In aged population, the association of thyroid hormones on physical performance, especially within their normal range, has yet to be elucidated. In this study, individuals with low serum free T3/free T4 were likely to have low muscle mass and impaired physical performance.
We aimed to evaluate the associations of muscle mass, strength, and physical performance with thyroid hormone in an aged euthyroid population from a community-based cohort.
We examined 918 men aged over 60 years and 1215 postmenopausal women from the Ansung cohort study. Appendicular skeletal muscle mass divided by square of height (ASM/ht2) was used as the muscle mass index. Hand grip strength was measured using a hydraulic dynamometer. Physical performance was assessed using the short physical performance battery (SPPB).
Participants with higher tertiles of free T3 and free T3/free T4 were younger and had higher ASM/ht2, stronger hand grip strength, and higher SPPB scores than those in the lower tertiles. In adjusted models, men within higher tertiles of free T3 had higher ASM/ht2 compared with those within lower tertiles (p = 0.033), whereas subjects with higher tertiles of free T4 had lower ASM/ht2 compared with those within lower tertiles (p = 0.043). Subjects within higher tertiles of free T3/free T4 had higher ASM/ht2 (p < 0.001) and better physical performance (p = 0.048) than those within lower tertiles after adjustments. However, free T3, free T4, or free T3/free T4 was not related to hand grip strength after adjustment for covariates.
Our results thus indicate that in an aged euthyroid population, low serum free T3/free T4 was a better index for low muscle mass and impaired physical performance than serum free T3 or free T4 alone.