Muscle health in the oldest-old with and without subclinical hypothyroidism: sarcopenia, low muscle strength, and inflammatory–metabolic correlates in a prospective cross-sectional study

Muscle health impairment is common in very old adults and may present as reduced muscle strength, low muscle mass, or both. Subclinical hypothyroidism is frequently observed in older populations; however, its relationship with muscle health remains unclear, particularly in adults aged 80 years and older. This study aimed to evaluate muscle health in oldest-old adults with and without subclinical hypothyroidism and to examine associations with body composition and inflammatory–metabolic parameters. This single-centre, prospective cross-sectional study included 115 adults aged 80 years and older attending a healthy ageing outpatient clinic between December 2024 and March 2025. Participants were classified according to the presence or absence of subclinical hypothyroidism. Body composition was assessed using bioelectrical impedance analysis. Appendicular skeletal muscle index (ASMI) and fat-free mass index (FFMI) were calculated to define low muscle mass. Handgrip strength was measured using a hand dynamometer, and low muscle strength was defined using sex-specific thresholds for adults aged 80 years and older. Firth penalized logistic regression was used to evaluate the association between subclinical hypothyroidism and FFMI-based low muscle mass. The mean age was 84 ± 3 years, and 59 participants (51.3%) were female. Subclinical hypothyroidism was present in 39 participants (33.9%). Low muscle strength was identified in 72 participants (62.6%), ASMI-based low muscle mass in 3 (2.6%), and FFMI-based low muscle mass in 16 (13.9%). Low muscle strength (64.1% vs. 61.8%, p = 0.842) and ASMI-based low muscle mass (2.6% vs. 2.6%, p = 1.000) did not differ by thyroid status. FFMI-based low muscle mass was more frequent in participants with subclinical hypothyroidism (28.2% vs. 6.6%, p = 0.004) and remained associated after adjustment for age and sex (adjusted OR 7.87, 95% CI 2.16–28.63; p = 0.002). Low muscle strength was highly prevalent in adults aged 80 years and older, regardless of thyroid status. Subclinical hypothyroidism was associated with FFMI-based low muscle mass, but not with low muscle strength or ASMI-based low muscle mass. The prevalence of low muscle mass differed according to the index used, highlighting the importance of considering the selected muscle mass measure when interpreting body composition in very old adults.