Iodine is the cause of more thyroid problems than all other food substances combined. You can get sick from eating either too much or too little iodine. In a few countries, especially in remote mountainous areas where the daily iodine intake is less than 25 micrograms per day, we find the most severe forms of hypothyroidism including cretinism. This tragic disorder results from extreme iodine deficiency - affected children are born with such marked mental and physical retardation, including deaf mutism and dwarfism, that they never can be productive members of their society.
The worldwide problem of iodine deficiency is much bigger and extremely serious. One billion people (one-fifth of the world's population) do not get enough iodine in their diets, and countless population studies have shown that they have thyroid-related problems. In addition to unsightly goiters, they experience increased infant mortality, infertility, impaired growth, and frequent or endemic hypothyroidism. Most harmful are the widespread milder forms of developmental retardation that cause poor performance in school and in the workplace. These factors are reflected in impaired social and economic development. In fact iodine deficiency is the leading cause of preventable mental deficiency in the world today.
The map shows that iodine deficiency is found almost everywhere in the world except North America. The state of dietary iodine in the former Soviet Union is not well known. However, there is some data that suggests that lack of iodine is common in some areas, and may have worsened the effect of the Chernobyl accident on inhabitants of that region. Lacking normal amounts of dietary iodine, their thyroid glands absorbed more radioactive iodine from fallout and thus increased their subsequent risks for thyroid failure and thyroid cancer.
An important international effort is now underway to eradicate iodine deficiency worldwide by the year 2000. Led by the International Council for the Control of Iodine Deficiency Disorders, such agencies as UNICEF, the World Health Organization, the World Bank, and the Thyroid Foundation of America are supporting programs to correct iodine deficiency by adding iodine to salt and city water supplies, and by individual treatments such as the administration of iodized oil orally or by injection. Here is a wonderful opportunity for you to help a most worthy cause. For more information contact the ICCIDD or TFA at the addresses or telephone numbers listed in the back of this book.
In contrast, inhabitants of the United States, Canada, Japan, and certain other countries eat more iodine than is really necessary. The Food and Nutrition Board of the National Research Council has recommended a daily intake of 150 to 300 micrograms of iodine per day. If you live in the United States your daily dietary iodine intake is probably between 200 and 700 micrograms per day, because extra iodine has been added to bread, milk, salt, and other foods that you consume frequently.
In countries where seaweed or dulse is eaten, the consumption of iodine may be much higher. One study found that the inhabitants of the Japanese island of Hokkaido who consume large quantities of a seaweed called kombu take in more than 200 milligrams (200,000 micrograms) of iodine per day, a thousand times the recommended daily requirement. Although there is a 10 percent incidence of thyroid enlargement or goiter on Hokkaido, the inhabitants do not appear to have an increased risk for other thyroid problems. Normal individuals seem to have the ability to control the amount of iodine that actually enters their thyroid gland even if their diet is supplemented with extra iodine. However, as shown in Table 4, if you have a problem with your thyroid, that problem could make you more likely to develop a change in thyroid function if you take in too much iodine in your diet or in other ways. That would be true, for example, if you ever had hyperthyroidism due to a generalized overactivity of your thyroid gland (diffuse toxic goiter or Graves' disease) or if you have a low-grade inflammation of your thyroid known as chronic lymphocytic thyroiditis (Hashimoto's disease). Either condition would give you a tendency to develop hypothyroidism if you were exposed to even a modest amount of extra iodine. In fact, patients who have had Graves' disease have become hypothyroid after ingesting as little as 18 milligrams of iodine per day.
Unborn babies are also very sensitive to iodine excess. Therefore, if you are pregnant and ingest large quantities of iodine in the form of a medication, kelp, or another form of seaweed, you risk having your baby born with a goiter and possibly with an underactive thyroid as well. A large goiter could compress the baby's windpipe and interfere with breathing. Moreover, since iodine can also be transmitted from mother to child in breast milk, you should avoid health foods and medications that contain extra iodine while nursing your baby.
An abrupt increase in dietary iodine can cause hyperthyroidism in people living in iodine-deficient areas of the world. "Epidemics" of hyperthyroidism have been seen in several countries when iodine was added to the national diet to correct a long-standing problem of wide-spread iodine deficiency. This happened in the U.S. in the 1920s when health authorities added iodine to salt.
Hyperthyroidism caused by excess iodine also has been observed in the United States and other parts of the world where dietary iodine is sufficient. In such areas, older patients with lumpy thyroids (nodular goiters) are those most likely to be affected by an increase in iodine intake. Moreover, since these patients tend to be elderly, they are more likely to have complications from the rapid pulse or irregular heart rhythm that may happen when their thyroid becomes overactive.
Therefore, if you have Graves' or Hashimoto's disease, are pregnant or nursing a baby, or if you have a nodular goiter, you should try to avoid an abrupt increase in your iodine intake. Do not eat kelp, and do read labels on bottles of vitamins and other medications. If you are having an x-ray for which a dye is given to you by mouth or by injection, find out if there is iodine in the x-ray dye. This is likely if the x-ray is of your kidney, spinal canal, gall bladder, or blood vessels. This is not to say you cannot take a medication that contains iodine or that you shouldn't have one of these special x-rays. Rather, if that is your situation, your doctor may choose to examine you after you take the medication or have the x-ray to be sure that your thyroid function has not changed.
The natural iodine of most foods is low. It is highest in seafoods, and there are variable amounts of bread, miIk, eggs, and meat. Fruits contain little iodine, as do vegetables with the exception of spinach. The exact amounts of iodine in these foodstuffs vary so widely and depend on so many factors that it is no longer possible to make a satisfactory list of the iodine content of foods. Instead, the message here is to eat a "regular diet" during pregnancy and while breast-feeding or if you know you have a thyroid condition. You do not need to avoid iodized salt, bread, and seafood—just don't take in extra iodine if possible in special foods like kelp.
Some medications which contain a lot of iodine are important treatment for critical problems. A good example is the heart drug amiodarone which physicians are using increasingly to stabilize life-threatening heart rhythm problems. Each tablet contains 75 milligrams of iodine and the average person taking the drug gets about 9 milligrams of extra iodine released into their bloodstream every day. If you live in the United States and need to take amiodarone, you have about a 20 percent chance of developing hypothyroidism, and a slight risk for hyperthyroidism. Since such a change in thyroid function may not occur immediately, your physician will likely check your thyroid periodically with a TSH blood test. Similarly, other foods or necessary medications can be used under your doctor's supervision.
If you or your physician want to know more about your iodine intake, the best thing to do is to measure the iodine content of your urine. In a general way the amount of iodine in your urine is equal to the amount you take in from all sources, including food, medications, and special x-ray dyes. Such a test could be helpful if you are pregnant and want to know exactly how much iodine you are taking in. However, in most cases that information is far less important than a measurement of your thyroid hormone and TSH blood levels.
Your thyroid is one of many endocrine glands, and autoimmune inflammations like those occurring in Hashimoto's thyroiditis may occur in these other glands too.