Thyroid hormones (THs) play a crucial role in the homeostasis of muscle function, such as myogenesis and energy metabolism, suggesting that the thyroid may be also involved in the entropic processes of muscle aging. The aim of the present study is to evaluate the effect of TH signaling on physical performance, muscle mass, and strength in a cohort of community-dwelling oldest-old subjects (> 90 years). The study population was selected in a rural area of central Italy (Mugello, Tuscany), and the design was cross-sectional. Four hundred seventy-five subjects (130 males and 345 females) were enrolled, representing about 65% of all the nonagenarians living in the Mugello area. After adjusting for multiple confounding factors (sex, age, diabetes, and levothyroxine administration), the lowest quartile of FT3/FT4 ratio distribution showed lower physical performance compared to the other quartiles (β ± SE: - 0.49 ± 0.12; p < 0.001), whereas the highest quartile of FT3/FT4 ratio was associated with higher skeletal muscle index (β ± SE: 1.11 ± 0.42; p = 0.009). In addition, the lowest quartile of FT4 showed a statistically significant higher handgrip strength (β ± SE: 1.78 ± 0.68; p = 0.009) compared to all other quartiles. This study demonstrates that nonagenarians with higher FT3/FT4 ratios had better preserved muscle function, therefore successfully overcoming the imbalance of homeostatic and entropic processes involved in muscle aging. However, we could not establish a cause-effect relationship due to the cross-sectional design of the study.
Keywords: Aging; Muscle mass; Muscle strength; Oldest-old; Physical performance; Rehabilitation; Thyroid hormone signaling.