Different Relationships Between Thyrotropin and Muscle Strength According to Sex and Age in Euthyroid Koreans (The 6th Korea National Health and Nutritional Examination Survey 2014-2015).

Changes in muscle mass and strength can be caused by multiple endocrine factors, including thyroid dysfunction. However, the relationship between thyroid function and muscle strength in euthyroid individuals is not clear. In the present study, we investigated the relationship between thyroid function and handgrip strength (HGS) in euthyroid Koreans, especially according to sex and age. This was a population-based, cross-sectional study from the 6th Korea National Health and Nutrition Examination Survey including 2894 Koreans (1487 men and 1407 women) aged ≥19 years. Serum thyrotropin (TSH) and free thyroxine (fT4) levels were measured. HGS was measured using a digital grip strength dynamometer. Serum TSH level, but not serum fT4 level, was significantly associated with HGS in men, but neither TSH nor fT4 level was associated with HGS in women. Serum TSH showed a negative association with HGS in men aged <65 years ( = 1376;  = -0.566,  = 0.008), but a positive association in men aged ≥65 years ( = 111;  = 1.158,  = 0.010) after adjusting for confounders. HGS decreased as TSH tertile increased in men aged <65 years ( for trend = 0.010), but increased with TSH tertile in men aged ≥65 years ( for trend = 0.009) after adjusting for confounders. Odds ratios (ORs) for the lowest quartile of HGS increased in the highest tertile of TSH among men aged <65 years (OR = 1.657 [95% confidence interval {CI} 1.099-2.500],  = 0.006) and decreased in the highest tertile of TSH among men aged ≥65 years (OR = 0.176 [CI 0.032-0.966],  = 0.034) after adjusting for confounders. In euthyroid Koreans, serum TSH levels were significantly associated with HGS only in men and this relationship was found to depend significantly on age. These findings suggest that age has a distinct influence on the effect of even subtle change of thyroid function on muscle strength in men.