TSH and free thyroxine concentrations are associated with differing metabolic markers in euthyroid subjects

Abstract

ObjectiveTo examine the association between thyroid function and the components of the metabolic syndrome and insulin resistance in an Hispanic population.DesignCross-sectional study.MethodsSubjects with no history of thyroid disease or diabetes were included. Thyroid function was stratified as euthyroid or subclinical hypothyroidism (SCH) status and subsequently by free thyroxine (FT4) and TSH tertiles. The association of the metabolic syndrome components (defined by 2004 Adult Treatment Panel III criteria) and insulin resistance with thyroid status, TSH, and FT4 were examined.ResultsA total of 3148 subjects were analyzed. The prevalence of SCH was 8.3%. The prevalence of the metabolic syndrome was similar in euthyroid and SCH patients (31.6 vs 32.06%, P=0.89). Total cholesterol was higher in patients with SCH (5.51±1.19 vs 5.34±1.05 mmol/l, P<0.032). Serum TSH values showed a positive correlation (adjusted for age and sex) with total cholesterol, triglycerides, and waist circumference. In contrast, FT4 showed a positive correlation with high-density lipoprotein cholesterol, and an inverse correlation with waist circumference, insulin, and HOMA-IR.ConclusionSCH is not associated with an increased risk for the metabolic syndrome (as conceived as a diagnostic category defined by the National Cholesterol, Education Program, Adult Treatment Panel III criteria). Despite this, low thyroid function (even in the euthyroid state) predisposes to higher cholesterol, glucose, insulin, and HOMA-IR levels. The combined use of TSH and FT4, compared with the assessment based on only FT4, is a more convenient approach to evaluate the association between thyroid function and metabolic variables.