What if a single trace mineral could influence your weight? Iodine, primarily known for its role in thyroid hormone production, has a more significant impact on your health than many realize. While iodine's main role in the body is to produce thyroid hormones, its influence extends beyond just the thyroid. It participates in normal growth and development, particularly in infants and children.
The Role of Iodine in Thyroid Function and Metabolism
All good thyroid health begins with the building block of thyroid hormone: iodine. “Iodine sufficiency is super important in maintaining proper thyroid hormone status,” says Hyesoo Lowe, MD, endocrinologist at the Thyroid Center. Iodine is an elementary micronutrient for human life. It exists in various forms in the human body, such as iodide atom (I−), molecular iodine (I2), triiodide (I3), iodine anion (HI2O−), and iodine-binding molecules (such as iodolipids). I− is a kind of reducing agent that can be oxidized by peroxidase enzymes to generate thyroid hormones (THs). Tri-iodothyronine (T3) and thyroxine (T4) are essential regulators of energetic metabolism. Without sufficient iodine, your thyroid gland simply cannot function optimally. These hormones are directly responsible for regulating your metabolic rate, which is the speed at which your body burns calories for energy.
When the body lacks sufficient iodine, the thyroid gland struggles to produce enough T3 and T4. This deficiency can lead to hypothyroidism, a condition characterised by a sluggish metabolism. One of the most common and frustrating symptoms of an underactive thyroid (hypothyroidism) is unexplained weight gain. Individuals with hypothyroidism often experience fatigue, which reduces their motivation for physical activity.
Iodine Deficiency: Symptoms and Risk Factors
Iodine is an essential mineral commonly found in seafood. Your thyroid gland uses it to make thyroid hormones, which help control growth, repair damaged cells and support a healthy metabolism. Unfortunately, up to a third of people worldwide are at risk of an iodine deficiency. Low levels of iodine can cause symptoms including neck swelling and weight gain. You may experience other symptoms of a low level of thyroid-stimulating hormone. Its symptoms are very similar to those of hypothyroidism, or low thyroid hormones. Since iodine is used to make thyroid hormones, an iodine deficiency means your body can’t make enough of them, leading to hypothyroidism.
Those at the highest risk include:
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- Pregnant people, who need to consume enough to meet their own daily needs, as well as the needs of their growing baby. The increased demand for iodine continues throughout lactation, as babies receive iodine through breast milk
- People who live in countries where there is very little iodine in the soil. This includes South Asia, Southeast Asia, New Zealand and European countries.
- People who don’t use iodized salt.
- People who follow a vegetarian or vegan diet.
On the other hand, iodine deficiencies are rare in the US, where there are sufficient levels of the mineral in the food supply.
An iodine deficiency can cause uncomfortable and even severe symptoms. They include swelling in the neck, pregnancy-related issues, weight gain and learning difficulties.
Here are some signs and symptoms of an iodine deficiency:
- Swelling in the front of the neck (Goiter): This is the most common symptom of an iodine deficiency. This is called a goiter and occurs when the thyroid gland grows too big. The thyroid gland is a small, butterfly-shaped gland in the front of your neck. It makes thyroid hormones upon receiving a signal from the thyroid-stimulating hormone (TSH). When blood levels of TSH rise, the thyroid gland uses iodine to make thyroid hormones. However, when your body is low in iodine, it can’t make enough of them. To compensate, the thyroid gland works harder to try to make more. This causes the cells to grow and multiply, eventually leading to a goiter. Fortunately, most cases can be treated by increasing your iodine intake. However, if a goiter hasn’t been treated for many years, it might cause permanent thyroid damage.
- Unexpected weight gain: It may occur if the body does not have enough iodine to make thyroid hormones. This is because thyroid hormones help control the speed of your metabolism, which is the process by which your body converts food into energy and heat. When your thyroid hormone levels are low, your body burns fewer calories at rest. Unfortunately, this means more calories from the foods you eat are stored as fat. Adding more iodine to your diet may help reverse the effects of a slow metabolism, as it can help your body make more thyroid hormones. Low iodine levels may slow your metabolism and encourage food to be stored as fat, rather than be burned as energy. This may lead to weight gain.
- Fatigue and weakness: Some studies have found that nearly 80% of people with low thyroid hormone levels, which occur in cases of iodine deficiency, feel tired, sluggish and weak. These symptoms occur because thyroid hormones help the body make energy. When thyroid hormone levels are low, the body can’t make as much energy as it usually does. This may cause your energy levels to plummet and leave you feeling weak. In fact, a study in 2,456 people found that fatigue and weakness were the most common symptoms among those with low or slightly low thyroid hormone levels. Low iodine levels may leave you feeling tired, sluggish and weak. This is because your body needs the mineral to make energy.
- Hair loss: When your thyroid hormone levels are low, your hair follicles may stop regenerating. Over time, this may result in hair loss. For this reason, people with an iodine deficiency may also suffer from hair loss. One study in 700 people found that 30% of those with low thyroid hormone levels experienced hair loss. However, other studies have found that low thyroid hormone levels only seem to cause hair loss in those with a family history of hair loss. If you experience hair loss because of an iodine deficiency, getting enough of this mineral may help correct your thyroid hormone levels and stop hair loss. An iodine deficiency may prevent hair follicles from regenerating. Fortunately, getting sufficient iodine can help correct hair loss that occurs due to an iodine deficiency.
- Dry, flaky skin: Some studies have found that up to 77% of people with low thyroid hormone levels may experience dry, flaky skin. Thyroid hormones, which contain iodine, help your skin cells regenerate. When thyroid hormone levels are low, this regeneration doesn’t occur as often, possibly leading to dry, flaky skin. Additionally, thyroid hormones help the body regulate sweat. People with lower thyroid hormone levels, such as those with an iodine deficiency, tend to sweat less than people with normal thyroid hormone levels. Given that sweat helps keep your skin moist and hydrated, a lack of sweat may be another reason why dry, flaky skin is a common symptom of iodine deficiency. Dry, flaky skin may occur with an iodine deficiency, as the mineral helps your skin cells regenerate. It also helps your body sweat and hydrates your skin cells, so an iodine deficiency can cause you to sweat less.
- Feeling cold: Some studies have found that over 80% of people with low thyroid hormone levels may feel more sensitive to cold temperatures than usual. Since iodine is used to make thyroid hormones, an iodine deficiency can cause your thyroid hormone levels to plummet. Given that thyroid hormones help control the speed of your metabolism, low thyroid hormone levels may cause it to slow down. A slower metabolism generates less heat, which may cause you to feel colder than usual. Also, thyroid hormones help boost the activity of your brown fat, a type of fat that specializes in generating heat. This means that low thyroid hormone levels, which may be caused by an iodine deficiency, could prevent brown fat from doing its job. Iodine helps generate body heat, so low levels of it may leave you feeling colder than usual.
- Slow heart rate: Too little of this mineral could cause your heart to beat slower than usual. A severe iodine deficiency may cause an abnormally slow heart rate. This could make you feel weak, fatigued, dizzy and possibly cause you to faint. An iodine deficiency may slow your heart rate, which may leave you feeling weak, fatigued, dizzy and at risk of fainting.
- Difficulties learning and remembering: A study including over 1,000 adults found that those with higher thyroid hormone levels performed better on learning and memory tests, compared to those with lower thyroid hormone levels. Thyroid hormones help your brain grow and develop. That’s why an iodine deficiency, which is required to make thyroid hormones, can reduce brain development. In fact, studies have found that the hippocampus, the part of the brain that controls long-term memory, appears to be smaller in people with low thyroid hormone levels. An iodine deficiency at any age may cause you to struggle to learn and remember things. One possible reason for this might be an underdeveloped brain.
- Pregnancy complications: Not consuming enough iodine throughout pregnancy and lactation may cause side effects for both the mother and baby. Mothers may experience symptoms of an underactive thyroid, such as a goiter, weakness, fatigue and feeling cold. Meanwhile, an iodine deficiency in infants may stunt physical growth and brain development. Furthermore, a severe iodine deficiency may increase the risk of stillbirth. Getting enough iodine is especially important for pregnant and breastfeeding women, as they have higher needs. An iodine deficiency may cause severe side effects, especially for the baby, such as stunted growth and brain development.
- Heavy and irregular menstrual bleeding: In one study, 68% of women with low thyroid hormone levels experienced irregular menstrual cycles, compared to only 12% of healthy women. Research also shows that women with low thyroid hormone levels experience more frequent menstrual cycles with heavy bleeding. This is because low thyroid hormone levels disrupt the signals of hormones that are involved in the menstrual cycle.
Recommended Daily Intake and Food Sources
The recommended daily intake (RDI) is 150 mcg per day. This amount should meet the needs of 97-98% of all healthy adults. However, pregnant or breastfeeding women need more. Pregnant women need 220 mcg daily, while lactating women need 290 mcg daily.
The foods below are excellent sources of iodine:
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- Seaweed, one whole sheet dried: 11-1,989% of the RDI
- Cod, 3 ounces (85 grams): 66% of the RDI
- Yogurt, plain, 1 cup: 50% of the RDI
- Iodized salt, 1/4 teaspoon (1.5 grams): 47% of the RDI
- Shrimp, 3 ounces (85 grams): 23% of the RDI
- Egg, 1 large: 16% of the RDI
- Tuna, canned, 3 ounces (85 grams): 11% of the RDI
- Dried prunes, 5 prunes: 9% of the RDI
Seaweed is usually a great source of iodine, but this depends on where it came from. Seaweed from some countries, such as Japan, are rich in iodine. Smaller amounts of this mineral are also found in a variety of foods like fish, shellfish, beef, chicken, lima and pinto beans, milk and other dairy products. The best way to get enough iodine is to add iodized salt to your meals. Half a teaspoon (3 grams) over the course of the day is enough to avoid a deficiency.
If you think you have an iodine deficiency, it’s best to consult your doctor. They will check for signs of swelling (a goiter) or take a urine sample to check your iodine levels. Your thyroid levels will likely also be checked if you are exhibiting any symptoms of underactive thyroid.
Iodine and Metabolic Syndrome
Metabolic syndrome (MetS) is defined as the combination of metabolic disorders including abdominal obesity, hypertension (HBP), dyslipidemia, and hyperglycemia. MetS is widely prevalent worldwide and poses serious issues such as cardiovascular disease (CVD), tumors, and total mortality. The development of MetS involves multiple factors, with research indicating that oxidative stress and chronic inflammatory conditions play a vital role. Changes in dietary habits are another contributing aspect to the prevalence of MetS. Recent research has examined the impact of iodine itself on the prevalence of metabolic disorders.
Clinical studies have demonstrated that iodine nutrition has an impact on metabolism. However, the mechanism involved in these effects is still poorly understood and requires further investigation. Due to varying dosages of iodine administration, durations of intervention, experimental animals, and thyroid hormone values, different in vivo studies have failed to reach a consensus.
Iodine's Impact on Obesity
In a large epidemiological study (TIDE) conducted in China, the prevalence of central obesity significantly decreased when the UIC was 300 μg/L or higher. The odds ratio (OR) for central obesity with an UIC of ≥800 μg/L was 0.797 (p < 0.05). Among school-age children in China, overweight children exhibited a lower UIC compared to children with normal weight. Women with obesity also demonstrated a significantly lower UIC in comparison to both themselves after undergoing bariatric surgery and women with normal weight. In a randomized controlled trial (RCT), the body fat percentage of the participants who consumed tablets containing iodine-reduced kelp powder showed a significant decrease in comparison to those who took the placebo. In a 4-week placebo-controlled study, seaweed fucoxanthin supplementation (1 mg/day) decreased waist circumference (WC) and fat mass in obese Japanese individuals. In addition, fucoxanthin supplementation (3 mg/day) decreased visceral fat, body mass index (BMI), and weight. However, mUIC was positively associated with obesity among Colombian women of reproductive age.
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Iodine's Influence on Diabetes
The association between UIC and the prevalence of diabetes exhibited a U-shaped curve in the TIDE study. In a cohort study involving 71,264 women, individuals with higher levels of iodine intake were found to be at a higher risk of developing type 2 diabetes mellitus (T2DM) in comparison to those with inadequate iodine intake. Adults in iodine-sufficient (IS, mUIC 200-299.99 μg/L) and iodine-excess (IE, mUIC ≥300 μg/L) areas exhibited higher blood glucose levels compared to those in iodine-adequate area (IA, mUIC 100-199.99 μg/L). Low UIC was also reported to be associated with an increased risk of elevated fasting plasma glucose (FPG) in females. A study revealed that patients with T2DM had lower UIC levels compared to healthy individuals. Furthermore, UIC showed a negative correlation with insulin resistance in subjects with T2DM. Higher iodine status may potentially protect against hyperglycemia during pregnancy. A study indicated that pregnant and lactating women in the IE area (mWIC >300 μg/L) had lower blood glucose levels and a lower prevalence of hyperglycemia. Furthermore, a higher concentration of iodine in the placenta was associated with a reduced incidence of gestational diabetes mellitus (GDM) among 471 pregnant women. However, Bell et al. did not find any correlation between UIC and the prevalence of GDM.
Iodine and Hypertension
In the TIDE study, researchers observed a U-shaped curve in the relationship between UIC and the prevalence of hypertension. The lowest point was found at a UIC range of 300-499 μg/L. Two studies conducted in China revealed that adults in IS and IE areas exhibited higher blood pressure levels compared to those in IA area. Meanwhile, iodine deficiency was identified as a risk factor for preeclampsia and hypertensive disease of pregnancy (HPD). However, a randomized case-control study revealed that wakame (Undaria pinnatifida) intake (5 g/day) in brown algae significantly decreased blood pressure in 36 older Japanese individuals with hypertension.
The Connection Between Iodine and Dyslipidemia
The National Health and Nutrition Examination Survey (NHANES) 2007-2012 reported that subjects with the lowest decile of UIC were more likely to be at risk for elevated total cholesterol (TC) (adjusted odds ratio (aOR) = 1.51) and elevated low-density lipoprotein (LDL) cholesterol (aOR = 1.58), compared to those with the highest decile of UIC. US adolescents with low UIC had a significantly higher risk of hypercholesterolemia, elevated non-high-density lipoprotein (HDL), and elevated LDL compared to those with normal UIC. RCTs have reported that iodine supplementation reduces hypercholesterolemia incidence in overweight women and also decreased serum LDL-C levels in overweight Japanese adults. Seaweed supplementation increased HDL-C levels and decreased TG levels in Korean patients with T2DM. A meta-analysis found that brown seaweed intake significantly decreased the levels of TC (mean difference (MD): −3.001; 95% CI: −5.770, −0.232) and LDL-C (MD: −6.519; 95% CI: −12.884, −0.154). However, there is an observational study that proposes the opposite conclusion. Two studies have found that the relationship between iodine status and dyslipidemia is either a U-shaped or inverted U-shaped curve.
Iodine and Hyperuricemia/Gout
So far, the relationship has only been reported in one epidemiological study, which found an inverse association between UIC and the prevalence of hyperuricemia and gout. Subjects in the IS and IE groups had a lower probability of having hyperuricemia and gout in comparison to those in the IA group.
Iodine and Mortality
Longitudinal data indicated an excess mortality in individuals with ID (UIC <100 μg/L) after adjusting for confounding factors. The HRs for all-cause mortality were 1.29 in individuals with UIC of 50-99 μg/L, and 1.71 in individuals with UIC of less than 50 μg/L. Iodine excess did not increase the risk of mortality. However, the NHANES III reported contradictory outcomes. In a median follow-up period lasting 19.2 years, having a UIC higher than 400 μg/L indicated an increased risk (HR = 1.19) for all-cause mortality. There was no observed correlation between low UIC and an increased risk of mortality.
Extrathyroidal Effects of Iodine
Sodium iodide symporter (NIS) is the most effective and specific transporter of I− expressed on thyroid follicular epithelial cells. Many other organs can also actively accumulate iodine, including the salivary glands, stomach, lactating mammary gland, ovary, prostate, and pancreas. In addition to NIS and Pendrin, new iodine transporters, including cystic fibrosis transmembrane conductance regulator (CFTR), anoctamin 1 (ANO1), and sodium multivitamin transporter (SMVT), have recently been discovered to be expressed in these organs. Therefore, iodine itself has extrathyroidal effects on organs that can take it up.
Iodine is a micronutrient that possesses antimicrobial properties. Iodine treatment in obese mice showed a weight-reducing effect and modified the gut microbiota, leading to an increase in pathogenic bacteria and a decrease in beneficial bacteria. Conversely, contrasting response patterns were observed in mice with normal weight. Another study found a significant relationship between the use of vulvar povidone iodine disinfection and the colonization of neonatal oral microbiota. Additionally, the intestinal microbiota also contributed to iodine absorption. There is considerable evidence indicating that iodine has extrathyroidal effects as an antioxidant, especially in breast diseases and certain tumors. Iodide has been found to be highly efficient in scavenging reactive oxygen species (ROS), thus reducing damage caused by free oxygen radicals. In lactating women, the iodine content in breast milk exhibits a negative correlation with the activity of catalase, superoxide dismutase (SOD), and glutathione peroxidase (GSH-Px), as well as adiponectin levels. As an obesity-related hormone, adiponectin also plays a crucial role in regulating insulin sensitivity.
Chronic inflammatory condition paves the way for the development of metabolic disorders. Iodine also has well-known anti-inflammatory and immunomodulatory effects. Both PENDRIN and NIS were expressed on the surface of human leukocytes. The application of sodium iodide (NaI) to leukocytes resulted in a significant rise in the production of both pro- and anti-inflammatory cytokines.
Maintaining Optimal Iodine Levels
In order to eliminate iodine deficiency diseases, most countries have implemented universal salt iodization (USI). However, excess iodine intakes can occur due to the consumption of iodized salt, drinking water, animal milk rich in iodine, specific types of seaweeds, iodine-containing dietary supplements, and from a combination of these sources. Some pharmaceuticals (like Amiodarone), disinfectants, and iodine-containing contrast media, can also be common sources of iodine. Over 90 percent of dietary iodine is absorbed under normal conditions. The recommended dietary intake of iodine is 150-299 μg/day. The appropriate range of median urinary iodine concentration (mUIC) should be 100-299 μg/L. Japanese average iodine consumption is 1,200 μg/day, which is 7.2 times higher than that of the British and 5.7 times higher than that of Americans. Iodine excess can increase the risk of hyperthyroidism and subclinical hypothyroidism. However, euthyroid individuals are usually tolerant to iodine-induced thyroid diseases as their thyroid function rapidly normalizes upon discontinuation of excessive iodine consumption. A moderately high iodine intake has been shown to be beneficial in reducing the incidence of breast and prostate cancer in some epidemiological studies.
While iodine is essential, avoiding consuming it excessively is equally important. Symptoms of iodine excess can include similar signs to deficiency, such as goitre (due to different mechanisms), digestive upset, skin rashes, and potentially more serious thyroid conditions. Therefore, it's vital to stick to recommended daily allowances unless otherwise advised by a healthcare professional.