Context: Despite apparent muscle weakness in overt or even subclinical hyperthyroidism, the effects of thyroid function in the reference range on muscle strength are unknown.
Objective: To investigate the association of serum TSH and free T4 with handgrip strength (HGS) in euthyroid elderly.
Design and setting: A nationally representative population-based, cross-sectional study from the Korea National Health and Nutrition Examination Surveys.
Participants: A total of 650 men aged ≥50 years and 533 postmenopausal women.
Main outcome measures: HGS was measured using a digital grip strength dynamometer, and low muscle strength was defined based on the Korean specific cutoff point of HGS (28.9 and 16.8 kg in men and women, respectively).
Results: After adjustment for confounders, lower serum TSH but not free T4 was associated with lower HGS in men (P = 0.032). Compared with men with high-normal TSH, those with low-normal TSH consistently showed 5.0% lower HGS (P = 0.027), with a linear decrease in HGS across decreasing serum TSH quartiles (P for trend = 0.018). Men with low muscle strength had 22.0% lower serum TSH than those without (P = 0.015), and the odds for the risk of low muscle strength was 3.76 times higher among men with low-normal TSH than it was among those with high-normal TSH (P = 0.021). However, these associations were not observed in postmenopausal women.
Conclusions: These results suggest that serum TSH level at the lower end of reference range may be associated with low muscle strength, especially in older men.