Abstract
Prior cross-sectional analyses have demonstrated an association between subclinical hypothyroidism and metabolic syndrome and selected components. However, the temporal relation between metabolic syndrome and declining thyroid function remains unclear. In a prospective study, an unselected cohort of 66,822 participants with and without metabolic syndrome were followed. A proportional hazards regression model was used to estimate hazard ratios (HRs) and 95% CIs for hypothyroidism. Exploratory analyses for the relation between components of metabolic syndrome and declining thyroid function were also undertaken. During an average follow-up of 4.2 years, the incident rates for subclinical hypothyroidism were substantially higher in participants who began the study with metabolic syndrome compared with metabolically normal controls. After controlling for risk factors, patients with metabolic syndrome were at a 21% excess risk of developing subclinical hypothyroidism (adjusted HR 1.21; 95% CI 1.03–1.42). When individual components were analyzed, an increased risk of subclinical hypothyroidism was associated with high blood pressure (1.24; 1.04–1.48) and high serum triglycerides (1.18; 1.00–1.39), with a trend of increasing risk as participants had additional more components. Individuals with metabolic syndrome are at a greater risk for developing subclinical hypothyroidism, while its mechanisms and temporal consequences of this observation remain to be determined.
Highlights
Subclinical hypothyroidism, defined as elevated TSH with free T4 concentrations at the lower end of the euthyroid range, affects approximately 4–10% of the general population[1, 2]
A total of 11,437 (16.6%) participants had met the criteria for metabolic syndrome and 1,151 (2%) had hypothyroidism, including 92 overt and 1,059 subclinical hypothyroidism
Overt and subclinical hypothyroidism were both significantly associated with metabolic syndrome with OR 1.89 and 1.48 respectively
Summary
Subclinical hypothyroidism, defined as elevated TSH with free T4 concentrations at the lower end of the euthyroid range, affects approximately 4–10% of the general population[1, 2]. Autoimmune disease is often the accepted cause of thyroid dysfunction, it is less clearer which risk factors may predispose or modify hypothyroidism In this regard it is important to note that many[12,13,14,15], but not all[16, 17], cross-sectional studies observed that metabolic syndrome and its components, including high blood pressure, elevated triglycerides level, obesity, and insulin resistance, and perhaps high serum cholesterol level, are closely related to subclinical hypothyroidism. Despite these known associations, the temporal relationships between subclinical hypothyroidism and assorted cardiovascular risk factors remain largely unexplored. The analyses examined further the effect of individual components of the metabolic syndrome on the occurrence of subclinical hypothyroidism
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