Free T3/T4 ratio predicts metabolic syndrome in euthyroid adults

Thyroid-stimulating hormone levels were associated with free thyroxine/triiodothyronine ratio among healthy, euthyroid adults, according to researchers in South Korea. They also reported that free thyroxine/triiodothyronine ratio, or FT3/FT4, was a better predictive marker for metabolic syndrome than TSH.

“Thyroid hormones have a broad spectrum of actions on the regulation of lipid and carbohydrate metabolism, blood pressure and energy expenditure,” So Young Park, of the department of internal medicine at Cheil General Hospital and Dankook University College of Medicine, Seoul, and colleagues wrote. “Numerous studies have tried to reveal the association between thyroid function and metabolic or cardiovascular disorders, such as obesity, metabolic syndrome and cardiovascular diseases. Currently, the interrelation between TSH, thyroid hormones and metabolic parameters is complex and has not been confirmed.”

The researchers performed a cross-sectional study of 132,346 adults (66,991 men and 65,355 women) aged 18 years and older. All participants had TSH, as well as free T3 and T4 levels, inside the institutional reference ranges. Park and colleagues measured TSH, free T3 and T4 levels along with such metabolic parameters as height, weight, waist circumference, blood pressure and serum cholesterol, triglyceride, high-density lipoprotein cholesterol, insulin and glucose.

After adjusting for age, BMI, smoking status and menopausal status, TSH was positively associated with the free T3/T4 ratio in both men and women. TSH and free T3/T4 ratio were associated with risk for metabolic syndrome parameters; however, the free T3/T4 ratio showed better predictive power than TSH.

“To conclude, we found a significant positive association between TSH level and the FT3/FT4 ratio in … euthyroid subjects,” the researchers wrote. “The higher FT3/FT4 ratio is associated with increased risk of metabolic syndrome parameters and insulin resistance. The FT3/FT4 ratio could be a better predictor for metabolic syndrome than TSH.”

"As measuring DIO2 activity is not simple, we propose that FT3/FT4 ratios can be considered an indicator of peripheral DIO2 activity,” Sihoon Lee, MD, PhD, said. “Also, FT3/FT4 ratios can be used as a predictor of metabolic syndrome."– by Andy Polhamus

Disclosure: The researchers report no relevant financial disclosures.