According to a study published in Clinical Endocrinology, lower skeletal muscle mass index is significantly associated with higher serum-free triiodothyronine levels in both men and women.
In this retrospective cross-sectional study of 36,655 euthyroid patients (average age, 36.4 years) who joined a medical health checkup program, researchers sought to evaluate the associations among thyroid hormone levels, body composition, and insulin resistance in patients with normal thyroid ultrasound findings. To do so, they analyzed serum thyroid hormone levels in association with body fat percentage, skeletal muscle mass index, and homeostatic model assessment of insulin resistance.
Via multiple linear regression analysis, the researchers determined that serum-free triiodothyronine levels exhibited a positive association with waist circumference and homeostatic model assessment of insulin resistance, but negative associations with body weight, body mass index, and lower skeletal muscle mass index among both men and women.
Serum thyroid-stimulating hormone levels also showed a positive association with homeostatic model assessment of insulin resistance in both men and women. However, the researchers gathered inconsistent results in regard to the association between serum-free thyroxine levels and anthropometric markers in men or women.
Several limitations were noted for this study. Primarily, because of its cross-sectional design, the researchers were unable to evaluate changes in thyroid hormone levels over time and to detect a cause-and-effect relationship.
Despite such limitations, the researchers unveiled new insight into associations between thyroid hormone levels and body composition in euthyroid patients, and stated, “Further studies are needed to understand the association between serum [free thyroxine levels] and obesity, as well as the underlying mechanisms of these associations.”
Kwon H, Cho JH, Lee DY, et al. Association between thyroid hormone levels, body composition, and insulin resistance in euthyroid subjects with normal thyroid ultrasound: The Kangbuk Samsung Health Study [published online July 27, 2018]. Clin Endocrinol (Oxf). doi: 10.1111/cen.13823