Introduction

Iodine is a non-metallic trace element 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 its oceans. In general, 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.

Food Sources

The iodine content of most foods depends on the iodine content of the soil in which it was raised because soil mineral content varies geographically. Seafood is rich in iodine because marine animals can concentrate the iodine from seawater. Certain types of seaweed (e.g., wakame) are also very rich in iodine.

Processed foods may contain slightly higher levels of iodine due to the addition of iodized salt or food additives, such as calcium iodate and potassium iodate. In the U.K. and Northern Europe, iodine levels in dairy products tend to be lower in summer when cattle are allowed to graze in pastures with low soil iodine content. The iodine content of foods can vary considerably.

Some important food sources of iodine

Mackerel	Shrimp	Mussels	Oysters	Cod
Seaweed	Milk	Potato	Salmon	Eggs

Recommended Dietary Allowance (RDA)

The European Union RDA for the general population is set at 130 µ g/day.

Inhibitors/stimulators:

The following food components have been found to stimulate the absorption of iodine:

Selenium – selenium deficiency can increase the effects of iodine deficiency. Iodine is essential for the synthesis of thyroid hormone, but selenium-dependent enzymes (iodothyronine deiodinases) are also required for the conversion of thyroxine (T4) to the biologically active thyroid hormone, triiodothyronine (T3).

Vitamin A and Iron – deficiencies of vitamin A or iron may also increase the effects of iodine deficiency.

Functions in the Body

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 converts 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. T4, the most abundant circulating thyroid hormone, can be converted to T3 by enzymes known as deiodinases in target tissues. In this manner, thyroid hormones regulate a number of physiological processes, including growth, development, metabolism, and reproductive function.

Deficiency

Iodine deficiency is now accepted as the most common cause of preventable brain damage in the world. According to the World Health Organization (WHO), iodine deficiency disorders (IDD) affect 740 million people throughout the world, and nearly 50 million people suffer from some degree of IDD-related brain damage. Major international efforts have produced dramatic improvements in the correction of iodine deficiency in the 1990's mainly through the use of iodized salt and iodized vegetable oil in iodine deficient countries.

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. In mild iodine deficiency, this adaptation 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 the development of the central nervous system, which is most active before and shortly after birth.

Toxicity

Acute iodine poisoning is rare and usually occurs only with doses of many grams. Symptoms of acute iodine poisoning include burning of the mouth, throat, and stomach, fever, nausea, vomiting, diarrhoea, a weak pulse, and coma.

It is rare for diets of natural foods to supply more than 2,000 µg of iodine/day, and most diets supply less than 1,000 µg/day. People living in the northern coastal regions of Japan, whose diets contain large amounts of seaweed, have been found to have iodine intakes ranging from 50,000 to 80,000 µg (50-80 mg) of iodine/day without noticeable site effects.

Regulation

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 decreases the sensitivity of the pituitary gland to TRH, limiting its secretion of TSH. 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 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.