The Impact of Iron and Selenium Deficiencies on Iodine and Thyroid Metabolism

Several minerals and trace elements are essential for normal thyroid hormone metabolism, e.g., iodine, iron, selenium, and zinc. Coexisting deficiencies of these elements can impair thyroid function. Iron deficiency impairs thyroid hormone synthesis by reducing activity of heme-dependent thyroid peroxidase. Iron-deficiency anemia blunts and iron supplementation improves the efficacy of iodine supplementation. Combined selenium and iodine deficiency leads to myxedematous cretinism.

The normal thyroid gland retains high selenium concentrations even under conditions of inadequate selenium supply and expresses many of the known selenocysteine-containing proteins. Among these selenoproteins are the glutathione peroxidase, deiodinase, and thioredoxine reductase families of enzymes. Adequate selenium nutrition supports efficient thyroid hormone synthesis and metabolism and protects the thyroid gland from damage by excessive iodide exposure. In regions of combined severe iodine and selenium deficiency, normalization of iodine supply is mandatory before initiation of selenium supplementation in order to prevent hypothyroidism. Selenium deficiency and disturbed thyroid hormone economy may develop under conditions of special dietary regimens such as long-term total parenteral nutrition, phenylketonuria diet, cystic fibrosis, or may be the result of imbalanced nutrition in children, elderly people, or sick patients.

Selenium and Iodine

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The Interaction of Selenium and Iodine

Selenium and iodine interact with each other in one key area: thyroid hormone. Because this hormone plays a key role in metabolism and development, it is important to have an optimal level of it. Let’s find out more about how the two minerals impact thyroid health.

Thyroid Health

The thyroid contains one of the higher concentrations of selenium per gram than other tissues, making sufficient levels important for thyroid health. This is likely due to several selenoproteins being involved in thyroid health, including glutathione peroxidase, thioredoxin reductases, and the three deiodinase enzymes.

On a physiological level, it makes sense why these two minerals are essential for thyroid health. Thyroid hormone is made of a compound of iodine and tyrosyl residues in thyroglobulin, which are coupled together to create T3 and T4. T4 is converted into the active form, T3, when the deiodinase enzyme, which is a selenoprotein, removes one of the iodine compounds. Type I and II deiodinase enzymes are stimulated by TSH to convert more of the active form of the thyroid hormone. The other selenoproteins help to protect against the generation of H2O2, which in excessive levels can lead to oxidative damage.

Therefore, for a healthy functioning thyroid, you must have sufficient levels of both of these minerals. With both of these minerals, it is key to keep the right balance; too much can be just as disastrous as too little. Both situations can contribute to dysfunction in thyroid hormone production, as well as thyroid autoimmune disorders. Additionally, there must be a balance between the two minerals. An excess of one might contribute to or exacerbate a deficiency in the other.

The recommended daily value and tolerable upper limit, both of which are listed above, are helpful guides to plan your optimal consumption of both minerals. You can measure your individual status of iodine and selenium using a urine test, and you can measure your selenium status through a blood test, to determine if you are at risk of a deficiency or excessive intake. One study found evidence to support the WHO’s recommendation for urinary iodine between 100 to 200 mug/l for thyroid health. Another study found a reduced risk for thyroid disease with serum selenium levels between 69 and 90.99 ug/L compared to the bottom and top quintiles. Discuss your test results and any symptoms you might have with your doctor or health practitioner to determine if you require a different intake level to meet your individual needs.

Some studies have found that supplementing with selenium can help with and might even reverse thyroid problems, including autoimmune thyroid. In one study, researchers looked at the effect of supplementing with 83 mcg selenomethionine per day for a period of four months in 192 participants, of which there were 96 supplemented and 96 control patients. Out of all participants, 33 or 17.2 percent restored normal thyroid activity. Those who were supplemented had a higher success rate, with 31.3 percent of treated patients restoring euthyroid compared to just 3.1 percent.

In another study reviewing patients with goiter in Pakistan, researchers reviewed the impact of selenium status and its impact on iodine deficiency. Compared to the controls, both males and females with goiter had a lower serum level of selenium and a higher urinary selenium. These patients also had lower levels of iodine compared to age-matched controls. The researchers concluded that the selenium levels might help to determine the severity of the hypothyroidism experienced by those deficient in iodine, but that more research was needed.

In a study on rats reviewing the impact of independent and simultaneous zinc, selenium, iodine deficiency on thyroid activity, the serums levels of both total T4 and free T4 were significantly lower and the TSH was higher in all groups that were iodine deficient. Serum T3 levels were lower in selenium and iodine, selenium and zinc, and zinc only deficient groups. Additionally, the groups that were deficient in just selenium or selenium and zinc had lower levels of thyroid glutathione peroxidase activity. However, when there was also a concurrent iodine deficiency, this enzyme had a greater activity than the controls.

Healthy Development

Sufficient levels of both iodine and selenium are essential for the healthy development of fetuses and young children, in part due to their importance to the creation of thyroid hormone. The thyroid hormone plays a key role in brain development in the womb. In the initial trimester, the fetus relies on thyroid hormone from the mother. Although the fetus starts to create its own hormone in the second trimester, even at birth, roughly 20 to 40 percent of T4 in cord blood is from the mother.

In one study reviewing data from the Generation R Study, the researchers compared the level of maternal thyroid hormones and the risk of verbal and non-verbal cognitive function and executive function in the children. The researchers did not find a connection between TSH and free T4 with any impact on language function, except for high levels of free T4 did predict a lower risk of having a delay in expressive language at 2.5 years old. However, having mild hypothyroxinaemia (a condition where TSH levels are within range but free T4 levels are low) prior to 18 weeks of gestation was correlated with having expressive language delay at 1.5 and 2.5 years of age (an OR risk of 1.44). Severe hypothyroxinaemia had an even greater risk, with an OR of 1.8. There was also a higher risk of delay in nonverbal cognitive function.  

Maternal hypothyroxinemia might also contribute to the development of ADHD in the child. In one population-based cohort study, the researchers found that children born to mothers with hypothyroxinemia had higher ADHD scores at 8 years old compared to children not exposed to it, even after adjusting for confounders. There was no association with subclinical hypothyroxinemia.  

Because of the importance of iodine to thyroid hormone production, severe iodine deficiency can cause developmental problems in the fetus, especially to the brain, and contributes to brain damage and cretinism. Even mild iodine deficiency might contribute to some cognitive dysfunction. In a longitudinal follow-up of an Australian cohort at 9 years old, the children who were born to mothers who had mild iodine deficiency (urinary levels under 150 ug/L), performed worse in spelling, grammar, and English-literacy compared to those who were not. There was a 10 percent reduction in spelling performance, 7.6 percent reduction in grammar, and 5.7 percent reduction in English literacy. Even after adjusting for certain other factors, the association remained. The cohort included children born prior to the implementation of an iodine fortification, so they experienced iodine insufficiency during gestation but not during early childhood. 

Although most studies pointing to this issue look at iodine deficiency, some studies have started to also look at the impact of selenium levels in the neurodevelopment of children. Sufficient selenium levels during pregnancy are also important to proper development of the child. In one prospective cohort study, there was a positive association between the level of selenium during the first trimester of pregnancy and the motor skills and language skills of the child at one year old. Another study also found a positive association between maternal levels of selenium and the development in their children at 1.5 years old. This was most evident in language comprehension and psychomotor development, especially in girls. 

How to Get Sufficient Levels of Selenium and Iodine 

Food Sources of Selenium 

Food sources are always a great place to start to ensure you have sufficient levels of selenium and iodine. There are several selenium-rich foods, especially if they come from soils that are rich in selenium. The richest sources of selenium from greatest to least include:  

– Brazil nuts (2,549.6 ug for 1 cup of whole Brazil nuts)
– Turkey breast
– Eggs
– Rockfish
– Tuna
– Sunflower seeds
– Sardines
– Mollusks 

Food Sources of Iodine:  

Although many people only think about iodized salt for their iodine, you can find it in other food sources. Fish and dairy products are some of the richest sources of iodine. For those who are vegetarian or vegan, or otherwise avoid these foods, seaweed is also a rich source of iodine, with some varieties having almost 2,000 percent of the daily recommended value. Fruits and vegetables also have some iodine, especially if they are grown in areas that have iodine-rich soil. If you do rely on iodized table salt, it is important to be careful that the salt levels do not increase your sodium levels too much.  

Supplements: What You Need to Know 

Supplements can be a great source of selenium and iodine if you are at risk of a deficiency and cannot consume sufficient levels through your food, especially if you live in an area deficient of these minerals in the soil. As discussed above, if you do choose to supplement, care should be taken to not end up with excessive amounts of selenium or iodine. With both, some of the same problems that occur with deficiency occur with excess levels. When determining whether you need to supplement with iodine, do not forget to accurately calculate how much you get through your salt if you choose iodized salt. 

Pregnant women, and those who plan to become pregnant, should discuss with their doctor whether they are at risk of an iodine and/or selenium deficiency, as well as a thyroid problem, to reduce the risk of problems with cognitive development.  

Selenium plays many key roles in the body, but one key role is its synergistic relationship with iodine to ensure thyroid health. If you live in an area with low levels of either mineral in the soil, or you are otherwise at risk of a deficiency, then talk with your doctor, nutritionist, or another medical professional about ways to increase your consumption and/or supplement so you benefit from the health benefits discussed above.