|Year : 2014 | Volume
| Issue : 2 | Page : 60-65
Diet and thyroid - myths and facts
Ruchita Sharma1, Shantanu Bharti2, K. V. S. Hari Kumar3
1 Department of Medicine, Command Hospital, Lucknow, Uttar Pradesh, India
2 Department of Psychiatry, Era Medical College, Lucknow, Uttar Pradesh, India
3 Department of Endocrinology, Command Hospital, Lucknow, Uttar Pradesh, India
|Date of Web Publication||6-May-2014|
K. V. S. Hari Kumar
Department of Endocrinology, Command Hospital, Chandimandir 134 107, Haryana
Source of Support: None, Conflict of Interest: None
Thyroid gland is located in the neck and synthesize thyroid hormones, which have an indispensable role in the metabolic functions of the body. Iodine is an essential compound for the synthesis of thyroid hormones and is mostly derived from the environment. Other important nutrients for the thyroid function include selenium, iron, Zinc and vitamin A. Dietary alterations of the micronutrients lead to structural and functional alterations in thyroid function. The alternative medical practitioners and nutritionists advocate plenty of dietary modifications without a scientific rationale. In this article, we review the role of dietary micronutrients in thyroid physiology and dispel few myths surrounding the same topic.
Keywords: Diet, goiter, hyperthyroidism, hypothyroidism, thyroid
|How to cite this article:|
Sharma R, Bharti S, Kumar KH. Diet and thyroid - myths and facts. J Med Nutr Nutraceut 2014;3:60-5
| Introduction|| |
Thyroid gland is an important endocrine gland located in the neck and is the site for production of thyroid hormones. The synthesis of thyroid hormones is under the hypothalamus-pituitary control similar to other hormonal axes. Iodine is a major substrate for the synthesis of thyroid hormones and is derived mostly from the environment.  The relation of iodine and goiter is known for many centuries leading to the universal salt iodination program to limit the detrimental effects of iodine deficiency.  Previous observations also suggest the presence of goitrogenic substances in the diet, identifying an important role of dietary constituents in thyroidology.  Thyroid disorders are very common in the society, affecting about 10-15% of the population. Universal salt iodization helped in the prevention of goiter and other iodine deficiency disorders. The persistence of goiter even after the iodinization leads researchers to explore the relation of other dietary nutrients in the thyroid physiology.  The micronutrients other than iodine, which play important role in thyroid hormone synthesis are selenium, iron and zinc.  The alterations in the dietary components may lead to thyroid dysfunction in predisposed individuals. In this article, we shall review the relation between diet and thyroid and try to dispel certain myths on this subject.
Thyroid physiology and dietary constituents
The hormones secreted from the thyroid gland are thyroxine (T4) and tryiodothyronine (T3). T3 is the active hormone and T4 acts as the prohormone. Only 20% of T3 is secreted from the thyroid and rest 80% comes from T4 deiodination in peripheral organs like liver and kidney.  Thyroid hormone regulates metabolic rate, body temperature and tissue growth. The thyroid gland is made of follicles, which are surrounded by a single layer of epithelial cell called the follicular cell. Follicle absorbs iodine and stores it in thyrolobulin a glycoprotein synthesized within thyroid cells. The thyroid hormone synthesis can be summarized in the four steps. Firstly, active uptake of circulating iodide to cytosol by sodium iodide symporter, secondly, oxidation of iodide by thyroid peroxidase and iodination of tyrosine residues on thyroglobulin molecule, creating mono and diiodotyrosines, thirdly, coupling of iodinated tyrosine residues to form T3 and T4 and forthly the step of proteolysis due to hydrolysis and releasing the T3 and T4 in cytosol. Iodine remains the major nutrient required for the synthesis of thyroid hormones. The second important micronutrient is selenium, which forms selenocysteine compounds and protects the thyroid from free radical damage.  Research has shown that iron, vitamin A and zinc also plays an important part of thyroid hormone synthesis.  Hence, dietary intake of all these micronutrients is essential for the functioning of the thyroid gland.
Iodine and thyroid
Iodine is essential for the iodination of tyrosine residues leading to the formation of thyroid hormones. A detailed discussion on this topic is beyond the purview of this article and readers are requested to peruse the recently published reviews. , Environmental iodine is the main source of iodine, that is present in soil, sea water, dairy products, seafood and eggs. Iodized salt and multivitamin tablets containing iodine are the other sources of iodine. Recommended dietary allowance for iodine for adult men and women is 150 ug and for pregnant and lactating women is 220 ug and 290 ug respectively. Urinary iodine level is the best measure to assess the iodine level in the body and iodine deficiency is diagnosed when the median iodine concentration is less than 50 ug/ml in population.  Iodine supplementation in the form of iodized salt and iodized oil has proven dramatic improvement in many developing countries. Certain population like pure vegetarian may not reach an adequate iodine intake even in countries considered iodine sufficient. A reduction in iodine intake can also be related to reduced salt intake for hypertension. Depending on availability of iodine, the thyroid gland is able to modify the use of iodine for thyroid hormone production. The failure of compensation in severely iodine deficient population results in hypothyroidism and developmental brain damage. In mild iodine deficiency, thyroid gland adapts by hyperfunctioning and prolonged hyperactivity leads to autonomous thyroid function.
Both the excess or deficient intake of iodine is associated with thyroid disorders in the population.  Iodine deficiency can lead to a variety of medical problems at all ages in the humans. Children of mothers having an iodine deficiency during pregnancy may have mental retardation, deaf mutism, spasticity and short stature. Congenital hypothyroidism due to iodine deficiency is the most common cause of preventable mental retardation in the world.  Other disorders include goiter, hypothyroidism and myalgia. Iodine deficiency in pregnant women is associated with miscarriage, stillbirth, preterm labor and congenital disorders in babies. The consumption of excess iodine is also associated with various thyroid disorders like hyperthyroidism, autoimmune thyroid disease and nodule formation. The safe upper limit of iodine intake is around 1.1 mg per day. One teaspoon of iodized salt contains 284 micrograms of iodine and a single gram of seaweed contains around 2 mg of iodine.
Selenium and thyroid
The thyroid gland is among the human tissues with the highest selenium content similar to other endocrine organs. Selenium is an integral part of selenoproteins and critical enzymes in thyroid hormone synthesis making this an essential micronutrient.  Selenoproteins (glutathione peroxidase and thioredoxin reductase) are responsible for the cellular antioxidative defense and protects the thyroid gland from damage due to hydrogen peroxide and reactive oxygen species. The key enzymes involved in the activation and inactivation of thyroid hormones (iodothyronine deiodinases) are also selenoproteins. Selenium deficiency exacerbates iodine deficiency and they both in combination contribute to the pathogenesis of hypothyroidism, goiter, autoimmune thyroid disease and myxedematous cretinism.  Selenium is found mainly in the soil and the consumption of balanced diet gives adequate selenium for thyroid hormone synthesis. The selenium rich foods in the diet are mushrooms, garlic, onions, eggs, beef liver, shellfish, wheat germ, sunflower seeds and sesame seeds. Selenium deficiency is seen in patients with poor gastrointestinal absorption, long term parenteral nutrition therapy and people residing in areas where the soil content is very low. Replacement of selenium is essential in these patients to prevent thyroid dysfunction. In patients with combined selenium and iodine deficiency, replacement of selenium alone worsens the condition further due to the peripheral conversion of T4 to T3 leading to a marked reduction in T4. Hence, in regions of combined iodine and selenium deficiency, iodine supplementation is mandatory prior to the initiation of selenium supplementation.
Iron and thyroid
Several minerals and trace elements including iron are essential for normal thyroid hormone metabolism. The thyroid function is impaired when the deficiency of these elements coexists in the body. Iron deficiency impairs the synthesis of thyroid hormones by reducing the activity of heme-dependent thyroid peroxidase. Population studies have revealed that nonanemic children responded swiftly to iodine supplementation with regard to goiter and TSH levels than anemic children.  Globally, iron and iodine deficiency coexist and the authorities have developed a double fortified salt (iodine and iron) to tackle the problem in an effective manner. 
Thyroid dysfunction and dietary alterations
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 and summarized in [Figure 1].
Hypothyroidism and goitre
Iodine deficiency is the major cause for thyroid deficiency as explained in the previous sections. Previous reports suggest that a vegetarian diet was associated with reduced risk of hypothyroidism than omnivorous diets in population studies with a follow up of 6 years.  The presence of goitrogenic substances in the diet is responsible for the persistent goiter in post iodization phase. The goitrogenic substances in the normal human diet are the vegetables from the Brassica genus (Cauliflower, Cabbage, Broccoli, Horseradish, Radish, Rapeseed and Turnips), cassava, which contains cyanogenic glucosides, flavonoids and organic residues in the soil.  Cabbage and cauliflower have high concentrations of glucosinolates, bamboo shoot and cassava were rich in cyanogenic glucosides and other vegetables like mustard, turnip and radish are rich in thiocyanate content. All the goitrogenic substances affect the thyroid hormone synthesis by inhibiting the iodine uptake and the activity of thyroid peroxidase.
Excess iodine in the diet may lead to transient hypothyroidism, known as Wolff-Chaikoff's effect.  This autoregulation of the thyroid hormone synthesis is done by reducing the organification of iodine and decreasing the uptake. This effect is short lived and thyroid gland usually escapes from this phenomenon. Failure to escape from the detrimental effects of iodine excess results in hypothyroidism and goiter. The predisposing factors for iodine induced hypothyroidism are extremes of age, autoimmune thyroid disease and past history of treated Graves' disease. Previous reports suggest that modification of dietary constituents resulted in the normalization of (TSH) thyroid-stimulating hormone in children with subclinical hypothyroidism. 
Graves' disease and hyperthyroidism
Dietary alterations affect the thyroid function and may lead to hyperthyroidism due to excess iodine consumption. The other important management issue is consumption of iodine free diet prior to the nuclear imaging and thyroid radio ablation. Jod-Basedow phenomenon describes the occurrence of hyperthyroidism following the excess consumption of iodine.  This phenomenon is seen in patients of endemic goiter, people with multinodular goiter, Grave's disease and thyroid adenoma. This effect is typically opposite of Wolff-Chaikoff's effect and is not described in normal thyroid glands. The small increase of iodine intake in patients with a thyroid disorders with no pituitary control leads to Jod-Basedow phenomenon.
The management of differentiated thyroid carcinoma includes total thyroidectomy followed by ablation of thyroid remnant. Remnant ablation is preceded by thyroid hormone withdrawal or by therapy with recombinant human (TSH) thyroid-stimulating hormone prior to dosing the patient. Another important strategy to increase the radioiodide uptake is to create an iodine deplete stage by giving iodine free diet. The low plasma iodide concentration results in the increased expression of the sodium iodide symporter and increases the subsequent uptake of radioiodide by the thyroid gland.  The dietary measures to achieve a low iodine diet are given in [Table 1]. The beneficial effect of a low iodine diet is well established prior to iodine scan, but the same is not studied much before pertechnate scan. A study published recently suggests that low iodine diet for two weeks prior to technetium scan is beneficial in patients with multinodular goiter and seafood consumers. 
Thyroid nodule is a common disorder in the clinical practice and different nutritional and environmental factors are responsible for the etiopathogenesis of the nodule. Iodine deficiency and smoking increase the risk, whereas alcohol consumption and use of oral contraceptives are associated with reduced risk of goiter and the thyroid nodule.  Thiocyanate in the tobacco smoke inhibits iodine uptake by competitive inhibition and is the mediator of the goitrogenic effect.
The incidence of thyroid cancer has been increasing over the past few decades for unexplained reasons. The alterations in dietary constituents and endocrine disruptors have been proposed as the contributing factors in the etiopathogenesis of thyroid cancer. Population studies exploring the link between dietary patterns and thyroid cancer revealed interesting findings. Dietary nitrite is an established risk factor for the thyroid cancer especially in males. Nitrate competitively inhibits iodide uptake, reduces the thyroid hormone synthesis leading to thyrotropinemia and thyroid hyperplasia with a potential to transform into thyroid cancer.  Nitrates also serve as the precursors for the formation of nitrosamines and nitrosamides which are potent carcinogenic substances. The highest concentration of nitrates is seen in leafy vegetables (spinach, lettuce) and beet-root. An increased risk of thyroid cancer was described with the consumption 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. ,,
The topic of thyroid and diet is incomplete without mentioning about the soy proteins, perchlorate and the influence of maternal diet on the thyroid physiology. Numerous research papers have shown that the constituents of maternal diet during pregnancy have a profound influence on the fetal thyroid system leading to obesity during adulthood. Obesity and thyroid disease are interlinked with important physiological changes in the hypothalamo pituitary axis during weight gain. The topic of thyroid and obesity is the genesis behind a lot of myths associated with the thyroid diet. The print literature and internet has lead to lots of confusion in the minds of thyroid patients about their diet. The prevalent myths and relevant scientific facts are given in [Table 2].
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.  The antithyroid effects of genistein are exaggerated by the iodine deficiency. Genistein inhibits the activity of thyroid peroxidase in a dose dependent manner. The effects of soy isoflavones on the thyroid are dependent on the underlying iodine status and thyroid function. A metanalysis on the subject concludes that soy isoflavones have no adverse effects in euthyroid, iodine-replete individuals. There exists little evidence that soy foods may increase the thyroxine dose in hypothyroidism by inhibiting the absorption of levothyroxine. The hypothyroid adult patients need not avoid soy foods, but have to ensure adequate iodine content in their diet.
Perchlorate and thyroid
Perchlorate competitively inhibits the uptake of iodide into the thyroid gland, and affects the thyroid hormone synthesis.  The major contributors of perchlorate in the diet are vegetables, dairy products, drinking water and infant food products. The thyroid inhibitory actions of perchlorate were utilized by using this in iodine induced hyperthyroidism. In normal conditions, the presence of small perchlorate levels in water and food does not lead significant change in thyroid function due to the large iodine stores. Hence, no dietary modification is necessary for protection against the perchlorate induced thyroid dysfunction.
Maternal diet and fetal thyroid
Thyroid hormones also play an important role as a regulator of fetal development and energy expenditure. The amount and timing of thyroid hormone release are regulated through tight regulation of genes expressed within the hypothalamic-pituitary-thyroid axis. A fine balance is maintained between the production of T3 and T4 through tissue specific expression of the deiodinase, iodothyronine (DIO) genes. Maternal high fat diet leads to alterations in the hepatic histone code leading to transcriptional and epigenetic alterations of the fetal thyroid axis. Thus, in utero exposure to high fat diet may lead to obesity in later life by disturbing the fetal thyroid homeostasis. 
Obesity and thyroid
Thyroid hormone has a major influence on carbohydrate and lipid metabolism. Obesity is seen in hypofunctioning thyroid gland and is contributed by multiple factors like reduced metabolic rate, fluid retention and mucopolysaccharide deposition.  Leptin is the product of the ob gene, that regulates the food intake and body fat and previous reports gave conflicting results about the relation between leptin and thyroid hormones. 
| Conclusion|| |
Dietary micronutrients play an important role in the synthesis of thyroid hormones. Iodine remains the cornerstone with increasing focus on other nutrients like selenium and zinc. Iodine consumption is a double edged sword with disorders emanating from both the deficiency and excess consumption. The best advice for all patients is to consume a healthy balanced diet, meet the daily iodine requirement and eliminate the most potent goitrogenic substances from the diet. There is a lot of hype behind the thyroid diet with limited scientific basis and it is advisable to follow the recommendations of the treating physician.
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[Table 1], [Table 2]