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Obesity and Thyroid: Which came first, the chicken or the egg?

The relationship between thyroid problems and obesity has been the subject of long and complex research in the health sector. The question that often arises is whether thyroid disorders, such as hypothyroidism, can be responsible for weight gain, or whether obesity, in turn, influences thyroid function. < /p>

This intriguing interaction between thyroid function and body weight raises fundamental questions about how these two factors relate and how they can impact people's overall health. To better understand these dynamics, it is necessary to examine the complex interactions between the thyroid endocrine system and body weight balance, which sheds light on important questions related to obesity and thyroid health.

With a shape that resembles that of a butterfly, the thyroid gland is positioned in the front part of the neck, just below the thyroid cartilage, often known as the Adam's Apple. This gland plays a crucial role in regulating the functions of vital organs, including the heart, brain, liver and kidneys, by secreting the hormones triiodothyronine (T3) and thyroxine (T4) upon TSH stimulation._11100000-0000-0000-0000- 000000000111_

With the increasing obesity epidemic worldwide, obesity has received increasing attention and is considered a significant public health challenge on a global scale due to its wide-ranging adverse consequences for human health, such as increased the risk of diabetes, cardiovascular disease and cancer.

The incidence of thyroid disorders, which mainly include thyroid dysfunctions and autoimmune thyroid diseases, is increasing in recent years. Thyroid dysfunctions include hyperthyroidism and hypothyroidism, both of which can be categorized into subclinical stages (with only changes in TSH) and manifest stages (with changes in both TSH and thyroid hormones).

The most common autoimmune thyroid diseases are characterized by the presence of autoantibodies against thyroid antigens, such as TSH receptor antibody (TRAb), thyroid peroxidase antibody (TPOAb) and thyroglobulin antibody (TGAb). They have two main subtypes: Graves' disease (GD) and Hashimoto's thyroiditis (HT), which present different clinical manifestations, although they share similar immunogenetic mechanisms.

Patients with thyroid disorders also have a high risk of other non-thyroid-related diseases, such as cardiovascular disease, cancer, obesity, and adverse pregnancy outcomes. Patients with thyroid dysfunction or Graves' disease require long-term medical treatment or follow-up to optimize prognosis. 

Identification of risk factors for thyroid disorders can help healthcare professionals recognize individuals at risk of developing or who already have subclinical thyroid disorders. This allows us to provide immediate treatment to improve patient outcomes and is critical to understanding the pathophysiological mechanisms underlying these thyroid disorders. 



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Obesity and Thyroid: What is the relationship?

Although studies reveal that immunological dysfunctions, environmental elements and genetic factors contribute to the pathogenesis of thyroid disorders, their pathology is still not completely clear. It is widely known that obesity is associated with hormonal changes, including thyroid-stimulating hormone (TSH) and thyroid hormones, as well as being accompanied by several endocrine and metabolic diseases. 

Clinically, it is known that untreated hypothyroidism can induce weight gain and a decrease in basal metabolism, leading us to propose the hypothesis that the relationship between obesity and thyroid disease may be bidirectional. Furthermore, if this relationship is truly bidirectional and if obesity does indeed influence the risk of thyroid disorders, it is not yet completely clear how obesity affects the risk of thyroid dysfunction and the impact on the risk of thyroid autoimmunity.

With this in mind, the study “The Impact of Obesity on Thyroid Autoimmunity and Dysfunction: A Systematic Review and Meta-Analysis”, published on October 1, 2019, in the Journal Frontiers in Immunology, raised this central question about thyroid disorders and obesity. With a total of 22 studies, it was observed that clinically it is easy to observe that patients with hyperthyroidism often lose a lot of weight and regain it after remission. 

In contrast, patients with hypothyroidism tend to gain some weight and lose it modestly after thyroid hormone replacement. So is it common sense to consider obesity as often secondary to hypothyroidism? Probably not, certainly weight gain related to hypothyroidism is caused by increased sodium reabsorption in the renal tubules, and where there is sodium there will certainly be water. 

In addition, documents show that this weight gain is approximately 4 to 5 kg, purely from water retention resolved after correction with a hormone duly calculated based on body weight (1.6 – 1.8 mcg/kg/day according to American Thyroid association). Or, considering Hashimoto's hypothyroidism, there will also be a dose change. 

There is no treatment for hypothyroidism with a dose of 25 mcg, unless you are an elderly patient, but that is a conversation for another time. Furthermore, the mechanisms by which hypothyroidism causes weight gain are supposedly achieved through changes in energy expenditure and appetite.


Thyroid Disorders: Villain or Consequence of Obesity?

Another important component in this association is leptin, which plays a role in chronic inflammation associated with interleukin 6, which can result in morphological changes in the thyroid and restrict the expression of thyroglobulin. This, in turn, can induce changes in thyroid hormone levels in obese individuals.

It is certainly believed that leptin, a hormone produced by adipocytes, also plays a role in chronic inflammation, which can result in morphological changes in the thyroid. Furthermore, leptin can restrict the expression of thyroglobulin sodium/iodine symport, which induces changes in thyroid hormone levels in obese individuals. 

Other studies also suggest that this state of chronic inflammation present in obesity can affect thyroid function, modulating the expression and activity of deiodinases. Have you heard about T3r in obese patients? Little clinical implication but physiology helps to understand. 

Therefore, we can suggest that the relationship between obesity and thyroid disorders is bidirectional, and obesity is significantly associated with hypothyroidism and TSH and TPO levels. These findings indicate that obesity prevention plays a crucial role in preventing thyroid disorders.


Bibliographic reference

Article: SONG, Rong-Hua; WANG, Bin; YAO, Qiu-Ming; LI, Qian; JIA, XI; ZHANG, Jin-An. The Impact of Obesity on Thyroid Autoimmunity and Dysfunction: a systematic review and meta-analysis. Frontiers In Immunology, [S.L.], v. 10, 1 Oct. 2019. Frontiers Media SA.

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