Thyroid hormones (THs) are critical regulators of important physiological processes within the human body, which become particularly evident in case of TH imbalance. Both deficiency and excess of THs are associated with severe disorders affecting different organs/systems.
Interestingly, hypothyroidism is associated with increased susceptibility to infectious diseases. Yet, a comprehensive understanding of the local control of TH action within the different components of the immune system is still missing. Furthermore, data on how THs impact the function of different immune cells are incomplete or controversial and this is an area of ongoing and important research[1].
The interplay of THs and the immune system involves a bidirectional crosstalk. On the one hand, pathological conditions related to severe illness or autoimmunity interfere with TH homeostasis. On the other hand, THs act as regulators of innate and adaptive immune cells, an imbalance in these supportive immune systems can lead to complex immune-mediated disorders. The functionality of THs in each individual may be influenced by several factors including nutritional status. Of increasing interest however, is the role of vitamin D in immunity as a whole and in particular the possible allied relationship with thyroid function and health.
Vitamin D in Autoimmune Thyroid Diseases
Autoimmune thyroid diseases are characterised by an immune attack of the thyroid gland. These conditions are the most common autoimmune disorders in general, with a prevalence of approximately 5%. Hashimoto’s thyroiditis, characterised by hypothyroidism, and Graves’ disease, characterised by hyperthyroidism, are the two main types of autoimmune thyroid diseases. Both conditions are T-cell-mediated autoimmune disorders confirmed by thyroid lymphocytic infiltration[2].
Work undertaken by Xu et al exploring the role of vitamin D and autoimmune thyroid disorders indicated that people with Graves disease were more likely to also have a deficiency of vitamin D[3]. A more recent meta-analysis expanded the association with Vitamin D deficiency and hypothyroidism, Hashimoto’s and Graves, albeit Vitamin D and Graves were shown only in older subjects[4].
It may be assumed that as the global biological actions of 1,25(OH)2D3 (VitD3) reflect an ability to interact functionally with the immune system by promoting immune tolerance and a shift from the pro-inflammatory setting to a more tolerogenic immune setting, that this mode of action may link to protective effects in all autoimmune diseases and inflammatory processes including those that affect the thyroid[5].
Vitamin D Supplementation
A 2012 review with a global perspective found that 6 to 47% of vitamin D intake may come from dietary supplements[6]. Consequently, without supplementation, vitamin D status strongly depends on endogenous vitamin D production which is also influenced by genetic determinants, latitude, season, skin pigmentation, body mass and lifestyle such as the use of sunscreen and clothing[7].
Other factors that interplay with the generation of an autoimmune thyroid response include endogenous factors (genetic factors, oestrogens, adipokines and body mass index etc.) and exogenous factors (nicotinism, iodine, selenium, zinc, iron, magnesium, tyrosine, B Vitamins, Vitamin A and Vitamin D deficiency, infections, stress, and nutrition status). These interplay across many systems with the role of the immune system and its capacity to induce and maintain tolerance to self-antigens being of primary importance.
Beyond D to Diet
Whilst this short review has focused its attention on the growing appreciation of the importance of vitamin D sufficiency in the optimisation of thyroid function and related immune tolerance, the thyroid is also susceptible to various nutritional deficits and environmental influences that fall within the control of the individual.
Nutrient demand by the thyroid is high, as it operates a key role in metabolic functionality and accordingly subclinical as well as frank deficiency can adversely impact on its capacity to manage the related essential metabolic needs.
According to current knowledge, gluten or lactose should be eliminated in the presence of food intolerances or diseases such as coeliac disease, and where non coeliac gluten sensitivity is present, gluten elimination is recommended[8]. One of the principles of a diet that reduces systemic as well as thyroidic inflammation is to limit saturated fatty acids, sugars, and refined carbohydrates, which have a pro-inflammatory effect[9]. Such an anti-inflammatory diet is represented, among others, by the Mediterranean diet. In a study by Ruggeri et al. [10], Hashimoto’s patients who adhered to the principles of the Mediterranean diet had lower oxidative stress parameters, which can have an impact on reducing the inflammatory process in the thyroid and other tissues.
Food selection and consumption is also recognised to either support or diminish immune competence. Many of the micronutrients needed for thyroid function are similarly required for immune function and a deficiency of these nutrients is common where dietetic imbalances are driven by consumer selection or food deprivation[11],[12]. While it is important to consider the quality of the diet in terms of its nutrient composition, the collective effects of a dietary pattern may be more relevant than a single nutrient or single food, as dietary patterns can impact inflammatory status which can modulate noncommunicable and infectious diseases risk. Dietary patterns that have high inclusion of foods such as fruits, vegetables, berries, and oily fish while being low in soft drinks, refined sweetened foods, salty snacks, and processed meat, are considered health-promoting and should always be seen as a primary mechanism of improving thyroid function ahead of individual nutrient supplementation[13].
Iodine and thyroid
No nutrition discussion of thyroid function would be complete without reference to oral supplementation of iodine. Iodine is an essential element for the production of THs. Both deficient and excess iodine intakes may precipitate in adverse thyroidal events. The thyroid gland has the capacity and holds the machinery to handle iodine efficiently when the availability of iodine becomes scarce, as well as when iodine is available in excessive quantities. The latter situation is handled by the thyroid by acutely inhibiting the organification of iodine, the so-called acute Wolff-Chaikoff effect. The Wolff-Chaikoff effect is an effective means of rejecting the large quantities of iodide and therefore preventing the thyroid from synthesising large quantities of thyroid hormones[14].
Iodine deficiency has long been known to cause thyroid issues, with the most dramatic effect noted in babies born to iodine-deficient mothers. In fact, iodine deficiency is among the most prevalent (and preventable) causes of mental impairment to this day, worldwide[15]. Oral supplementation with Iodine has been proposed from 150ug a day to 50mg per day. This scale of variation reflects the differing states of thyroid need which when using amounts over 12gms per day should be assessed by a health care practitioner knowledgeable in this field. It has in the last few years attracted a closer level of investigation in the functional medicine field and whilst there are compelling arguments for and against the use of high dose iodine i.e. 50mg daily or more, there are also clear cases of benefit.
Conclusion
In concluding this short review on thyroid health and immunity, it’s clear that thyroid hormones are integral to physiological processes and immune function. The interplay between thyroid function, nutrient status—particularly vitamin D—and autoimmunity offers a promising avenue for research and clinical practice.
In practice emphasising a holistic approach, particularly anti-inflammatory dietary patterns like the Mediterranean diet, in managing thyroid health should be a cornerstone of thyroid and overall health support. Iodine is of course an essential mineral in thyroid hormone production and is likely to be required in the management of dysfunction, but should be applied via careful supplementation.
References
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