Serum free thyroxine concentration is associated with metabolic syndrome in euthyroid subjects
Ladan Mehran1, Atieh Amouzegar, Maryam Tohidi
11 Endocrine Research Center, Research Institute for Endocrine Sciences, Shahid Beheshti University of Medical Sciences , Tehran, I. R. Iran .
Context:
The association between thyroid hormones within the euthyroid range and metabolic syndrome is not clear.
Aim:
This study evaluated the relationship between thyroid hormones and metabolic syndrome and its components in euthyroid subjects.
Design And Setting:
This is a cross-sectional population based study conducted within the framework of the cohort of the Tehran Thyroid Study.
Participants:
Out of 5786 subjects aged ≥ 20 years, 3755 euthyroid subjects without a history of thyroid disorders, diabetes, or use of steroids or lipid-lowering agents were investigated.
Outcome Measures:
Body weight, waist circumference (WC), and blood pressure (BP) were measured. Serum concentrations of lipids and lipoproteins, fasting blood glucose (FBG), insulin, free T4 (FT4), and thyrotropin (TSH) were assayed. Metabolic syndrome was determined by definition of the Joint Interim Statement adjusted for the Iranian population.
Results:
After adjustment for age, sex, and smoking, serum FT4 was significantly associated with high-density lipoprotein cholesterol (HDL-C), low-density lipoprotein cholesterol (LDL-C), triglycerides (TGs), WC, systolic BP, and diastolic BP; the associations that remained significant after further adjustment for body mass index (BMI) and the homeostasis model assessment index for insulin resistance (HOMA-IR) except for HDL-C. Serum TSH was associated only with TGs after adjustments for age, sex, smoking, and BMI, an association that disappeared after further adjustment for HOMA-IR. Serum FT4 was negatively associated (β=-0.02, 95% confidence interval [CI: -0.03, -0.01]) and TSH was positively associated (β = 0.03 [CI: 0.01, 0.04) with insulin resistance. The prevalence of metabolic syndrome decreased from 30.1% in the lowest FT4 tertile to 22.4% in the highest FT4 tertile (p<0.001). The prevalence of other metabolic syndrome components decreased significantly from higher to lower FT4 tertiles. Higher FT4 values were associated with lower odds of metabolic syndrome (OR = 0.96 [95% CI: 0.92, 0.99]; p=0.01).
Conclusion:
In euthyroid subjects, FT4, rather than TSH, is associated with risk of metabolic syndrome and its components.
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