The Association of Thyroid Stimulating Hormone Among the Reference Range and its Change with the Incidence of Metabolic Syndrome in a 5-Year Prospective Cohort Study

Objective:
Although a number of studies have explored the association between thyroid stimulating hormone (TSH) among the normal range and the prevalence of metabolic syndrome (MetS), the conclusions are conflicting. We aimed to investigate the association of TSH among the reference range and its change (∆TSH) with the incidence of MetS.

Methods:
In this prospective population-based cohort study, 1501 participants with baseline TSH among the reference range were divided into three groups by tertile cut-off values and 1386 participants with the normal TSH levels both in 2014 and 2019 were divided into three groups by ∆TSH in tertiles. Binary logistic regression models were used to estimate the associations of TSH and ∆TSH with MetS and its components. The reported association was measured using the odds ratios (ORs) and 95% confidence intervals (CIs).

Results:
Compared with the lowest baseline TSH group, the highest group of TSH was associated with a higher incidence of MetS (OR 1.659; 95% CI 1.089, 2.525) in women after adjusting for important confounders. Among the individual components of MetS, the highest group of TSH was associated with central obesity (OR 1.746; 95% CI 1.135,2.685) in women. There was a lack of association between ∆ TSH and MetS in both sexes. However, a significant relationship was found for the highest group of ∆ TSH and central obesity (OR 1.887; 95% CI 1.144, 3.114) in men. Moreover, the middle (OR 2.526; 95% CI 1.374, 4.645) and high groups (OR 2.057; 95% CI 1.104, 3.833) of ∆TSH were associated with hypertriglyceride in women.

Discussion/Conclusion:
Our results suggest that an increase in TSH levels even among the normal range is related to a higher risk of MetS and certain components. It is important to monitor basic TSH as well as the dynamic changes of TSH.