Thyroid Function as a Predictor of Handgrip Strength Among Middle-Aged and Older Euthyroid Adults: The TCLSIH Cohort Study

Objectives: Thyroid hormones (THs) play a critical role in skeletal muscle function, such as protein synthesis and energy metabolism, suggesting that thyroid function may be involved in the decline of muscle strength. Studies regarding the long-term effects of THs on muscle strength are limited. Therefore, this large-scale longitudinal study aimed to explore how TH levels were associated with handgrip strength (HGS) among middle-aged and older euthyroid adults. Design: Longitudinal population-based cohort study (approximately 4-year follow-up period, median: 3.0 years). Setting: The Tianjin Chronic Low-grade Systemic Inflammation and Health Cohort Study, Tianjin, China. Participants: A total of 2152 participants were enrolled, and annual measures were conducted in this longitudinal study. Measures: Chemiluminescence immunoassay was used to measure free triiodothyronine (FT3), free thyroxine (FT4), and thyroid-stimulating hormone (TSH). HGS was measured using a handheld digital dynamometer. Multiple linear regression analysis was used to evaluate the association between THs, FT3/FT4 ratio, TSH levels, and annual change in HGS and weight-adjusted HGS. Results: After adjusting for multiple confounding factors, significant associations between FT3 levels, FT3/FT4 ratio, and annual change in HGS [standard regression coefficient (SRC) = 0.073, P < .01, and and SRC = 0.059, P = .01, respectively] were observed. However, no significant difference was observed between FT4, TSH, and annual change in HGS (SRC = 0.021, P = .34; and SRC = -0.017, P = .44, respectively). Similar associations between FT3, FT4, FT3/FT4 ratio, TSH, and weight-adjusted HGS were observed. Conclusions and implications: This longitudinal study is the first to demonstrate that high-normal FT3 levels and higher FT3/FT4 ratios significantly predict annual change in HGS and weight-adjusted HGS and that FT4 and TSH levels are not associated with HGS among middle-aged and older euthyroid subjects. The findings suggest that new therapeutic approaches aimed at FT3 levels and FT3/FT4 ratios may be proposed to maintain muscle strength. (C) 2019 AMDA - The Society for Post-Acute and Long-Term Care Medicine.