Iodine is important for thyroid health, fetal growth, mental health, and much more. However, iodine intake is decreasing, especially in certain populations. So, what are the impacts of low iodine intake, who is at risk, and how is iodine deficiency treated? Let’s take a read!
Iodine is required to synthesize the thyroid hormones T3 and T4. Because of iodized salt programs, the United States has been considered iodine-replete since around the 1920s. However, iodine intake has been decreasing since 1971, when the National Health and Nutrition Examination Surveys (NHANES) began monitoring iodine status. In 1971, iodine levels determined from urine tests were 320 mcg/L. By 2010, average iodine concentration dropped to 144 mcg/L. Below 100 ug/L is considered deficient for an adult, while anything below 150 mcg/L is below optimal for pregnant women.
Iodine deficiency affects more than 2.2 billion people worldwide. In the Western world, iodine deficiency affects multiple demographics more than many practitioners realize:
- In the NHANES studies, pregnant women and women of childbearing age had the lowest median iodine levels
- In a study of Boston mothers, 47% of breast milk iodine samples were insufficient
- In Europe, it is estimated that up to 44% of the population may be iodine deficient
- In school-aged children in New Zealand, a whopping 83% is estimated to be iodine deficient
Patients are not consuming enough iodine-rich foods
Table salt is fortified with iodine, but this method of mass supplementing may no longer be sufficient. First, although the consumption of salty, processed food has skyrocketed over the past several decades, these products are generally not made with iodized salt. Second, sea salt, which is becoming more popular, doesn’t contain iodine either. Furthermore, due to fear of hypertension, many patients are instructed to severely restrict salt intake, which may be misguided.
Most of the world’s iodine is found in the oceans. Therefore, the richest dietary sources are from the sea: Sea vegetables, such as kombu, nori, and kelp, as well as saltwater fish and fish heads. Regular consumption of these sources are rare in the United States. Dairy products contain a little iodine, but most of it comes from iodophor, a cleanser used to sterilize milk tanks, and from cow feed, which is supplemented with iodine in the United States. Eggs are another minor source of iodine.
Who is at risk for iodine deficiency?
Considering the best dietary sources of iodine, specific demographics are susceptible to iodine deficiency.
Vegans and vegetarians
Veganism is increasingly popular, comprising 0.5% to 6% of the U.S. population, according to various estimates. Estimates of vegetarians range from 2% to 10% of the population. Some of the most nutrient-dense foods are properly sourced animal products, and so it should come as no surprise that vegetarians and vegans are at risk for a number of nutrient deficiencies, including vitamins B12, A, and D, as well as calcium, zinc, and iodine. Vegans are especially susceptible to iodine deficiency. In the literature, vegans repeatedly have low iodine intake and/or iodine deficiency. The only exceptions were vegans who consumed seaweed, a very rich source of iodine.
Vegans and vegetarians may also consume relatively high quantities of vegetables from the genus Brassica, including broccoli, kale, Swiss chard, and Brussels sprouts. These vegetables contain goitrogens, which in large doses inhibit iodine uptake by the thyroid. Therefore, even above-adequate iodine intake may not be enough to counter the goitrogen effects.
The ideal Paleo diet should include saltwater fish and sea vegetables, but if these aren’t consumed on a regular basis, Paleo dieters may be susceptible to iodine deficiency, especially if they are not consuming any dairy products. In one study that compared a Paleolithic diet to the Nordic nutrition recommendations for two years, the Paleo dieters had significantly lower iodine status. Patients on a Paleo diet should be sure they are consuming adequate iodine-rich foods.
During pregnancy, iodine requirements increase by 50%, due to an increased glomerular filtration rate and the needs of the fetus. If women aren’t consciously making an effort to consume iodine-rich foods, their diets may fall short. Between 1971 and 2008, the percentage of pregnant women with severely deficient urinary iodine levels (<50 mcg/L) increased from 4% to 15%. A fetus’s thyroid is not developed until after the first trimester, and so the fetus is completely dependent upon the mother’s properly working thyroid, including replete iodine. Iodine is especially important for fetal brain development and proper myelination of the central nervous system.
Those with high exposure to and/or suboptimal detoxification of environmental toxins
Environmental toxins may contribute to iodine deficiency in Western populations. Perchlorates, thiocyanates, and nitrates all compete with iodine in the sodium-iodine symporter present in thyroid and lactating breast tissue. Therefore, people with high exposure to these toxins or people who have trouble properly detoxing these substances may have trouble incorporating iodine into the thyroid. Perchlorate is used as an oxidizer for rocket fuels and a propellant in explosives. In the NHANES 2001–2002 study, perchlorates were present in all 2,820 urine samples. Several studies have found no association with perchlorate levels and thyroid function, except when also in the context of iodine deficiency.
Thiocyanates are found in vegetables in the genus Brassica, as discussed above, but they are also present in cigarette smoke. Nitrates are found in drinking water, processed meats, and some leafy and root vegetables.
Risks of iodine deficiency
Iodine deficiency disrupts thyroid function and inhibits proper production of T3 and T4, which can lead to a variety of health problems.
Subclinical hypothyroidism (SCH) is generally diagnosed from high levels of thyroid-stimulating hormone (TSH) but normal T3 and T4 levels. Symptoms of this condition are varied and can include mental slowing, depression, dementia, weight gain, dry skin, hair loss, cold intolerance, hoarse voice, irregular menstruation, infertility, muscle stiffness, and pain. Untreated SCH can lead to clinical hypothyroidism and swelling of the thyroid (goiter). SCH during pregnancy increases the risk of miscarriage, gestational diabetes, and preterm delivery. The prevalence of SCH during pregnancy may be as high as 15%, but proper diagnosis is challenging. Appropriate levels of TSH during different trimesters are not well established. Due to the suppressed immune system that accompanies pregnancy, thyroid antibodies, which may actually indicate an autoimmune thyroid disease versus iodine deficiency, may not be detectable.
Especially important for developing brains, iodine is critical for babies in utero and young children. Mild iodine deficiency during pregnancy is associated with decreased IQ and increased risk of attention deficit disorder in children. Severe iodine deficiency can result in cretinism, which is characterized by mental retardation, dwarfed state, bone dystrophy, and low basal metabolism. Iodine supplementation during pregnancy has been shown to reduce neonatal mortality and improve IQ scores.
Iodine concentrations are higher in breast tissue even than in the thyroid, to provide plenty of iodine for the breastfeeding infant. There is some evidence in animals that iodine-deficient breast tissue is more susceptible to the effect of carcinogens and therefore increased risk of cancer. The Japanese, who probably have the highest consumption of iodine in the world, have less than one-third the incidence of breast cancer than do Americans. When these women emigrate to the United States and adopt the Standard American Diet, their breast cancer risk rises to that of other American women.
Treatment for low iodine
Many conventional doctors measure iodine using a spot urine check. However, this will not give an accurate picture of iodine levels. One study reported that 12 or more spot urine checks are needed to achieve precision within 20%. First, a 24-hour urine iodine test involves collecting and then testing entire urine output for 24 hours. Second, a hair iodine test is a good marker for long-term iodine status. Last, serum thyroglobulin (not the antibody), reversely correlates well with iodine levels.
Patients who are iodine-deficient need to increase both iodine and selenium intake, preferably from foods. Selenium helps protect against iodine toxicity by ensuring that glutathione peroxidase activity in the thyroid is adequate to avoid cytotoxicity of hydrogen peroxide. Selenium is also required for the deiodinase enzymes that convert T4 to T3. Good dietary sources of selenium include Brazil nuts, crimini mushrooms, cod, shrimp, tuna, halibut, salmon, scallops, chicken, eggs, shiitake mushrooms, lamb, and turkey. To increase iodine intake, kelp flakes can be used in place of salt while other forms of seaweed can be used for flavoring foods.
Over-supplementing with iodine can contribute to Hashimoto’s disease or hyperthyroidism, especially when accompanied by selenium deficiency. Long-term supplementation with selenium is also not advisable, since it leads to fatigue, GI upsets, mild nerve damage, and more. If supplementation is necessary, start with a small dose of iodine, one 325 mcg kelp tablet per day. Increase the dose by one tablet every three weeks up to 3 tablets a day, unless hyperthyroid-like or detox symptoms occur. Once improves are seen, the dose can slowly be scaled down again. Very short-term supplementation of selenium, 200 mcg per day for a couple months, can be safe but requires extra precaution.
Adapted from: Chris Kresser
Nutrition Tip of the Day
A small handful of nuts or seeds can be a satisfying and healthy snack! Look for unsalted or lightly salted nuts. Almonds, hazelnuts, peanuts, pecans, pistachios and walnuts are all good choices.