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Decrease in free T4 increases metabolic syndrome risk
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The risk for metabolic syndrome may rise with decreasing serum free thyroxine levels, particularly in adults without obesity, according to findings published in Thyroid.
Fereidoun Azizi, MD, professor of internal medicine and endocrinology at Shahid Beheshti University of Medical Sciences in Iran, and colleagues evaluated data from the Tehran Thyroid Study on 2,393 adults (61% women; mean age for women, 36.4 years; mean age for men, 39.9 years) to determine the effect of thyroid hormone variation in the subclinical and euthyroid range on the incidence of metabolic syndrome over 10 years.
Through follow-up, 16.48% of participants developed metabolic syndrome from 2002 to 2005, 9.94% from 2006 to 2008 and 13.42% from 2009 to 2011.
After adjustment for age, sex, smoking, BMI and homeostasis model of assessment for insulin resistance (HOMA-IR), thyroid-stimulating hormone was positively associated with total cholesterol (P < .001), LDL cholesterol (P < .001) and triglyceride (P = .005) levels and waist circumference (P < .001). After adjustment, serum free T4 was negatively associated with total cholesterol (P = .011) and LDL cholesterol (P = .006) levels and with waist circumference (P = .019); serum free T4 was positively associated with systolic blood pressure (P = .006).
After adjustment, free T4 was associated with lower odds of large waist circumference (OR = 0.49; 95% CI, 0.35-0.69) and high triglyceride level (OR = 0.57; 95% CI, 0.41-0.78) and with higher odds of elevated BP (OR = 1.35; 95% CI, 1.05-1.74). No association was observed between any of the metabolic syndrome components and TSH.
Lower odds of metabolic syndrome were associated with free T4 in the unadjusted model and after adjustment for age, sex and smoking (OR = 0.59; 95% CI, 0.39-0.9). The odds for metabolic syndrome were not associated with TSH before or after adjustment.
In a subgroup analysis of participants with obesity (n = 354) and without obesity (n = 2,039), free T4 was a predictor of metabolic syndrome in participants without obesity only after adjustment for age, sex and smoking status (P = .007) and after further adjustment for HOMA-IR (P = .03).
“A lower normal [free] T4 was associated with incident [metabolic syndrome] and a few of its components in an iodine-sufficient Iranian population,” the researchers wrote. “The current survey gives clinical implications as the detection of low normal [free] T4 levels could indicate metabolically unhealthy subjects with combined risk factors who can benefit from early screening and medical intervention. Lower normal [free] T4 values in non-obese subjects, especially in the presence of other risk factors, may be associated with the development of [metabolic syndrome] in the future. More well-designed studies should definitely be conducted to confirm these findings and in case of similar findings, implementation of clinical trials are warranted to ascertain whether maintaining [free] T4 values within upper normal ranges could manage insulin resistance and metabolic abnormalities in euthyroid subjects.” – by Amber Cox
Disclosure: The researchers report no relevant financial disclosures.
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