1. What is iodine? – Iodine is a chemical element (as are oxygen, hydrogen, and iron). It occurs in a variety of chemical forms, the most important being: iodide (I-); iodate (IO3-), and elemental iodine (I2). It is present in fairly constant amounts in seawater but its distribution over land and fresh water is uneven. Deficiency is especially associated with high new mountains (e.g., Himalayas, Andes, Alps) and areas of frequent flooding, but many other areas are also deficient (e.g., Central Africa, Central Asia, much of Europe).
2. What does iodine do? – Iodine is an essential part of the chemical structure of thyroid hormones. The thyroid is a butterfly-shaped gland in the front part of the neck. It makes two hormones (thyroxine (T4) and triiodothyronine (T3)). The thyroid hormones are released into the bloodstream and carried by it to target organs, particularly the liver, kidneys, muscles, heart, and developing brain.
3. Why do we need iodine? – Because thyroid hormones are essential to life. The thyroid hormones act in target organs by influencing many different chemical reactions, usually involving manufacture of key proteins. The body must have proper levels of thyroid hormone to work well.
4. How much iodine should we get? – Several international groups have made recommendations, which are fairly similar. ICCIDD, WHO, and UNICEF recommend the following daily amounts: age 0-7 years, 90 micrograms (mcg); age 7-12 years, 120 mcg; older than 12 years, 150 mcg; and pregnant and lactating women, 200 mcg.
A recent report by the Food and Nutrition Board, Institute of Medicine, National Academy of Sciences, USA, offers similar recommendations. It calculates an “Estimated Average Requirement” and from that derives an RDA (Recommended Daily Allowance). However, occasionally sufficient data are not available and instead an “Adequate Intake” is calculated, which may be set higher than the RDA would be, for safety. The recommendations for daily intake are as follows: the AI for infants 0-6 months is 110 mcg iodine and 7-12 months, 130 mcg; the RDA’s are: 1-8 years old, 90 mcg; 9-13 years, 120 mcg; 14 and older, 150 mcg; pregnancy, 220 mcg; lactation, 290 mcg. The Food and Nutrition Board also sets the tolerable upper limits of the daily iodine intake as 1.1 mg (1100 mcg) for adults, with proportionately lower levels for younger age groups.
5. Where do we get iodine from? – Most of it comes from what we eat and drink. Seafood is usually a good source because the ocean contains considerable iodine. Freshwater fish reflect the iodine content of the water where they swim, which may be deficient. Other foods vary tremendously in iodine content, depending on their source and what may have been added. Plants grown in iodine-deficient soil do not have much iodine, nor do meat or other products from animals fed on iodine-deficient plants. Because the breast concentrates iodine, dairy products are usually a good source, but only if the cows get enough iodine.
Iodized salt is a special case. With only a few isolated exceptions, edible salt (sodium chloride) does not naturally contain iodine. Iodine is added deliberately as one of the most efficient ways of improving iodine nutrition. The amount added varies widely in different regions. In Canada and the United States, iodized salt contains 100 ppm (parts per million, same as 100 mcg/gram) as potassium iodide (equals 77 ppm as iodide), so two grams of salt contains approximately the daily recommended amount of 150 mcg iodine. In the United States, you can buy salt that is either iodized or not iodized, and the price is the same; about 50% of all the salt sold in the U.S. is iodized. In Canada, all table salt is iodized. Most other countries add from 10 to 40 mcg iodine per gram of salt (10-40 ppm). Daily salt intake varies greatly in different parts of the world, ranging from two to five grams in many western countries to 20 grams in some others. An average figure may be 10 grams per day.