The Thyroid Diet: Is There Such a Thing?

Angela M. Leung, MD, MSc

Disclosures

August 27, 2018

Thyroid disease includes hypothyroidism, hyperthyroidism, and thyroid nodules, and is commonly seen in most primary care and outpatient endocrinology offices. In routine clinical practice, patients will often inquire about dietary changes they can make to treat or reverse their thyroid disease. The evidence is strongest for adequate but not excessive iodine intake to benefit thyroid health in general, as well as selenium supplementation for patients with Graves disease. Aside from these, we have sparse scientific data showing that dietary changes can significantly benefit hypo- or hyperthyroidism.

Here, we will discuss what we know about what some patients might call a "thyroid diet" and cover the most popular nutrition questions we get asked by patients with thyroid disease.

But first, a quick review.

The Most Common Thyroid Diseases

Hypothyroidism and Hashimoto Thyroiditis

Hypothyroidism affects up to 7% of the general population.[1] Although endemic iodine deficiency is the leading cause of hypothyroidism globally, the most common etiology of hypothyroidism in the United States is Hashimoto thyroiditis, an autoimmune condition characterized by positive serum thyroid autoantibody titers and lymphocytic infiltration of the thyroid, which may lead to destruction of thyroid follicular cells, thereby increasing the risk for hypothyroidism.

Levothyroxine as thyroid hormone replacement remains one of the most commonly prescribed medications in the United States.[2] Patients should be counseled that many foods and substances interfere with the intestinal absorption of oral levothyroxine[3] and that taking their medication separately from mealtimes is advised.[4]

The majority of patients with hypothyroidism experience symptomatic improvement upon treatment with thyroid hormone replacement. However, some may continue to report hypothyroid symptoms despite the achievement of biochemical euthyroidism.[5,6] Others are dissatisfied with the current available options and seek alternative therapies.[7,8] As such, patients may seek dietary interventions to ameliorate symptoms or reverse their hypothyroidism.

Popular interventions include supplementation with various micronutrients, vitamins, or minerals, or restricted intake of certain foods or food groups. Given the role of Hashimoto thyroiditis in the development of hypothyroidism, possible intervention to specifically decrease serum thyroid autoantibody titers is a common inquiry. Euthyroid patients with Hashimoto thyroiditis may inquire about the levelroles of iodine and selenium supplementation, as well as the potential risks of ingesting cruciferous vegetables and soy.

Hyperthyroidism

Hyperthyroidism is present in up to 1.3% of the general population.[1] Graves disease is the leading cause of hyperthyroidism worldwide, but other etiologies include toxic nodular goiter and the hyperthyroid phase of thyroiditis. Conventional therapies for the treatment of hyperthyroidism are antithyroid medications, radioactive iodine treatment, and thyroid surgery, each with their respective potential risks and benefits.[9]

Serum thyroid-stimulating antibodies drive the onset and course of Graves disease. Dietary iodine and selenium are micronutrients that can modify these thyroid antibody titers. Additionally, iodine status is an important consideration in individuals with thyroid nodules who are at risk for iodine-induced hyperthyroidism.

Thyroid Nodules

Thyroid nodules are often incidentally detected on radiologic imaging. Criteria based on a combination of nodule size and sonographic characteristics inform the decision to pursue a thyroid nodule fine-needle aspiration biopsy, as there is an overall risk for malignancy of only 7%-15% in all nodules.[10] Thyroid surgery is recommended if malignancy is highly suspected or confirmed, or if benign nodules are large enough to be associated with compressive symptoms to the anterior neck. Biopsy-benign or nonsuspicious nodules are monitored by ultrasound at regular intervals. Given the chronicity of monitoring needed for benign nodules, patients will commonly seek dietary or other modalities to decrease the size of their thyroid nodules.

Specific Dietary Influences on Thyroid Health

Iodine

The production of thyroid hormone within the thyroid follicular cell requires adequate levels of circulating iodide taken in through the diet as either iodide, iodate, or organically-bound iodine. The US Recommended Dietary Allowance (RDA) for iodine is 150 µg/day in adults, and 220 µg/day and 290 µg/day in pregnant and lactating women, respectively.[11] Common dietary sources of iodine include iodized salt, seafood (including seaweed and fish), and some breads and grains.

Iodine is not required to be labeled on food packaging; thus, dietary sources may be difficult to identify. Iodine nutrition in the United States has been adequate, but only marginally so.[12] Although eating a regular diet should meet nutritional iodine needs, some individuals may require supplementation in order to achieve RDA goals. Individuals with restricted diets, such as vegetarians and vegans, are at higher risk for inadequate iodine intake because vegetables are not a rich source of iodine.[13]

Some "iodine for thyroid health" tablets, which are commonly available over the counter, may contain several hundred-fold the daily recommended amount of iodine in just a single dose. Other products labeled "for thyroid support" include tablets or liquid supplements containing spirulina (a superfood derived from blue-green algae) or kelp.

When counseling patients, I stress that taking supplements with such high iodine content is unlikely to help their thyroid health, and in fact may even pose harm. In some individuals, excess iodine exposure or ingestion may induce hyperthyroidism or hypothyroidism,[14] and chronic iodine excess may induce autoimmune thyroiditis, as highly iodinated thyroglobulin is immunogenic.[15] Iodine-induced thyroid dysfunction is more common in those who have a history of endemic iodine deficiency or preexisting thyroid disease.[16] For this reason, the American Thyroid Association recommends avoiding supplements containing >500 µg/day of iodine.[17]

Goitrogens

The term "goitrogen" refers to any substance that can produce goiter, an enlargement of the thyroid gland. This is usually accomplished through effects that decrease thyroidal iodine, but goitrogenic substances can also act by inhibiting any of the other components of normal thyroid hormone production. The most common examples of dietary goitrogens are cruciferous vegetables and soy products.

Cruciferous Vegetables

Cruciferous vegetables are defined as those in the Brassica genus and include broccoli, cabbage, Brussels sprouts, kale, turnips, cauliflower, collard greens, and bok choy. They are rich in glucosinolates, compounds that produce sulforaphane and the phenethyl and indolylic isothiocyanates associated with anticancerous properties.[18] However, glucosinolates also include the metabolite thiocyanate, which inhibits thyroid hormone synthesis.[18] Thus, although eating cruciferous vegetables certainly has health benefits, frequent consumption of large quantities may induce or exacerbate hypothyroidism.

Data on the amount of cruciferous vegetable consumption needed to adversely affect thyroid function are limited. In a study of five euthyroid volunteers who ingested 15.2 oz of a commercial kale juice twice per day for 7 days, mean 6-hour thyroid radioiodine uptake decreased by 2.52% compared with baseline values, and serum thyroid function tests were unchanged.[19] It would be interesting to see the results of a study with longer-term ingestion, perhaps with larger amounts of kale consumed.

In an extreme example, one case report described the development of myxedema coma in an 88-year-old Chinese woman who consumed 1.0-1.5 kg of raw bok choy daily for several months in an attempt to improve her diabetes control.[20]

On a practical level, I tell both euthyroid and hypothyroid patients that although these data suggest that frequent intake of large amounts of cruciferous vegetables may decrease thyroid hormone production, no rigorous clinical studies exist to support the need to stop eating them. I advise a well-balanced diet that includes eating cruciferous vegetables in reasonable amounts. The problem is that we currently have very little evidence of what is "reasonable" in regard to cruciferous vegetable consumption and thyroid health.

Soy

Dietary soy products—including soy milk, tofu, soy sauce, tempeh, and miso—contain isoflavones. Because isoflavones can inhibit the action of thyroid peroxidase, which is required for thyroid hormone synthesis, it has been proposed that dietary soy intake may increase the risk for hypothyroidism in euthyroid individuals or that a higher dose of thyroid hormone replacement may be required in patients being treated for hypothyroidism.

The available literature shows that in euthyroid individuals living in iodine-replete areas, consumption of soy probably has no adverse effects on serum thyroid function.[21] An exception is when soy-based infant formula is used for neonates with congenital hypothyroidism; an increase in dose of levothyroxine may be required to adequately address thyroid hormone needs.[22]

I tell my adult patients that a reasonable, normal amount of soy consumption is generally safe. Furthermore, there is no reason to avoid soy if a patient with known hypothyroidism is being treated with thyroid hormone replacement.

Other Trace Minerals

The important role of iodine in thyroid health is well understood. In contrast, data on other trace minerals and their effects on thyroid status are more inconsistent.[23]

Selenium. Selenium is a micronutrient important for thyroid hormone metabolism. The US RDA of selenium in men and nonpregnant, nonlactating women is 55 µg.[24] The richest dietary sources of selenium are seafood and organ meats. Typical sources in the US diet are breads, grains, meat, poultry, fish, and eggs. Brazil nuts are also rich in selenium, with a single nut providing up to 90 µg.

The tolerable upper intake level for selenium is 400 µg/day.[24] Although selenium toxicity is not commonly encountered in routine clinical practice, symptoms include nausea; nail discoloration, brittleness, and loss; hair loss; fatigue; irritability; and foul breath (often described as "garlic breath").

Some studies have shown benefit from selenium supplementation in individuals with autoimmune thyroid disease, and low selenium levels have been associated with increased risk for goiter and thyroid nodules in European women.[25] In areas of severe selenium deficiency, supplementation up to 100 µg/day may be beneficial.[26] From the available evidence, however, routine selenium supplementation in individuals following unrestricted diets for the purpose of treating Graves disease,[27,28] decreasing serum thyroid antibody titers, or maintaining normal thyroid function is mostly unsupported.

I don't generally recommend selenium supplementation to my patients for the sole purpose of benefiting thyroid health. One exception is patients with mild Graves ophthalmopathy, in whom selenium supplementation can improve quality of life and the course of ocular disease.[29] Supplementation in these patients is recommended by the European Thyroid Association/European Group on Graves' Orbitopathy as a 6-month course.[30]

Zinc, copper, and magnesium. The roles of zinc, copper, and magnesium in thyroid hormone synthesis and metabolism are even less well defined. In the US National Health and Nutrition Examination Survey (2011-2012), levels of zinc, copper, and selenium were inconsistently associated with free or total serum thyroid hormone levels,[31] whereas a meta-analysis of eight studies suggested a relationship between levels of selenium, copper, and magnesium with thyroid cancer.[32] Some patients may ask about these minerals and how they affect thyroid health. Given the available evidence, supplementation of these trace minerals solely for the purpose of promoting thyroid function is not generally supported.

Other Dietary Considerations for Thyroid Health

Finally, a number of other dietary factors that have much less rigorous—and in some cases, absent—scientific data are suggested to affect thyroid health. Coffee, tea, and alcohol appear to have no effect on thyroid cancer risk,[33] although coffee decreases the absorption of oral levothyroxine in individuals being treated for hypothyroidism.[34] The potential benefit of vitamin D as a preventive or therapeutic agent for various thyroid diseases remains unclear.[35]

Popular in the functional medicine community are interventions to treat leaky gut syndrome, the theory of increased intestinal permeability leading to various diseases. Gluten-free diets, sugar-free diets, and probiotics are advocated for promoting thyroid health.

Although one small study demonstrated decreased serum thyroid antibody titers among 34 women who followed a gluten-free diet for 6 months,[36] published data in the scientific literature on the effects of these interventions on thyroid health are lacking. I discuss with my patients that much remains unknown about thyroid disease and that these are areas of uncertainty in modern medicine for which continued research is still needed.

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