What Are the Functions of Iodine in the Body?

Although your body needs only small amounts of iodine, its role in your body is anything but small. Iodine is critical for the production of your thyroid hormone, which has many important roles. The thyroid hormone is essential in adults and during fetal life and childhood, when it supports the normal development of the brain and other organs. Iodine may have other key roles in the body, although these are not yet fully understood.

Iodine has many important roles. (Image: Clive Streeter/Dorling Kindersley/Getty Images)

Iodine and Your Thyroid

Your thyroid gland produces two hormones, triiodothyronine, or T3, and thyroxine, or T4. According to the Institute of Medicine, iodine makes up about 59 percent of T3 and 65 percent of T4 by weight. These hormones have extensive roles throughout your body, and iodine is essential to produce them. Thyroid hormones act on every type of cell in your body, raising the overall level of cellular activity -- called the metabolic rate. In most cells, it does this by increasing the number of energy-producing components, or mitochondria. Ultimately, your thyroid hormone has many wide-ranging effects, including keeping your body temperature at a normal level, helping regulate blood glucose and helping mobilize stored fats and other nutrients when you need extra energy.

Iodine and Fetal Development

Getting enough dietary iodine is crucial during all phases of human development, including before birth. A woman's adequate intake of iodine during pregnancy helps ensure normal production of thyroid hormones, which are required for early formation and growth of fetal organs. Without enough thyroid hormone, the risk of miscarriage, stillbirth and birth defects increases. Insufficient iodine during pregnancy can also cause an infant's weight to be abnormally low at birth.

Adequate iodine intake and normal thyroid hormone levels are especially important for fetal brain development. Without enough iodine, nerve cells in the fetal brain could grow at a reduced rate and slow production of a myelin -- a substance essential for conduction of nerve impulses. A study in the July 2013 issue of "Lancet" evaluated iodine levels in about 1,000 pregnant women and tested their children at ages 8 and 9 for verbal intelligence and reading ability. Children whose mothers had the lowest iodine levels during pregnancy were most likely to score poorly on the tests.

Iodine After Birth

After birth, it's crucial for a baby or growing child to take in enough iodine, either through breast milk or formula, and later as part of the diet. This helps keep thyroid hormone in a normal range and ensures the child's body is able to utilize energy and grow at a normal rate. With inadequate iodine intake and low thyroid hormone levels, a child could grow slowly, develop muscular disorders and have other physical problems.

An infant or child also needs to consume enough iodine for continued growth of his brain and nerves. A 2010 review in "Indian Journal of Endocrinology and Metabolism" reported that iodine deficiency during childhood can cause poor learning, slow mental development or speech or hearing problems. In adults, iodine remains important for normal brain function. Iodine deficiency and low thyroid hormone, or hypothyroidism, often causes slow mental function, sensitivity to cold, muscle weakness or intestinal problems.

Iodine in Other Organs

The salivary glands, stomach lining, parts of the eyes and other organs also take up iodine. Although iodine's roles outside the thyroid are not fully understood, research summarized in the August 2013 issue of "Thyroid" suggests that iodine may act as an antioxidant, removing potentially harmful substances called free radicals from your tissues and thereby reducing the risk of cancer and other disorders. For example, a report published in the April 2005 issue of "Journal of Mammary Gland Biology and Neoplasia" reported that diets high in iodine are associated with low breast cancer rates. The authors suggest that iodine might prevent abnormal growth and division of breast cells, but further research is still needed to confirm this.