What it does in the body

Thyroid gland
Iodine is a component of the thyroid hormones triiodothyronine and thyroxin, which determine the metabolic rate of the body. This affects the body's conversion of food into energy and also the way energy is used.

Thyroid hormones are vital for growth and development of all organs, especially the brain, reproductive organs, nerves, bones, skin, hair, nails and teeth. The thyroid is involved in protein manufacture, cholesterol synthesis, carbohydrate absorption and the conversion of carotene to vitamin A. Thyroxin is an important regulator of body weight.

Absorption and metabolism
Iodine is rapidly absorbed from the gut. Excesses are excreted in the urine.

Deficiency
Iodine deficiency leads to various illnesses which are known as iodine deficiency disorders and include hypothyroidism, goiter and cretinism. Intakes of less than 50 mcg per day induce deficiency.

Hypothyroidism and goiter
When body iodine stores are exhausted, the thyroid gland in the neck is influenced by the pituitary gland to increase its activity and can become enlarged. This swelling is known as a goiter. Other symptoms of hypothyroidism include fatigue, apathy, drowsiness, sensitivity to cold, lethargy, muscle weakness, weight gain and coarse skin. Young men and women in iodine-deficient areas are at the greatest risk of developing goiter.

Deficiency of iodine is an important world health problem but is relatively rare in industrialized countries due to iodine fortification of salt. Those who still suffer from goiter may do so because they eat too many foods which block iodine utilization. These foods are known as goitrogens and include raw cabbage, kale, turnips, peanuts, soybeans and cauliflower. Drugs such as disulfiram, thiouracil, thiourea and sulfonamide can also inhibit the thyroid gland and lead to deficiency.

A reduction of salt in the diet, combined with a growing consumption of manufactured food prepared using low iodine salt, may lead to an increased risk of deficiency in areas where there is little natural iodine.

Cretinism
Severe iodine deficiency in a mother's diet during pregnancy increases the risk of miscarriage and stillbirth. If the baby survives to term, it is likely to suffer irreversible mental retardation. This is known as cretinism and is a major cause of preventable intellectual impairment in low iodine areas. Mildly iodine-deficient children have learning disabilities and poor motivation. The developing fetus, newborn and young children are most susceptible to the effects of an iodine- deficient diet, and treatment before conception or in early pregnancy is essential to prevent irreversible damage. Breast milk contains more iodine than formula milk and premature babies who are formula-fed may be at risk of deficiency.

Breast disorders
Iodine deficiency may play a role in fibrocystic breast disease. Hypothyroidism and iodine deficiency may also increase the risk of breast cancer, as a higher incidence of disease has been found in iodine-deficient areas.¹

Sources
Good sources of iodine include vegetables grown in iodine-rich soil, kelp, onions, milk, milk products, salt water fish and seafood. The iodine content of vegetables varies widely with the iodine content of the soil in which they are grown. The table below can be used as a guide.

Sodium or potassium iodide is added to table salt in many countries including the USA, Switzerland, Australia and New Zealand. Salt used in the processing and refining of foods is not usually iodized. Potassium iodate is used in the baking of some bread.

The average iodine intake in the USA is over 600 mcg per day.

Recommended dietary allowances

Supplements
Iodine supplements come in various forms including ammonium iodide, calcium iodide, potassium iodide and kelp. People who live in low soil iodine areas who restrict the salt in their diet and do not eat fish may benefit from iodine supplements.

Toxic effects of excess intake
Symptoms of acute poisoning from ingestion of iodine (rather than iodide) are mainly due to its corrosive effects on the gastrointestinal tract and include vomiting, abdominal pain and diarrhea. Other symptoms may include metallic taste, sore teeth, gum and mouth, and severe headache. Eventually the kidneys fail to produce urine. A fatal dose is 2 to 3 g of iodine. Treatment is with large volumes of milk and starch solutions with 1 percent solution of potassium thiosulfate.

Toxic effects from the iodide form of iodine are rare and may include a reduction of thyroid hormone secretion, acne, and inflammation of the salivary glands when doses reach 1500 mcg. Dietary intake of iodine should not exceed 1000 mcg per day for any length of time. Toxic symptoms may result from high intakes which occur as part of medical treatment with iodine as iodides. Patients may become hypersensitive after prolonged oral administration.

Topical application of iodine-containing disinfectants may lead to hypothyroidism in newborn babies.

A disorder known as hyperthyroidism of Graves disease is due to an overactive thyroid. It is not due to over-consumption of iodine, but happens as a result of a disruption in the mechanisms that control thyroid hormone function.

Therapeutic uses of supplements
Supplemental iodine is used to treat iodine deficiency disorders. On a large scale, this is often given in the form of iodized salt or as an iodized oil injection.

Fibrocystic breast disease
Some studies have shown that iodine treatment can relieve the symptoms of fibrocystic breast disease. In 1993, Canadian researchers published a review of trials using iodine replacement therapy to treat fibrocystic breast disease. Preparations used included sodium iodide, protein-bound iodide and molecular iodine. Beneficial effects were seen with all the treatments, but molecular iodine was found to be the most beneficial.² Thyroid hormone replacement therapy may also be beneficial.

Other uses
Iodine is an antiseptic and can be used to kill bacteria and fungi. Iodine used topically as a douche is effective against a wide range of organisms including candida and chlamydia. Excessive use should be avoided since some iodine will be absorbed into the system and can cause suppression of thyroid function. Iodine tablets are frequently used to disinfect water.

Iodine can also be used to prevent radioactive damage to the thyroid gland. In nuclear accidents, radioactive iodine is released into the atmosphere and can be taken up by the thyroid, possibly causing cancer. Immediate treatment with iodine prevents this uptake.

Cautions
Potassium iodide supplements should be used with caution in cases of dehydration, acne, heat cramps, adrenal insufficiency, and cardiac disease. Prolonged use during pregnancy is not advisable.

1 Smyth PP. Thyroid disease and breast cancer. J Endocrinol Invest, 1993 May, 16:5, 396-401

2 Ghent WR; Eskin BA; Low DA; Hill LP Iodine replacement in fibrocystic disease of the breast. Can J Surg, 1993 Oct, 36:5, 453-60