Thyroid diet Myths and Facts

Prof. Dr. Robin Maskey is an endocrinologist and internal medicine expert, currently serving as the Head of the Department of Internal Medicine and Chair of the Fellowship in Endocrinology at B.P. Koirala Institute of Health Sciences (BPKIHS), Dharan. A Fellow of the American College of Physicians, the Royal College of Physicians (Edinburgh), and the Indian College of Physicians, Dr. Maskey has over two decades of experience in clinical care, teaching, and research. He has received specialized training in endocrinology from renowned centers in India, Singapore, and the USA. His key interests lie in diabetes education, thyroid disorders, and endocrine emergencies, with a strong commitment to patient-centered care and public health advocacy. Dr. Maskey is also the Chief Editor of the Journal of Diabetes and Endocrine Association of Nepal, President of Diabetes Nepal (Dharan Chapter), and an Executive Board Member of the International Society of Endocrinology.

In 1928 Chesney et al postulated that rabbits fed on cabbages developed goiter due to deficiency of micronutrients and this theory was later on also postulated by various authors in humans subsequently. Thyroid disorders are the most common endocrine disorder affecting 10-15% of the population. The patients of thyroid diseases often ask their physicians about thyroid diet. But there are a lot of myths on thyroid diet often misled by the junk pseudoscientific articles mentioning that certain substances in the diet cause thyroid disorders.
We all are aware that universal salt iodization prevents goiter and other iodine deficiency disorders. But persistence of goiter even after the iodination has made researchers think about the relation of other dietary nutrients in the thyroid physiology. Iodine followed by selenium, iron, vitamin A and zinc are the major micronutrients to help the body to produce enzymes, proper growth and development and protect the thyroid from free radical damage. Hence, dietary intake of all these micronutrients is essential for the functioning of the thyroid gland.
The common thyroid disorders namely (hypothyroidism, hyperthyroidism, thyroid cancer) have a relevant dietary alteration in the clinical course as explained below.
The thyroid disorders may be divided clinically into four common syndromic presentations. They include hypothyroidism, hyperthyroidism, thyroid swelling (diffuse or nodule) and thyroid cancer. Each of these thyroid disorders has a relevant dietary alteration in the clinical course as explained below.

Hypothyroidism
The hypothyroidism occurs after iodine exposure due to thyroid autoantibodies and lymphocytic infiltration in the thyroid gland. The two population based studies done in Denmark from 1997-2016 and 1997 -2017 showed higher incidence of hypothyroidism and thyrotoxicosis after iodine fortification with mild and moderate iodine deficiency. In another Chinese study the safe upper level of iodine intake in euthyroid Chinese adults demonstrated the development of subclinical hypothyroidism in people taking 800 μg of iodine per day. Deficiency was associated with neurological underdevelopment, goiter, and hypothyroidism and excess was associated with both hyperthyroidism and hypothyroidism.

The effect of cruciferous vegetables on the thyroid gland depends on the amount of isothiocyanate and sulforaphane (progoitrins) by the action of the enzyme myrosinase or thioglycosidase (derived from adjacent plant cells) released on mastication which decrease iodine transport into the thyroid follicular cells by competitively inhibiting sodium-iodide symporter, the type of brassica ingested and its glucosinolate contents, cooking method and baseline iodine status of the individual. So well-cooked brassica vegetables can be taken freely in individuals with or without thyroid disorders, but one should be cautious while taking extraordinary amounts of raw brassica.

In routine clinical practice, patients suffering from thyroid diseases often enquire regarding dietary modifications for treatment or for reversal of their disease from their health care providers. There are a lot of myths surrounding this topic and people (including physicians) are often misled by the junk pseudoscientific articles available online and in various sensational “health magazines”.

Myth 1
“People with Thyroid Disorders Should Avoid All Cruciferous Vegetables”

Fact
– Cruciferous vegetables (broccoli, cauliflower, cabbage, kale, Brussels sprouts) contain glucosinolates and can interfere with thyroid function, but should be in very high amounts.
– For most people with healthy thyroid function, the thyroid gland has mechanisms to deal with goitrogens, if iodine levels are adequate.
– Cooking these vegetables removes thiocyanate, isothiocyanate, which have goitrogenic properties and make them safer to eat in typical portions.
– The bottom line: If you have a healthy thyroid and normal iodine levels, goitrogenic foods are not a concern. However, if you have iodine deficiency or an existing thyroid condition, you may need to limit their intake, especially raw.

Myth 2
“Iodine supplements can cure hypothyroidism”.

Fact
• Iodine is crucial for the production of thyroid hormones, but iodine deficiency is rare in developed countries due to iodized salt and iodine-rich foods (like sea food and dairy).
• Most cases of hypothyroidism are due to autoimmune conditions like Hashimoto’s thyroiditis, not iodine deficiency.
• Taking iodine supplement when there is adequate iodine can actually worsen thyroid problems , particularly in autoimmune thyroid disease.
• Excessive iodine can disrupt thyroid function and may trigger an autoimmune response in some people (Wolf Chaikof phenomenon).
• The bottom line: Iodine supplementation should only be considered if a deficiency is diagnosed, and it should be done under medical supervision.

Myth 3
“Gluten-Free Diet Cures Hypothyroidism”

Fact
– There is no scientific evidence that a gluten-free diet cures hypothyroidism.
– People with Hashimoto’s (an autoimmune thyroid disease) who are also gluten-sensitive or have celiac disease may benefit from a gluten-free diet to reduce inflammation.

Hyperthyroidism
Dietary alterations affect the thyroid function leading to hyperthyroidism due to excess iodine consumption. A study suggests that low iodine diet should be taken two weeks prior to technetium scan in patients with multinodular goiter and seafood consumers.

Thyroid Cancer
Thyroid nodules and cancer occur due to different nutritional environmental factors and endocrine disruptors. Thiocyanate in the tobacco smoke inhibits iodine uptake by competitive inhibition and is the mediator of the goitrogenic effect.
Dietary nitrite causes thyroid cancer by competitively inhibiting iodide uptake thereby decreasing thyroid hormone synthesis leading to thyrotropinemia and thyroid hyperplasia with a potential to transform into thyroid cancer. An increased risk of thyroid cancer has also been seen with use of of cruciferous vegetables (thiocyanate consumption), butter, cheese, starchy food, pasta, meat and poultry products and a decreased risk with non-cruciferous vegetables (especially carrots, green salad and citrus fruits), iodized salt, ham and fish.

Endocrine Disruptors
One of the most common endocrine disruptors that may impair thyroid function is Bisphenol A (BPA) which is an organic chemical compound that belongs to the group of phenols used in the production of plastics, seafood, canned fish, and canned vegetables including polycarbonates and epoxy resins. BPA can affect thyroid function by impairing pituitary and thyroid gene expression, inducing cellular toxicity and have an antagonistic effect on thyroid receptors.18

Myth 4
“Soy Foods Harm Thyroid Function”

Fact
– Soya contains phytoestrogens and goitrogens, which in theory could affect thyroid function by interfering with iodine uptake but they do so when consumed in large amounts only.
– The primary concern comes with levothyroxine. Soy has been shown to potentially interfere with the absorption of levothyroxine if consumed in large amounts or at the same time as medication.
– The bottom line: No need to avoid soy entirely, but if you are on thyroid medication, it’s best to space out soy consumption and medication by at least 4 hours.

Soy protein and thyroid
Soy-containing foods and supplements are widely consumed for possible health benefits that include prevention of cancer, dyslipidemia, cardiovascular disease and osteoporosis. Genistein is the major soy isoflavone which has estrogenic and goitrogenic activity that inhibits the activity of thyroid peroxidase in a dose dependent manner.
The current consensus has mentioned hypothyroidism patients can safely consume soy-based foods unless they are already deficient in iodine but a case report has highlighted that the ingestion of soy proteins in close temporal proximity to levothyroxine can interfere with the absorption of the latter.

Recent concept of Thyroid Diet
The concept of symbiosis and its role in the pathogenesis of autoimmune thyroid disease is thought to disrupt the intestinal epithelial barrier, increase intestinal permeability and, therefore, increase the translocation of bacteria and bacterial products across the gut. 21 This translocation is referred to as a “leaky gut”. Through small studies, an association has been suggested between “leaky gut” and autoimmune thyroid diseases, both Graves’ disease and Hashimoto’s thyroiditis.

Myth 5:
“You should avoid dairy if you have thyroid disease.”