Iodine

What is it?

Iodine is an essential trace element for humans. The average adult body contains between 20 and 50 mg iodine, and more than 60 per cent of this is concentrated in the thyroid gland situated at the base of the neck. The rest is in thyroid hormones in the blood, ovaries and muscles. Worldwide soil distribution of iodine is extremely variable and food grown in areas of low iodine does not contain enough of the mineral to meet requirements. Such areas include a band across the middle of the USA, the Midlands and South West England, and areas of China, Continental Europe, Russia and South America.

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.

Deficiencies

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. (See page 609 for more information.) 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.

Food	Amount	Iodine (mcg)
Mackerel	150g	255
Mussels	150g	180
Cod	150g	165
Kipper	150g	107
Whiting	150g	100
Yogurt	150g	95
Milk	560g	86
Cockles	50g	80
Fish fingers	75g	75
Pilchards in tomato sauce	100g	64
Scampi	150g	62
Herring	150g	48
Beer	560g	45
Plaice	150g	42
Prawns	150g	42
Eggs	70g	37
Sardines, canned in oil	150g	35
Trout	150g	24
Kidney	150g	23
Liver	150g	22
Tuna	150g	21
Bacon	150g	18
Cheese	40g	18
Potato chips	265g	13

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

Recommended dietary allowances (RDA)

	Men	Women	Pregnancy	Lactation
USA	150 mcg	150 mcg	175 mcg	200 mcg
UK	140 mcg	140 mcg	-	-
Australia	150 mcg	120 mcg	150 mcg	200 mcg

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 per cent 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.