Iodine supplementation during pregnancy: a public health challenge

Iodine deficiency remains the most frequent cause worldwide, after starvation, of preventable mental retardation in children. It causes maternal hypothyroxinemia, which affects pregnant women even in apparently iodine-sufficient areas, and often goes unnoticed because L-thyroxine (T₄) levels remain within the normal range, and thyroid-stimulating hormone (TSH) is not increased. Even a mild hypothyroxinemia during pregnancy increases the risk of neurodevelopmental abnormalities, and experimental data clearly demonstrate that it damages the cortical cytoarchitecture of the fetal brain. The American Thyroid Association (ATA) recommends a supplement of 150 μg iodine/day during pregnancy and lactation, in addition to the use of iodized salt. We discuss the importance of iodine supplementation to ensure adequate T₄ levels in all women who are considering conception and throughout pregnancy and lactation.

Introduction

The human brain contains billions of stereotypically organized neurons that theoretically would endow all humans with similar intellectual capabilities. However, clear differences between individuals can be seen. The development and organization of the brain depends on genetic and epigenetic factors that can act both before and after birth, resulting in variability of brain architecture and function. These factors influence the intellectual development of each individual, but epigenetic factors are the only factors so far that have been shown to be modified. It is known that under adequate and enriched stimulation during development the brain becomes more complex and increases its capacity to process information. Adverse factors during gestation and early postnatal development, such as malnutrition, hormonal imbalances, alcohol, drugs, environmental contaminants, stress and active or passive smoking, cause irreversible alterations in the development of the central nervous system (CNS) of fetuses and children, often resulting in severe intellectual deficits 1, 2, 3, 4.

The thyroid hormones L-thyroxine (T₄) and 3,5,3′-triiodo-L-thyronine (T₃) are among the fundamental factors that contribute to normal development of the CNS through genomic and nongenomic actions in neurons and glial cells [2]. Thyroid hormones are iodinated amino acids, released in the thyroid gland by thyroid-stimulating hormone (TSH)-induced proteolysis of the iodinated thyroglobulin. Therefore, iodine is an essential component of thyroid hormones. The amount of iodine intake per day, which varies according to the age and physiological state of individuals (Table 1), is therefore crucial for the thyroid gland to produce adequate amounts of thyroid hormones. In particular, iodine intake is fundamental during gestation and lactation, because in these developmental periods the mother is the only source of T₄ and iodine for the fetus and iodine for the neonate 1, 3, 4, 5, 6.

Indeed, iodine deficiency has been recognized since 1986 as the most frequent cause worldwide, after starvation, of preventable mental defects, affecting to different extents about 1 billion people [7]. The accumulated evidence was so overwhelming that the World Summit for Children, held at the United Nations in New York, included in its 1990 plan of action the elimination of iodine deficiency disorders (IDD) by the year 2000 [8]; these resolutions were supported at two other later World Conferences. However, these actions are progressing slowly [9]. It can thus be stated that, as far as iodine nutrition is concerned, we can now consider the following human rights of the child:

• Every child has the right to an adequate supply of iodine to ensure his (or her) normal development. The right of the unborn child is of particular importance.

• Every mother has the right to adequate iodine nutrition to ensure that her unborn child experiences normal mental development.

These statements can be derived from the resolutions of the Convention on the Rights of the Child, United Nations Assembly, New York, 1989; The World Summit for Children, United Nations, New York, 1990 [8]; The World Conference on Macronutrients: Elimination of ‘Hidden Hunger’, Montreal, 1991 endorsed by United Nations Children's Fund (UNICEF), World Health Organization (WHO), Food and Agricultural Organization of the United Nations (FAO) and the World Conference on Nutrition (WHO, FAO), Rome, 1992.