Nutrition and the thyroid gland

Patients often ask about a “diet for the thyroid”:

• What changes can they make to their diet?
• Which dietary supplements can they take?
• What should they avoid because it is considered toxic to the thyroid?

They ask all of this in the hope of treating or reversing a thyroid condition. Endocrinologists are also often asked questions such as:

• How can I improve my hypothyroidism symptoms beyond conventional medical treatment?
• How can I lower my thyroid antibody levels?
• How much of a particular food or dietary supplement might be too much for my thyroid?

So below I will go through the most popular nutrition questions relating to how the thyroid gland works.

Iodine

To make thyroid hormones, the body needs adequate levels of iodine in the bloodstream, which we take in from food, water or supplements. Iodine deficiency puts a person at risk of developing hypothyroidism, or of their hypothyroidism getting worse.

The Institute of Medicine of the US National Academies recommends 150 µg of iodine daily for adults, and 220 µg and 290 µg respectively for pregnant and breastfeeding women.

Dietary sources of iodine include iodised salt, fish and seafood, as well as seeds and grains.

It is important to note that sea salt, despite its name, does not contain iodine unless it has been artificially fortified with iodine.

Measuring iodine in the urine or blood reflects iodine intake over the last few days only, so it is not easy to work out whether someone’s iodine intake is adequate.

For people whose iodine intake is low (for example, because they avoid seafood or live in areas where the soil is low in iodine), a dietary supplement of 150 µg a day is recommended, especially for women of childbearing age.

Take care with supplements labelled “for a healthy thyroid” or “for thyroid support” that do not state how much iodine they contain, because a single tablet may contain as much as a hundred times the recommended daily dose.

Taking too much iodine can suppress thyroid function (temporary hypothyroidism), or do the opposite, making the thyroid produce large amounts of thyroid hormones (temporary hyperthyroidism).

There is also evidence that a chronically excessive iodine intake can trigger autoimmune thyroiditis, because iodinated thyroglobulin is immunogenic. For these reasons, the American Thyroid Association recommends avoiding supplements containing more than 500 µg of iodine per day.

Goitrogens

The term “goitrogens” refers to any substance that can make the thyroid enlarge (goitre). This includes any substance that can reduce the iodine available for thyroid function, or that blocks any substance involved in normal thyroid hormone production. The most common examples of goitrogenic foods are cruciferous vegetables and soya products.

Cruciferous vegetables

These include broccoli, cabbage, cauliflower, Brussels sprouts, turnips, kale, collard greens and bok choy (pak choi). They are rich in glucosinolates and other substances that interfere with various stages of thyroid hormone production.

Eating these vegetables in normal amounts is certainly good for your health, but eating too much of them over a long period can lead to (or worsen) hypothyroidism. There is very little evidence on how much of these vegetables it takes to suppress thyroid function and cause a goitre.

An extreme example is the case of an 88-year-old Chinese woman with diabetes who went into myxoedema coma (caused by extreme hypothyroidism) after eating 1–1.5 kg of bok choy a day for several months in an attempt to control her diabetes.

A recent randomised study in volunteers with normal thyroid function, who were given broccoli juice for 12 weeks, found that neither their thyroid function nor their antibody levels were affected compared with the control group.

So, although older evidence led to the assumption that frequently eating large amounts of cruciferous vegetables affects thyroid function, the current view is that there is no need at all to stop eating these healthy foods, and there is no recommended upper limit on how much you eat.

Soya

Soya products include soya milk, soy sauce, miso, tempeh and tofu. They contain isoflavones, polyphenolic compounds that are classed as phyto-oestrogens because of their mild, oestrogen-like action.

Isoflavones inhibit thyroid peroxidase, which is involved in making thyroid hormones, and so it has been suggested that eating soya may carry a risk of hypothyroidism. It has also been observed that people with hypothyroidism who consume large amounts of soya need a slightly higher dose of levothyroxine. Newborn babies with congenital hypothyroidism who are fed soya formula also need a small increase in their dose.

In short, eating soya in normal amounts is appropriate, and for people who are already being treated for hypothyroidism it causes no problem at all, because their dose is adjusted to their needs.

Trace elements

Selenium

Selenium is an important element for the metabolism of thyroid hormones. The recommended daily intake of selenium is 55 µg. The foods richest in selenium are fish and seafood, as well as offal. Bread, cereals, meat, poultry and eggs are also typical sources. The upper limit for selenium intake is 400 µg a day.

Selenium toxicity is very rare in everyday medical practice. The symptoms are: nausea, discoloured and brittle nails, hair loss, tiredness, irritability and bad breath.

Most of the evidence on the link between selenium and thyroid disease concerns autoimmune thyroiditis, where selenium supplements appear to have beneficial effects. Low selenium levels have been linked to a higher rate of goitre and nodules. Although selenium intake appears to be associated with a fall in antibody levels, it has not been shown to offer any long-term benefit to the outcome of pregnancy in pregnant women who are antibody-positive.

The recommendation to take selenium mainly concerns people with thyroid eye disease. The European Thyroid Association and the European Group on Graves’ Orbitopathy (EUGOGO) recommend 200 µg daily for at least 6 months for these patients.

Zinc, copper and magnesium

Their role in the production and metabolism of thyroid hormones has not yet been clarified. Zinc, copper and selenium levels have been linked, though not clearly, with thyroid hormone levels in the blood, while a deficiency of these elements, as well as of magnesium, has been linked with the development of thyroid cancer. More evidence is needed before taking them as supplements can be recommended.

Fluoride

Fluoride is found in our environment, and we also take it in artificially from fluoridated water (to prevent tooth decay). It is also found in tea, processed foods, dental products, supplements and insecticides. Animal studies have found that a raised blood fluoride level is associated with hypothyroidism, but this is not certain in humans. The level of fluoride intake above which thyroid health is harmed is not known.

Gluten, diets and the thyroid

Because dietary approaches such as gluten-free or sugar-free diets, combined with taking probiotics, are very popular, it is worth looking at the scientific evidence on how they affect thyroid health.

Gluten-free diets

Gluten intolerance (coeliac disease) is an autoimmune disease of the digestive system, just as Hashimoto’s thyroiditis is an autoimmune disease. It has been observed that people with coeliac disease are three times as likely to develop autoimmune thyroiditis as the general population.

This is because, in general, when someone has one autoimmune disease, they are more prone to developing a second one. It is not that one disease affects the other; rather, the same autoimmune mechanism that causes one autoimmune disease can more easily cause a second one too. This link raises the following questions:

• How does a gluten-free diet affect thyroid autoimmunity?
• If patients with coeliac disease avoid gluten, are they less likely to develop Hashimoto’s thyroiditis?

A large study showed that cutting out gluten for one year in patients with coeliac disease did not change their thyroid hormone levels, their antibodies or their ultrasound findings. At present there is no evidence that avoiding gluten in people who do not have coeliac disease brings any benefit to thyroid function, or to their health more generally.

Personally, however, I believe that by not eating gluten we cut out many high-calorie foods and replace them with others, such as pulses, quinoa and so on, which contain plenty of trace elements that protect the thyroid.

Other dietary habits

Neither a sugar-free diet nor the use of probiotics appears to affect the thyroid. Interestingly, though, patients with Hashimoto’s tend to eat more animal fat and processed meat (cold meats), while people who did not develop Hashimoto’s eat more red meat, wholegrain cereals and vegetable oils.

Coffee, tea and alcohol do not affect the thyroid, but they make it harder for people being treated for hypothyroidism to absorb their levothyroxine.

Conclusions

The relationship between diet and thyroid disease is particularly complex, and it should be discussed with your endocrinologist, taking into account your specific condition and your eating habits. In summary, the recommendations based on strong evidence are as follows:

1. 150 µg of iodine a day is recommended for people who are not adequately nourished, for pregnant women, for women planning a pregnancy and for breastfeeding women.
2. Don’t avoid cruciferous vegetables, such as broccoli, cabbage and cauliflower, or soya, as long as you eat them in reasonable amounts and at a reasonable frequency.
3. In thyroid eye disease, selenium has beneficial effects.
4. As for other dietary trends and approaches (gluten-free diets, etc.), there is no evidence that they offer any benefit to the thyroid.