Vitamin B12 and Vitamin D Levels in Patients with Autoimmune Hypothyroidism and Their Correlation with Anti-Thyroid Peroxidase Antibodies

The Power of Vitamin D: Is the Future in Precision Nutrition through Personalized Supplementation Plans?

In the last few decades, vitamin D has undeniably been one of the most studied nutrients. Despite our ability to produce vitamin D through sunlight exposure, its presence in several natural food sources and fortified foods, and its widespread availability as a dietary supplement, vitamin D deficiency is a serious public health problem, affecting nearly 50% of the global population. Low serum levels of vitamin D are being associated with increased susceptibility to numerous health conditions, including respiratory infections, mental health, autoimmune diseases, and different cancer types. Although the association between vitamin D status and health is well-established, the exact beneficial effects of vitamin D are still inconclusive and indefinite, especially when considering the prevention and treatment of different health conditions and the determination of an appropriate dosage to exert those beneficial effects in various population groups. Therefore, further research is needed. With constant improvements in our understanding of individual variations in vitamin D metabolism and requirements, in the future, precision nutrition and personalized supplementation plans could prove beneficial.

Acute Motor Axonal Neuropathy in Lupus Nephritis

A prevalent clinical scenario is provided in this case study, in which a 22-year-old lady with a five-year history of lupus nephritis with acute motor axonal neuropathy presents for therapy. The patient received immunomodulator medication and steroids to control her symptoms to keep up with her everyday life despite the absence of comorbidities such as hypertension, diabetes, and hypothyroidism. No laboratory measures were changed, including hemoglobin, serum creatinine, or thyroid function. Examining the nervous system indicated a potentially harmful consequence, underscoring the significance of prompt investigation and treatment. This research highlighted the importance of attention in cases with lupus nephritis, showing how early medical care can prevent serious neurological problems and contribute to the patient's general well-being.

FT4-to-FT3 ratio is a novel prognostic marker in subacute combined spinal cord degeneration patients

Objectives

The FT4-to-FT3 ratio (FFR) variations in patients with subacute combined spinal cord degeneration (SCSD) as a potentially useful prognostic indicator are still unknown. This study aimed to investigate the changes of FFR as a potentially valuable prognostic predictor in patients with SCSD.

Methods

This study included 144 consecutive SCSD patients who received standard diagnostic and therapeutic procedures between January 2015 and December 2021 and were admitted to the Department of Neurology at the First Affiliated Hospital of Bengbu Medical University. At the time of admission, we gathered data on all patients' demographics, daily routines, previous chronic conditions, medication histories, and other clinical details. For the purpose of measuring FFR, blood samples were specifically taken within 48 h of admission. The degree of neurological impairment of patients was assessed using the functional disability scale at the time of admission. At 6 months following discharge, the Modified Rankin Scale (mRS) was used to evaluate the clinical prognosis. To evaluate the relationship between the FFR and the risks of a poor outcome (mRS > 2), univariate and multivariate logistic regression analysis was utilized. The significance of the FT4/FT3 ratio in predicting the clinical outcomes in SCSD patients 6 months after discharge was assessed using the area under curve-receiver operating characteristic (AUC-ROC).

Results

About 90 patients (62.5%) of the 144 patients had poor outcomes, while 54 (37.5%) had favorable outcomes. Higher FFR at admission was independently linked to higher odds of a poor outcome, according to a logistic analysis. With an optimized cutoff value of >2.843, the FFR exhibited the maximum accuracy for predicting a poor outcome, according to the AUC‒ROC curve (AUC 0.731, P < 0.001; sensitivity, 77.8%; specificity, 83.3%). FFR was identified as an independent predictor of poor outcomes by multivariate logistic regression (OR, 2.244; 95% CI, 1.74-2.90; P < 0.001).

Conclusions

We discovered that in patients who had a bad result 6 months after discharge, the FFR had dramatically increased at the time of admission, providing a unique prognostic marker in patients with SCSD.

Relationship between thyroid function and dietary inflammatory index in Hashimoto thyroiditis patients

Inflammation is closely related to the changes of thyroid function in Hashimoto thyroiditis patients. Certain nutrients or dietary habits can alter the levels of autoantibodies in Hashimoto thyroiditis. However, it remains unclear whether dietary inflammation affects thyroid function in patients with Hashimoto thyroiditis. The purpose of this study was to assess the relationship between dietary inflammation and thyroid function in Hashimoto thyroiditis patients using data from the National Health and Nutrition Examination Survey. We employed weighted multivariable linear regression, subgroup analyses, and interaction analysis to explore the relationship between thyroid function and dietary inflammatory index. We found that dietary inflammatory index was positively correlated with TSH and total T4. Interaction analysis found an interaction between urinary iodine concentration and FT3, but subgroup analysis for different levels of urinary iodine concentration did not get statistically significant results. Hashimoto thyroiditis patients with more pro-inflammatory diet habits had higher levels of TSH and TT4. In order to prevent hypothyroidism more effectively in patients with Hashimoto thyroiditis, it is essential to control dietary inflammation. However, it is still necessary to design a better prospective cohort study to verify the causal relationship.

Do Dietary Supplements Affect Inflammation, Oxidative Stress, and Antioxidant Status in Adults with Hypothyroidism or Hashimoto's Disease?-A Systematic Review of Controlled Trials

This systematic review aims to summarise the results of controlled trials on dietary supplements (DS) usage and inflammation, oxidative stress, antioxidant status, and thyroid parameter improvement in hypothyroidism (HT)/Hashimoto's thyroiditis (AIT) patients. The study protocol was registered with PROSPERO (no. CRD42022365149). A comprehensive search of the PubMed, Scopus, and Web of Science databases resulted in the identification of nineteen randomised controlled trials and three non-randomised studies for the review; three studies examined the effect of supplementation with vitamin D, twelve studies-with selenium, and seven studies-with other DS. Based on very limited evidence, the lack of influence of vitamin D supplementation on inflammatory parameters was found, while no studies have examined oxidative stress and antioxidant status parameters, and only one provided results for a single thyroid parameter after an intervention. Some evidence was found proving that selenium supplementation may decrease inflammation and improve thyroid parameters, but reaching a conclusion about its influence on oxidative stress and antioxidant status is not possible because of the insufficient number of studies. Additionally, due to examining other DS (e.g., multicomponent, Nigella sativa, and genistein) only in single studies, conclusions cannot be drawn. Further long-term, high-quality randomised controlled trials are necessary to better understand the influence of DS on inflammation, oxidative stress, and antioxidant status, as well as their potential to improve thyroid gland function in HT/AIT patients.

A Study on Vitamin B12 Levels in Hypothyroid Patients Presenting to a Tertiary Care Teaching Hospital

Background and aims The prevalence of vitamin B12 deficiency is found to coexist in hypothyroid patients, causing the persistence of symptoms concomitant to both diseases even on adequate thyroxine supplementation. Primary objective To study vitamin B12 levels in patients with hypothyroidism. Secondary objective To study the clinical profile of patients with hypothyroidism with special reference to anemia, and to study the association between vitamin B12 deficiency with anti-thyroid peroxidase (anti-TPO) antibodies and anti-thyroglobulin (anti-Tg) antibodies in patients with hypothyroidism. Methods and results A single-centric cross-sectional study was carried out over a period of one year. Among 100 hypothyroid patients, 68% were found to be vitamin B12 deficient, among whom 73.5% were females. Of patients with raised anti-TPO antibodies, 78.6% had vitamin B12 deficiency (p = 0.01), while 78% of patients with raised anti-Tg antibodies were vitamin B12 deficient (p = 0.07). The Pearson correlation coefficient (r) of vitamin B12 with anti-TPO and anti-Tg antibodies was -0.302 (p = 0.002) and -0.253 (p = 0.011), respectively. Conclusion There is a predilection of hypothyroid patients toward developing anemia, with vitamin B12 deficiency as a major etiology. This finding can be correlated with the hematopoietic action of thyroid-stimulating hormones as well as autoimmune thyroid disease predisposing to pernicious anemia.

Impact of Vitamin D on Immunopathology of Hashimoto's Thyroiditis: From Theory to Practice

Hashimoto's thyroiditis (HT) is a common autoimmune disease affecting the thyroid gland, characterized by lymphocytic infiltration, damage to thyroid cells, and hypothyroidism, and often requires lifetime treatment with levothyroxine. The disease has a complex etiology, with genetic and environmental factors contributing to its development. Vitamin D deficiency has been linked to a higher prevalence of thyroid autoimmunity in certain populations, including children, adolescents, and obese individuals. Moreover, vitamin D supplementation has shown promise in reducing antithyroid antibody levels, improving thyroid function, and improving other markers of autoimmunity, such as cytokines, e.g., IP10, TNF-α, and IL-10, and the ratio of T-cell subsets, such as Th17 and Tr1. Studies suggest that by impacting various immunological mechanisms, vitamin D may help control autoimmunity and improve thyroid function and, potentially, clinical outcomes of HT patients. The article discusses the potential impact of vitamin D on various immune pathways in HT. Overall, current evidence supports the potential role of vitamin D in the prevention and management of HT, although further studies are needed to fully understand its mechanisms of action and potential therapeutic benefits.

Vitamin B12 levels in thyroid disorders: A systematic review and meta-analysis

Background and aims

Numerous studies have found an association between vitamin deficiency and thyroid disorders (TD). The presence of anti-parietal cell antibodies is indicative of reduced ability to absorb vitamin B12. Thus, this study reviewed the existing studies with the objective of assessing differences in the serum levels of vitamin B12 among patients with and without TD, the frequency of vitamin B12 deficiency in patients with TD, and the presence of anti-parietal cell antibodies in patients with TD.

Methods

A meta-analysis of random-effects model was conducted to calculate pooled frequencies, mean differences (MD), and their respective 95% confidence intervals (CI). We identified 64 studies that met our inclusion criteria (n = 28597).

Results

We found that patients with hypothyroidism had lower vitamin B12 levels than healthy participants (MD: -60.67 pg/mL; 95% CI: -107.31 to -14.03 pg/mL; p = 0.01). No significant differences in vitamin B12 levels were observed between healthy participants and patients with hyperthyroidism (p = 0.78), autoimmune thyroid disease (AITD) (p = 0.22), or subclinical hypothyroidism (SH) (p = 0.79). The frequencies of vitamin B12 deficiency among patients with hypothyroidism, hyperthyroidism, SH, and AITD were 27%, 6%, 27%, and 18%, respectively.

Conclusions

Patients with hypothyroidism had lower levels of vitamin B12 than healthy participants. No significant differences were observed between vitamin B12 levels and hyperthyroidism, AITD, or SH.

Systematic Review Registration

https://www.crd.york.ac.uk/prospero/display_record.php?RecordID=324422, identifier (CRD42022324422).

Correlation Between Vitamin B12 Deficiency and Autoimmune Thyroid Diseases

BACKGROUND

Autoimmune thyroid diseases (AITD) are the most prevalent organ-specific autoimmune disorders. Vitamin B12 plays an important role in the proper functioning of the immune system. The aim of this study was therefore to investigate the correlation between vitamin B12 deficiency and AITD.

MATERIALS AND METHODS

A total of 306 patients (aged 18-65 years, mean - 37.6 ± 11.3 years and comprising 87 males and 219 females) were studied retrospectively (observational study). Patients were divided into groups: with and without vitamin B12 deficiency, and with and without AITD. Differences between groups were evaluated by Fisher's exact test for qualitative variables and by Student's t-test for quantitative variables. Correlations for quantitative factors were determined by the Pearson correlation coefficient and for qualitative factors by Spearman correlation analysis. The sensitivity and specificity of vitamin B12 deficiency for AITD were calculated by ROC analysis.

RESULTS

The vitamin B12 level was significantly lower in patients with AITD (and 200.70 + 108.84) compared to controls (393.41+150.78 p<0.0001). Patients with vitamin B12 deficiency were characterized by significantly higher mean values of anti-TPO (236.60+455.74) compared to controls (39.51+165.57 p<0.0001). Vitamin B12 levels were inversely correlated to anti-TPO levels (r=- 0.233, p<0.001). Roc analysis of vitamin B12 as a diagnostic test for AITD gave the area under curve as 0.881 (95% CI: 0.839-0.924), a sensitivity of - 0.947, a specificity of - 0.768, and a cutoff value of - 178.9.

CONCLUSION

The vitamin B12 level correlates significantly to AITD. The concentration of vitamin B12 should therefore be determined in patients with autoimmune thyroiditis as a diagnostic test with high sensitivity and good specificity.

Oxidative stress in Hashimoto's thyroiditis: possible adjuvant therapies to attenuate deleterious effects

A number of studies have shown that oxidative stress is related to the pathogenesis of several immunological diseases, such as Hashimoto's thyroiditis (HT), although there is no plausible mechanism to explain it. Thus, we aimed at hypothesizing and providing some possible mechanisms linking oxidative stress to autoimmunity aspects and its implications for HT, as well as adjuvant therapeutic proposals to mitigate the deleterious effects. Our hypothesis is that deficient eating habits, autoimmune regulator gene predisposing gene, dysbiosis and molecular mimicry, unfolded proteins and stress in the endoplasmic reticulum, and thymus involution appear to be the main potential factors leading to HT oxidative stress. Likewise, we show that the use of minerals selenium and zinc, vitamins D and C, as well as probiotics, can be interesting adjuvant therapies for the control of oxidative damage and poor prognosis of HT. Further clinical trials are needed to understand the real beneficial and side effects of these supplements.

The Relationship between Gastrointestinal Health, Micronutrient Concentrations, and Autoimmunity: A Focus on the Thyroid

Currently, there is a lack of understanding of why many patients with thyroid dysfunction remain symptomatic despite being biochemically euthyroid. Gastrointestinal (GI) health is imperative for absorption of thyroid-specific nutrients as well as thyroid function directly. This comprehensive narrative review describes the impact of what the authors have conceptualized as the “nutrient–GI–thyroid axis”. Compelling evidence reveals how gastrointestinal health could be seen as the epicenter of thyroid-related care given that: (1) GI conditions can lower thyroid-specific nutrients; (2) GI care can improve status of thyroid-specific nutrients; (3) GI conditions are at least 45 times more common than hypothyroidism; (4) GI care can resolve symptoms thought to be from thyroid dysfunction; and (5) GI health can affect thyroid autoimmunity. A new appreciation for GI health could be the missing link to better nutrient status, thyroid status, and clinical care for those with thyroid dysfunction.

Metabolic Characteristics of Hashimoto's Thyroiditis Patients and the Role of Microelements and Diet in the Disease Management-An Overview

Hashimoto’s thyroiditis (HT) is the most common autoimmune disease and the leading cause of hypothyroidism, in which damage to the thyroid gland occurs due to the infiltration of lymphocytes. It is characterized by increased levels of antibodies against thyroid peroxidase and thyroglobulin. In this review, we present the metabolic profile, the effectiveness of micronutrient supplementation and the impact of dietary management in patients with HT. For this current literature review, the databases PubMed, Cochrane, Medline and Embase were reviewed from the last ten years until March 2022. This article provides a comprehensive overview of recent randomized controlled trials, meta-analyses, and clinical trials. Many patients with HT, even in the euthyroid state, have excess body weight, metabolic disorders, and reduced quality of life. Due to frequent concomitant nutritional deficiencies, the role of vitamin D, iodine, selenium, magnesium, iron and vitamin B12 is currently debated. Several studies have underlined the benefits of vitamin D and selenium supplementation. There is still no specific diet recommended for patients with HT, but a protective effect of an anti-inflammatory diet rich in vitamins and minerals and low in animal foods has been suggested. There is insufficient evidence to support a gluten-free diet for all HT patients. Pharmacotherapy, along with appropriate nutrition and supplementation, are important elements of medical care for patients with HT. The abovementioned factors may decrease autoantibody levels, improve thyroid function, slow down the inflammatory process, maintain proper body weight, relieve symptoms, and prevent nutritional deficiencies and the development of metabolic disorders in patients with HT.

ESPEN micronutrient guideline

BACKGROUND

Trace elements and vitamins, named together micronutrients (MNs), are essential for human metabolism. Recent research has shown the importance of MNs in common pathologies, with significant deficiencies impacting the outcome.

OBJECTIVE

This guideline aims to provide information for daily clinical nutrition practice regarding assessment of MN status, monitoring, and prescription. It proposes a consensus terminology, since many words are used imprecisely, resulting in confusion. This is particularly true for the words "deficiency", "repletion", "complement", and "supplement".

METHODS

The expert group attempted to apply the 2015 standard operating procedures (SOP) for ESPEN which focuses on disease. However, this approach could not be applied due to the multiple diseases requiring clinical nutrition resulting in one text for each MN, rather than for diseases. An extensive search of the literature was conducted in the databases Medline, PubMed, Cochrane, Google Scholar, and CINAHL. The search focused on physiological data, historical evidence (published before PubMed release in 1996), and observational and/or randomized trials. For each MN, the main functions, optimal analytical methods, impact of inflammation, potential toxicity, and provision during enteral or parenteral nutrition were addressed. The SOP wording was applied for strength of recommendations.

RESULTS

There was a limited number of interventional trials, preventing meta-analysis and leading to a low level of evidence. The recommendations underwent a consensus process, which resulted in a percentage of agreement (%): strong consensus required of >90% of votes. Altogether the guideline proposes sets of recommendations for 26 MNs, resulting in 170 single recommendations. Critical MNs were identified with deficiencies being present in numerous acute and chronic diseases. Monitoring and management strategies are proposed.

CONCLUSION

This guideline should enable addressing suboptimal and deficient status of a bundle of MNs in at-risk diseases. In particular, it offers practical advice on MN provision and monitoring during nutritional support.

Conformational Changes in the BSA-LT4 Complex Induced by the Presence of Vitamins: Spectroscopic Approach and Molecular Docking

Levothyroxine (LT4) is known for its use in various conditions including hypothyroidism. LT4 interaction with serum albumin may be influenced by the presence of vitamins. For this reason, we investigated the effect of vitamin C, vitamin B12, and folic acid on the complex of Bovine Serum Albumin with LT4 (BSA-LT4). UV-Vis spectroscopy was used to monitor the influence of vitamins on the BSA-LT4 complex. Fluorescence spectroscopy revealed a static quenching mechanism of the fluorescence of BSA-LT4 complex by the vitamin C and folic acid and a combined mechanism for vitamin B12. The interaction of vitamin C and folic acid with BSA-LT4 was moderate, while the binding of vitamin B12 was much stronger, extending the storage time of LT4 in blood plasma. Synchronous fluorescence found that the vitamins were closer to the vicinity of Trp than to Tyr and the effect was more pronounced for the binding of vitamin B12. The thermal stability of the BSA-LT4 complex was more evident, but no influence on the stability of BSA-LT4 complex was obtained for vitamin C. Molecular docking studies showed that vitamin C and folic acid bound the same site of the protein, while vitamin B12 bonded to a different site.

Is There An Association Between Vitamin B12 Level and Vitamin D Status in Children?

Vitamin B12 (vit B12) or cobalamin is a water-soluble vitamin that acts as a coenzyme and is effective in the formation of a healthy nervous system and erythrocytes. In its deficiency, except fatigue and weakness, may cause serious signs and symptoms such as macrocytic anemia, cognitive-developmental delay, irreversible neurological damage. In this study, hemoglobin, leukocyte, thrombocyte, iron, iron-binding, ferritin, and vit B12 levels were evaluated retrospectively during the routine controls of 689 children 5 to 18 years old without any complaints. In addition, the relationship between serum 25-Hydroxyvitamin D (vit D) levels and vit B12 levels, which were evaluated in 487 of the cases, was investigated. Vit B12 deficiency was diagnosed in a total of 160 (23.2%) cases, 99 (24.4%) girls and 61 (21.4%) boys. Vit B12 level was significantly lower in children over 12 years old. Vitamin D deficiency was found to be 49.3% in 487 cases and was significantly higher in the group under 12 years and girls. The frequency of anemia was 7.2% in this group. There was a significant relationship between vit D deficiency and vit B12 deficiency. In conclusion, vit B12 deficiency is common in school-age children, especially in the adolescent age group. It should be known that vit D deficiency may also be present in vit B12 deficiency cases. More comprehensive studies are needed to reveal the relationship between B12 and vit D deficiency.

Vitamin B12 (Cobalamin) Deficiency in Overt and Subclinical Primary Hypothyroidism

Background:

B12 (cobalamin) deficiency has been reported in hypothyroid patients with variable prevalence rates thus routine screening of hypothyroid patients was recommended by some and discouraged by others. We aimed to assess the prevalence of B12 deficiency among hypothyroid patients and to evaluate for pernicious anemia and celiac disease as etiologies.

Methods:

A total 133 patients were included. Thyroid hormones and thyroid peroxidase (TPO) autoantibodies were measured. Serum B12 was measured and if deficient, intrinsic factor antibodies (IFAB) and tissue transglutaminase (tTG) antibodies were evaluated.

Results:

Our study included 45 patients with overt hypothyroidism (OH), 48 patients with subclinical hypothyroidism (SCH), and 40 patients as controls. Mean age was 34.3 years and 82% were females. TPO antibodies were positive in 73.5% of OH and 51.1% of SCH patients. B12 deficiency was detected in 33.3%, 47.9%, and 37.5% of OH, SCH, and controls, respectively with no significant difference (P = .334). Borderline-to-low B12 level was more prevalent in the OH and the SCH groups compared to controls (68.9%, 85.4%, and 57.5%, respectively; P = .014). Among B12-deficient hypothyroid patients, 7.5% had positive IFAB and 13.3% had positive tTG antibodies. We did not find a significant association of TPO positivity and B12 deficiency (OR, 0.69; 95% CI 0.3-1.57; P = .147).

Conclusion:

We did not find a higher prevalence of B12 deficiency among hypothyroid patients nor an association with TPO positivity. Borderline B12 levels were more prevalent among hypothyroid patients.

Vitamin D deficiency in thyroid autoimmune diseases

The role of vitamin D in the human body is a complex one, proven by the many studies performed related to this aspect. Data from the literature on the correlation between vitamin D deficiency and thyroid autoimmune pathology, although present and increasing in the last 10 years, have failed to establish exactly whether or not there is a link between them. The aim of the study was to assess the status of vitamin D in patients with autoimmune thyroid disease; and to determine if there is a correlation between parameters such as: thyroid stimulating hormone (TSH), free thyroxine (FT4), ATPO; and vitamin D levels. Therefore, we performed a retrospective study in which we included 60 patients, 32 with autoimmune thyroid pathology and 28 patients with negative antithyroid antibodies. The average age of those in the case group was 58 years old and 52 years old for the control group. Female sex was predominant in both groups of patients, 97% in the study group and 71% in the control group. Vitamin D values in patients with thyroid autoimmune pathology in our study were on average higher than those obtained in the control group (patients with negative thyroid antibodies), without finding a statistically significant difference between the values of the two groups of patients (p = 0.197). The study shows a high prevalence of vitamin D deficiency in both groups of patients (with or without autoimmune thyroid disease), the highest rate being observed among patients without autoimmune pathology, but without a statistically significant difference between values.

Effect of Micronutrients on Thyroid Parameters

Micronutrients are involved in various vital cellular metabolic processes including thyroid hormone metabolism. This study aimed to investigate the correlation between serum levels of micronutrients and their effects on thyroid parameters. The correlation of serum levels of micronutrients and thyroid markers was studied in a group of 387 healthy individuals tested for thyroid markers (T4, T3, FT4, FT3, TSH, anti-TPO, RT3, and anti-Tg) and their micronutrient profile at Vibrant America Clinical Laboratory. The subjects were rationalized into three groups (deficient, normal, or excess levels of micronutrients), and the levels of their thyroid markers were compared. According to our results, deficiency of vitamin B2, B12, B9 and Vit-D25[OH] ( $p<0.05$ ) significantly affected thyroid functioning. Other elemental micronutrients such as calcium, copper, choline, iron, and zinc ( $p<0.05$ ) have a significant correlation with serum levels of free T3. Amino acids asparagine (r = 0.1765, $p<0.001$ ) and serine (r = 0.1186, $p<0.05$ ) were found to have a strong positive correlation with TSH. Valine, leucine, and arginine ( $p<0.05$ ) also exhibited a significant positive correlation with serum levels of T4 and FT4. No other significant correlations were observed with other micronutrients. Our study suggests strong evidence for the association of the levels of micronutrients with thyroid markers with a special note on the effect of serum levels of certain amino acids.

Environmental Factors Affecting Thyroid-Stimulating Hormone and Thyroid Hormone Levels

Thyroid hormones are necessary for the normal functioning of physiological systems. Therefore, knowledge of any factor (whether genetic, environmental or intrinsic) that alters the levels of thyroid-stimulating hormone (TSH) and thyroid hormones is crucial. Genetic factors contribute up to 65% of interindividual variations in TSH and thyroid hormone levels, but many environmental factors can also affect thyroid function. This review discusses studies that have analyzed the impact of environmental factors on TSH and thyroid hormone levels in healthy adults. We included lifestyle factors (smoking, alcohol consumption, diet and exercise) and pollutants (chemicals and heavy metals). Many inconsistencies in the results have been observed between studies, making it difficult to draw a general conclusion about how a particular environmental factor influences TSH and thyroid hormone levels. However, lifestyle factors that showed the clearest association with TSH and thyroid hormones were smoking, body mass index (BMI) and iodine (micronutrient taken from the diet). Smoking mainly led to a decrease in TSH levels and an increase in triiodothyronine (T3) and thyroxine (T4) levels, while BMI levels were positively correlated with TSH and free T3 levels. Excess iodine led to an increase in TSH levels and a decrease in thyroid hormone levels. Among the pollutants analyzed, most studies observed a decrease in thyroid hormone levels after exposure to perchlorate. Future studies should continue to analyze the impact of environmental factors on thyroid function as they could contribute to understanding the complex background of gene-environment interactions underlying the pathology of thyroid diseases.