficits -- spasticity, poor balance and deafness. In women at risk of a troubled pregnancy, a thorough thyroid function evaluation early in pregnancy would certainly seem prudent. Also, an available source of iodine within the diet or by dietary supplements (e.g. iodine fortified vitamins) should be routine. In addition, attention needs to be given to the thyroid hormone status of the newborn infant; and in selected cases to the infant several weeks after birth to make certain the infant's thyroid gland continues to function properly.
There are no longer any questions about the absolute requirement of thyroid hormone for adequate brain development. Both pregnant women and their physicians need to recognize this on the "check list" of things to consider to foster normal development of the developing brain.
In summary, a controversy continues to exist about whether all pregnant women should have their thyroid function routinely evaluated early in pregnancy; or, should only women who are "at risk" of poor thyroid function be evaluated? In any case, most experts agree that routinely an iodine supplement is a good idea.
1 Haddow J.E. et al. Maternal Thyroid Deficiency During Pregnancy and Subsequent Neuropsychological Development of the Child NEJM 1999; 341:549-555
© UCP Research & Educational Foundation, February 2000