LAUSR

To the Editor: Metabolic syndrome (MetS) is defined as a set of metabolic disorders including abdominal obesity, hyperglycemia, hypertension, and dyslipidemia. As thyroid hormones are essential for cellular energy homeostasis and regulation, the levels of serum thyroid hormones are considered to be associated with various metabolic parameters. Over the past decade, the association between thyroid hormones and MetS has been extensively studied in euthyroid subjects, leading to inconsistent results. Some studies suggested that a high level of thyrotropin (TSH) contributed to the increased incidence of MetS or unfavorable metabolic parameters, whereas other studies found no such correlation. Moreover, some researches have shown that free thyroxin (FT4) levels were inversely proportional to various unfavorable metabolic parameters or MetS, while other studies suggested that FT4 levels have no such association or even a positive association with some unfavorable metabolic components after adjustment. These conflicting results may result from different designs, settings, race, exclusion criteria, adjustments, statistical analyses, and definitions ofMetS and population iodine intake. Based on these, our study aimed to evaluate the association and causal relationship between thyroid hormones andMetS in people with normal thyroid function through standard epidemiological and Mendelian randomization analysis.