Reciprocal Association between Thyroid Function and Metabolic Syndrome: A Review Article

Introduction: Thyroid dysfunction can impair fat and glucose metabolism, blood pressure, and body weight. On the other hand, metabolic syndrome (MetS) can be associated with an increased risk of hypothyroidism. Due to the high prevalence of both disorders and the possibility of synergistic metabolic detrimental consequences in combination, this review study aimed to investigate the reciprocal association between thyroid function and MetS. Materials and Methods: Articles in PubMed and Web of Science written in English were searched from 1991 to 2021 using search terms hypothyroidism, hyperthyroidism, thyroid hormones, thyroxine, thyroid-stimulating hormone, and metabolic syndrome in titles and abstracts. Results: Hypothyroidism and even changes in thyroid hormones within the normal range are associated with disorders of various metabolic risk factors; however, the risk of developing MetS in people with both euthyroidism and subclinical hypothyroidism remains a controversial issue. Hyperthyroidism may be associated only with insulin resistance and impaired blood sugar levels (dysglycemia). Although the effect of various metabolic disorders such as obesity and diabetes on thyroid function has been observed, the results are contradictory regarding the prevalence and incidence of thyroid dysfunction in patients with MetS and healthy individuals. Conclusion: The association between MetS, thyroid function, and thyroid hormones is reciprocal and complex and may be affected by age, gender, insulin resistance, smoking, iodine intake, and inflammatory markers.