Iodine is a trace element found in seawater, rocks and in some types of soil. Good food sources include sea fish and shellfish. Iodine can also be found in plant foods such as cereals and grains but the levels vary depending on the amount of iodine in the soil where the plants are grown.

Iodine is an essential trace element; the thyroid hormones thyroxine and triiodotyronine contain iodine. In areas where there is little iodine in the diet—typically remote inland areas where no marine foods are eaten—iodine deficiency gives rise to goiter (so-called endemic goiter), as well as cretinism, which results in developmental delays and other health problems. While noting recent progress, the medical journal The Lancet noted, "According to WHO, in 2007, nearly 2 billion individuals had insufficient iodine intake, a third being of school age. Thus iodine deficiency, as the single greatest preventable cause of mental retardation, is an important public-health problem."

It helps make the thyroid hormones. These hormones help keep cells and the metabolic rate healthy. Iodine deficiency is rare in industrialized countries such as the United States, due to the enrichment of table salt and cattle feed with iodine. But deficiency is common in developing countries where supplementation may be considered. India is the most outstanding, with 500 million suffering from deficiency, 54 million from goiter, and two million from cretinism.

Iodine deficiency is one of the leading cause of preventable mental retardation, producing typical reductions in IQ of 10 to 15 IQ points. It has been speculated that deficiency of iodine and other micronutrients may be a possible factor in observed differences in IQ between ethnic groups.

Cretinism is a condition associated with iodine deficiency and goitre, commonly characterised by mental deficiency, deaf-mutism, squint, disorders of stance and gait, stunted growth and hypothyroidism. Paracelsus was the first to point out the relation between goitrous parents and mentally retarded children. As a result of restricted diet, isolation, intermarriage, etc., as well as low iodine content in their food, children often had peculiar stunted bodies and retarded mental faculties, a condition later known to be associated with thyroid deficiency. Diderot in his 1754 Encyclopédie described these patients as "crétins". In French, the term "crétin des Alpes" also became current, since the condition was observed in remote valleys of the Alps in particular. The word cretin appeared in the English language in 1779.

You should be able to get all the iodine you need by eating a varied and balanced diet. The below doses are based on scientific research, publications, traditional use, or expert opinion. Many herbs and supplements have not been thoroughly tested, and safety and effectiveness may not be proven. Brands may be made differently, with variable ingredients, even within the same brand. The below doses may not apply to all products. You should read product labels, and discuss doses with a qualified healthcare provider before starting therapy. The Recommended Dietary Allowance (RDA) is 150 micrograms daily in adults ages 18 and older (220 micrograms daily for pregnant women, 290 micrograms daily for breastfeeding women). The Tolerable Upper Intake Levels (UL) for adults ages 18 and older is 1,100 micrograms daily.

Iodine preparations used orally (by mouth) or topically (on the skin) are generally considered to be safe in healthy non-allergic individuals when used in recommended amounts, not exceeding tolerated upper limits. Higher amounts taken acutely or chronically may result in adverse effects. Acute iodine poisoning is rare and generally occurs only with doses of many grams. Symptoms may include burning of the mouth, throat, and stomach, fever, nausea, vomiting, diarrhea, cardiovascular compromise, and loss of consciousness/coma.

Chronic iodism, also known as iodide intoxication, may cause eye irritation, eyelid swelling, unpleasant/metallic taste, burning or swelling of the mouth/throat, soreness of the gums/teeth, increased salivation, gastrointestinal upset, diarrhea, anorexia, flu-like symptoms, sneezing, cough, pulmonary edema (fluid in the lungs), confusion, headache, fatigue, depression, numbness, tingling, pain, weakness, muscle aches, easy bruising, irregular heart beat, or acne-like skin lesions. Prolonged excess intake of iodide can lead to thyroid gland dysfunction including hypo- or hyperthyroidism, parotiditis, thyroid gland hyperplasia (enlargement), thyroid adenoma, goiter, autoimmunity, and elevated thyroid stimulating hormone (TSH) levels.