The following subgroups showed significant associations between serum Klotho (ln transformation) levels and hypothyroidism: 75 < age < 80 years (OR = 0.41, 95% CI 0.19–0.88), female (OR = 0.66, 95% CI 0.44–0.97), non-Hispanic White (OR = 0.64, 95% CI 0.43–0.94), no diabetes (OR = 0.65, 95% CI 0.45–0.94), no cardiovascular disease (OR = 0.69, 95% CI 0.48–0.99), 25 < BMI < 30 (OR = 0.43, 95% CI 0.25–0.76), eGFR ≤ 90 (ml/min/1.73m2) (OR = 0.71, 95% CI 0.50–0.97). This evidence concerns the gene KL and hypothyroidism.