Linus Pauling Institute on "Iodine"

http://lpi.oregonstate.edu/infocenter/minerals/iodine/

Iodine deficiency is now accepted as the most common cause of preventable brain damage in the world.

Iodine, a non-metallic trace element, is required by humans for the synthesis of thyroid hormones. Iodine deficiency is an important health problem throughout much of the world.

Most of the earth's iodine is found in oceans, and iodine content in the soil varies with region.

The older an exposed soil surface, the more likely the iodine has been leached away by erosion. Mountainous regions, such as the Himalayas, the Andes, and the Alps, and flooded river valleys, such as the Ganges, are among the most severely iodine-deficient areas in the world (1).

Function

Iodine is an essential component of the thyroid hormones, triiodothyronine (T₃) and thyroxine (T₄), and is therefore essential for normal thyroid function. To meet the body's demand for thyroid hormones, the thyroid gland traps iodine from the blood and incorporates it into thyroid hormones that are stored and released into the circulation when needed. In target tissues, such as the liver and the brain, T₃, the physiologically active thyroid hormone, can bind to thyroid receptors in the nuclei of cells and regulate gene expression. In target tissues, T₄, the most abundant circulating thyroid hormone, can be converted to T₃ by selenium-containing enzymes known as deiodinases. In this manner, thyroid hormones regulate a number of physiologic processes, including growth, development, metabolism, and reproductive function (1, 2).

The regulation of thyroid function is a complex process that involves the brain (hypothalamus) and pituitary gland. In response to thyrotropin-releasing hormone (TRH) secretion by the hypothalamus, the pituitary gland secretes thyroid-stimulating hormone (TSH), which stimulates iodine trapping, thyroid hormone synthesis, and release of T₃ and T₄ by the thyroid gland. The presence of adequate circulating T₄ and T₃ feeds back at the level of both the hypothalamus and pituitary, decreasing TRH and TSH production (diagram). When circulating T₄ levels decrease, the pituitary increases its secretion of TSH, resulting in increased iodine trapping as well as increased production and release of both T₃ and T₄. Iodine deficiency results in inadequate production of T₄. In response to decreased blood levels of T₄, the pituitary gland increases its output of TSH. Persistently elevated TSH levels may lead to hypertrophy (enlargement) of the thyroid gland, also known as goiter (see Deficiency) (3).

Deficiency

Iodine deficiency is now accepted as the most common cause of preventable brain damage in the world.

The spectrum of iodine deficiency disorders (IDD) includes mental retardation, hypothyroidism, goiter, and varying degrees of other growth and developmental abnormalities (1, 4). WHO estimated that over 30% of the world’s population (2 billion people) has insufficient iodine intake as measured by urinary iodine excretion below 100 micrograms (mcg)/liter (5); urinary iodine is an indicator of iodine status. Moreover, an estimated 31.5% of school-age children (6-12 years old) worldwide (266 million total children) has insufficient iodine intake (5). Major international efforts have produced dramatic improvements in the correction of iodine deficiency in the 1990s, mainly through the use of iodized salt in iodine-deficient countries (6). Today, 70% of households in the world use iodized salt (7). For more information on the international effort to eradicate iodine deficiency, visit the Web sites of the International Council for the Control of Iodine Deficiency Disorders (ICCIDD) or the WHO.

Thyroid enlargement, or goiter, is one of the earliest and most visible signs of iodine deficiency. The thyroid enlarges in response to persistent stimulation by TSH (see Function). In mild iodine deficiency, this adaptative response may be enough to provide the body with sufficient thyroid hormone. However, more severe cases of iodine deficiency result in hypothyroidism. Adequate iodine intake will generally reduce the size of goiters, but the reversibility of the effects of hypothyroidism depends on an individual's stage of development. Iodine deficiency has adverse effects in all stages of development but is most damaging to the developing brain.

In addition to regulating many aspects of growth and development, thyroid hormone is important for myelination of the central nervous system, which is most active before and shortly after birth (2, 6).

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