Zinc, Selenium and Iodine in Thyroid and Immune Health

Trace elements are needed in tiny amounts, measured in milligrams or micrograms a day, yet they sit at the centre of some of the body’s most important processes. Zinc, selenium and iodine are frequently discussed together because they converge on 2 systems of great interest in functional practice: the immune system and the thyroid. They also share a characteristic that practitioners must respect. The range between too little and too much is narrow, and the harms of excess are as real as those of deficiency.

Zinc

What it does

Zinc is a structural or catalytic component of hundreds of enzymes and many transcription factors. It is essential for cell division, immune cell development and function, wound healing, taste and smell, reproductive function and growth. Zinc deficiency impairs both innate and adaptive immunity, increasing susceptibility to infection.

Who is at risk

Zinc deficiency is common in parts of the world where diets are based on cereals high in phytates and low in animal foods. In high income countries, those at risk include people with malabsorption, such as coeliac disease and inflammatory bowel disease, people with chronic diarrhoea, heavy alcohol users, older adults with poor intake, people with chronic kidney disease, and some vegetarians and vegans. Certain medicines, including some diuretics, increase zinc loss.

Assessment

Serum or plasma zinc is the most commonly used marker, but it falls during inflammation, infection and pregnancy and varies with time of day and recent meals, making it an imperfect measure of status. Interpretation requires context, including an inflammatory marker.

Evidence for supplementation

• Childhood diarrhoea: zinc reduces duration and severity in low income settings and is recommended by the World Health Organization alongside oral rehydration.
• Common cold: some trials of zinc lozenges started within a day of symptoms suggest a shorter illness, but results vary with formulation and dose, and side effects such as nausea and altered taste are common.
• Deficiency states: including acrodermatitis enteropathica, a rare genetic condition, and deficiency due to malabsorption.
• Age related macular degeneration: zinc is part of antioxidant formulations shown in a large trial to slow progression in people with intermediate or advanced disease.

The hazard of excess

Long term high dose zinc supplementation can cause copper deficiency, because zinc induces a protein in intestinal cells that binds copper and prevents its absorption. Copper deficiency can cause anaemia, low white cell counts and a serious myelopathy resembling B12 deficiency. Cases have been linked to high dose supplements and to the excessive use of zinc containing denture adhesives. High zinc intake can also impair immune function, the very system it is often taken to support.

Selenium

What it does

Selenium is incorporated into selenoproteins, including glutathione peroxidases, which protect cells against oxidative damage, and the iodothyronine deiodinases, which convert thyroid hormone to its active form. The thyroid contains more selenium per gram than any other tissue, reflecting the need to protect it from the hydrogen peroxide used in hormone synthesis.

Geography matters

Selenium intake depends heavily on the selenium content of soil, which varies widely around the world. Severe deficiency in low selenium regions of China was associated with Keshan disease, a cardiomyopathy, which declined after selenium supplementation programmes. Parts of Europe and New Zealand have relatively low selenium soils, whereas parts of the Americas have high selenium soils. Brazil nuts are extremely rich in selenium, and their content varies greatly.

Selenium and the thyroid

In autoimmune thyroiditis, a systematic review of trials found that selenium supplementation reduced thyroid peroxidase antibody levels in people taking thyroid hormone. Whether this translates into better clinical outcomes, such as reduced progression or improved symptoms, is not established. In mild Graves orbitopathy, a randomised trial found that selenium improved quality of life and slowed progression, and some specialist guidelines recommend a course of selenium in this setting, particularly in areas of low selenium intake.

The hazard of excess

Selenium has one of the narrowest margins between adequate and toxic intake of any nutrient. Chronic excess, selenosis, causes hair and nail loss, skin changes, a garlic odour on the breath and neurological symptoms. In a trial in a population with adequate selenium intake, selenium supplementation was associated with an increased risk of type 2 diabetes, and a large trial of selenium for prostate cancer prevention found no benefit. Supplementation is therefore most justified where intake is low, and should be avoided in people who are already replete.

Iodine

What it does

Iodine is an essential structural component of thyroid hormones. Without sufficient iodine the thyroid cannot make enough hormone, leading to goitre and hypothyroidism. During pregnancy and early childhood, iodine deficiency impairs brain development and is one of the leading preventable causes of intellectual impairment worldwide.

Global picture

Universal salt iodisation has dramatically reduced iodine deficiency, but it remains a problem in some regions, and mild deficiency occurs in some high income countries, particularly among pregnant women. A study in the United Kingdom found that children of mothers with mild iodine deficiency in pregnancy had lower scores on measures of verbal intelligence and reading. Groups at particular risk include vegans who do not use iodised salt, people who avoid dairy and fish, and people who use non iodised speciality salts.

Assessment

Urinary iodine concentration is used to assess the iodine status of populations, but a single spot urine sample is not a reliable measure of an individual’s status because intake varies greatly from day to day. Thyroid function tests and dietary history are more useful in individuals.

The hazard of excess

Excess iodine can cause both hypothyroidism and hyperthyroidism in susceptible people, including those with autoimmune thyroid disease or nodular goitre. Sources of excess include kelp and seaweed supplements, some traditional remedies, iodine containing contrast agents and the antiarrhythmic drug amiodarone. Some seaweeds contain iodine at levels many times the tolerable upper intake in a single serving.

Where they meet

A practical approach

1. Assess risk from diet, geography, medical conditions and medicines before considering supplements.
2. Test where tests are informative and interpret them in context, particularly zinc with inflammation.
3. Correct deficiency at appropriate doses and for defined periods.
4. Check all supplements for overlapping content; zinc, selenium and iodine appear in many multivitamins and thyroid support products.
5. Avoid high dose, long term supplementation without a clear indication and monitoring.
6. Remember that for iodine, iodised salt is often the simplest and safest solution.

An illustrative case

A 42 year old woman with Hashimoto thyroiditis, treated with levothyroxine, has been taking a thyroid support supplement containing iodine and kelp, a selenium supplement, and zinc at a high dose for immunity, after reading about each online. She has also started eating several Brazil nuts a day. Her TSH has become unstable, and she has developed anaemia with a low white cell count.

Her doctor finds a low copper level and a high zinc level. The zinc is stopped and copper status is monitored. The kelp product is stopped, because excess iodine may be destabilising her thyroid, and the selenium is reduced, given her supplement and Brazil nut intake. Her TSH stabilises and her blood counts recover. Every supplement she had taken was aimed at a real physiological role; together they had caused harm.

Precision with small numbers

Zinc, selenium and iodine illustrate a principle that applies to all trace elements: they are essential in small amounts and harmful in excess. The practitioner’s role is to identify the people who need more, supply it carefully, and protect everyone else from well intentioned overdose.