Iodine is also needed for the production of thyroid hormones. The thyroid (a small gland in the front of the neck), which contains 80% of the body’s iodine pool, converts iodine into the thyroid hormones thyroxine (T4) and triiodothyronine (T3). These hormones are released into the bloodstream, controlling the body’s metabolism.
As established by the National Research Council’s Food and Nutrition Board, the revised 1989 Recommended Dietary Allowance (RDA) for iodine is 40 mcg for infants, increasing to 150 mcg for adults and children age 11 and older. The RDA for pregnant and lactating women increases to 175 and 200 mcg respectively.
Harrison’s Principles of Internal Medicine reports that average U.S. iodine daily intake ranges from approximately 0.5–1.0 mg. According to the Merck Manual of Diagnosis and Therapy, less than 20 mcg per day of iodide results in iodine deficiency; iodide intake 20 times greater than the daily requirement (2 mg) results in chronic iodine toxicity.
Iodine is available from a variety of food sources, drugs, and most commercial vitamin preparations. Some seafood and sea vegetables provide good sources of dietary iodine, including: canned sardines, canned tuna, clams, cod, haddock, halibut, herring, lobster, oyster, perch, salmon, sea bass, and shrimp.
Dulse, kelp, and seaweed are also sources of dietary iodine. If grown in iodine-rich soil, foods including asparagus, green peppers, lettuce, lima beans, mushrooms, pineapple, raisins, spinach, summer squash, Swiss chard, turnip greens, and whole wheat bread may provide good sources of dietary iodine.
Animal products can also provide a source of iodine, especially if the animals are fed iodine-enriched foods or salt: beef, beef liver, butter, cheddar cheese, cottage cheese, cream, eggs, lamb, milk, and pork. Some foods such as breads may contain iodine additives.
In some parts of the world, iodized oil supplements and water iodination provide other means of iodine supplementation. Many countries, however, still have insufficient iodine supplementation programs.
Iodine has several medical applications. Typically, in conjunction with drug therapy, iodine may be used to treat goiter (an enlargement of the thyroid gland), symptoms of hypothyroidism (diminished production of thyroid hormone), and hyperthyroidism (increased production of the thyroid gland).
It may also be used as an expectorant in cough medications. Applications of iodine to conditions including arteriosclerosis, arthritis, and angina pectoris have also been noted. Iodine tinctures (dilute mixtures of alcohol and iodine) or Betadine are used as antiseptics to kill bacteria in skin cuts.
Atomidine (a product containing iodine trichloride and other unlisted ingredients) is also sold as an antiseptic. Atomidine taken orally in minute cyclic doses is also recommended as a glandular stimulant and purifier.
Some research has shown that oral iodine supplements have antifibrotic and anti-inflammatory effects. Commonly reported studies have also suggested that iodine deficiency may be a factor in fibrocystic breast disease (FBD), a catch-all term that describes general, often normal, lumpiness of the breast.
Clinical trials on women diagnosed with FBD found that, even in women showing normal thyroid function, thyroid hormone supplementation produced results including decreased breast pain and decreased breast nodules.
Some early research also correlated higher incidence of breast, endometrial, and ovarian cancers with hypothyroidism and/or iodine deficiency. However, others have noted that low levels of selenium, which is more classically associated with cancer, were also present in the women in these studies.
Iodine is used in several compounds for a variety of medical testing. For example, it may be used in x-rays of the gallbladder or kidneys or in cardiac imaging. It is used as a diagnostic tool to examine the thyroid gland’s output.
A common test measures thyroid radioactive iodine uptake (RAIU). Trace amounts of radioactive iodine (I123 or I131) are used to test thyroid function.
Together with blood tests, examining how much iodine is taken up by the thyroid gland helps physicians diagnose hypothyroid conditions (when the thyroid takes up too little iodine) and hyperthyroid conditions (when it takes up too much). Radioactive iodine therapy is also used for treating thyroid disease and cancer.
Radioactive iodine can cross the placenta, causing severe dysfunction and damage to the fetus’s thyroid gland. Current Medical Diagnosis and Treatment 2000 notes that nursing mothers should discontinue nursing for a period of time after receiving test or treatment doses of radioactive iodine.
One study published in the Journal of the American Medical Association (JAMA) in May 2000 reported radiation exposure to family members of non-pregnant, non-nursing outpatients from I131 treatment to be well below limits mandated by U.S. Nuclear Regulatory Commission (NRC) guidelines. Medical professionals may also prescribe low iodine diets in combination with radioactive iodine tests or treatments.
Too much or too little iodine intake results in a wide spectrum of disorders that are addressed by adjusting iodine intake. Too much iodine can result in toxicity.
Goiter can result from iodine deficiency. Certain substances called goitrogens can also induce goiter by interfering with thyroid functioning. Some foods have goitrogenic tendencies, as do certain drugs, for example, thiourea, sulfonamides, and antipyrine.
As listed by Prescriptions for Nutritional Healing and other sources, foods containing substances that can prevent the utilization of iodine when eaten in large quantities include Brussels sprouts, cabbage, cauliflower, kale, millet, mustard, peaches, peanuts, pears, pine nuts, soybeans, and turnips. Limiting consumption of these foods may be recommended for persons with an underactive thyroid.
Iodine deficiency can also result in serious irreversible disorders and, as of May 2000, is considered a major global health problem by organizations such as the World Health Organization (WHO) and the United Nations Children’s Fund (UNICEF).
According to the International Council on Control of Iodine Deficiency Disorders (ICCIDD), IDDs are the most common cause of preventable brain damage and mental retardation worldwide.
IDD results in cretinism (a form of stunted growth) and problems in movement, speech, and hearing. A pregnant woman with an iodine deficiency risks miscarriage, stillbirth, and mental retardation of her baby.
As of 1999, the WHO called IDD a significant public health problem in 130 countries. The ICCIDD reported 1.6 billion people worldwide at risk for IDDs, and 50 million children suffering from some degree of IDD. Although not common, iodine deficiency is on the rise in the United States.
In 2002, the United Nation’s Children’s Fund announced a pledge to eliminate iodine deficiency in the world by 2005, citing the problem as a major cause of psychiatric and learning disabilities.
Excess iodine is typically excreted, and output can be measured in the urine. Regular excessive iodine intake is needed for toxicity. Excess iodine, when used as a supplement or in drug therapy, may reduce thyroid function. Although more commonly associated with iodine deficiency, goiter can also result from too much iodine due to thyroid hyperactivity.
Additionally, high amounts of iodine from sources such as overuse of iodized salt, vitamins, cough medications, kelp tablets, or from medical testing can cause effects including rapid pulse, nervousness, headaches, fatigue, a brassy taste in the mouth, excessive salivation, gastric irritation, and hypothyroidism.
Acne can appear or become worse. Some iodine-sensitive individuals may have an allergic reaction to iodine, often a skin rash. A physician may recommend that high iodine foods be removed from the diet of those who are iodinesensitive.
Similar side effects have also been observed in some women participating in studies on iodine and diagnosed FBD. Radioactive iodine has been implicated in producing thyroid dysfunction and thyroid cancer.