In euthyroid men and women, higher levels of serum free triiodothyronine are associated with lower skeletal muscle mass index and greater levels of insulin resistance, according to study findings published in Clinical Endocrinology.
“Thyroid hormones play crucial roles in the control of energy homeostasis and thermogenesis and can therefore influence body composition,” Eun-Jung Rhee, MD, PhD, of the division of endocrinology and metabolism at Kangbuk Samsung Hospital in Seoul, South Korea, and colleagues wrote in the study background. “Hypothyroidism is associated with weight gain, whereas hyperthyroidism is associated with increased metabolic rate and weight loss. Changes in thyroid function may contribute to the development of metabolic complications, even when thyroid hormone levels are within the normal range.”
In a retrospective, cross-sectional study, Rhee and colleagues analyzed data from 36,655 euthyroid adults without a history of thyroid disease participating in the health checkup program at the Health Promotion Center at Kangbuk Samsung Hospital between 2012 and 2014 (mean age, 36 years; 49% women; mean BMI for men, 24.3 kg/m²; mean BMI for women, 21.3 kg/m². Participants completed a questionnaire and underwent measurements of weight, height and waist circumference, with body composition measurements obtained via segmental bioelectric impedance. Researchers also measured body fat mass and muscle mass, serum free triiodothyronine (FT3), free thyroxine (FT4) and thyroid stimulating hormone (TSH), fasting plasma glucose, serum insulin and insulin resistance via homeostatic model assessment of insulin resistance (HOMA-IR). Researchers used linear regression analyses to assess relationships between thyroid hormone levels and anthropometric parameters, adjusted for age, systolic blood pressure, diastolic blood pressure, HDL and LDL cholesterol, triglyceride levels and glucose level.
Within the cohort, mean waist circumference was 85.6 cm for men and 74.9 cm for women; mean body fat mass was 16.7 kg for men and 16.3 kg for women; mean body fat percentage was 22.4% for men and 28.8% for women. Mean HOMA-IR was 1.7 for men and 1.3 for women. For thyroid hormones, mean FT3 levels was 5.1 pmol/L for men and 4.55 pmol/L for women; mean FT4 level was 17 pmol/L for men and 15.61 pmol/L for women; mean serum TSH was 1.98 mIU/L for men and 2.1 mIU/L for women.
Researchers found that serum FT3 levels were positively associated with waist circumference and HOMA-IR and negatively associated with body weight, BMI and skeletal muscle mass index, with results persisting after adjustment for cardiometabolic factors in both men and women (adjusted R2 = 0.086 and 0.095, respectively).
Researchers also observed that serum FT4 levels were positively associated with body weight; however, FT4 was inversely associated with BMI, skeletal muscle mass index and HOMA-IR in men only (adjusted R2 = 0.073). In women, serum FT4 was positively associated with waist circumference and HOMA-IR and inversely associated with body weight (adjusted R2 = 0.087).
In assessed TSH levels and their association with body composition parameters, only HOMA-IR was positively associated with TSH levels in men (adjusted R2 = 0.003), whereas serum TSH was positively associated with waist circumference and HOMA-IR in women (adjusted R2 = 0.016).
“In the current study, the association between serum FT4 levels and anthropometric markers showed inconsistent results in men and women,” the researchers wrote. “Further studies are needed to understand the association between serum FT4 levels and obesity, as well as the underlying mechanisms of these associations.” – by Regina Schaffer
Disclosures: The authors report no relevant financial disclosures.