Summary: 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.
Purpose: 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.
Methods: 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).
Results: 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.
Conclusion: 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.