TG and metabolic syndrome: In summary, we show that lower-normal FT4 (FT4 ≤ 16.0 pmol/L) is an independent risk factor for MetS, and through a combination of TSH and FT4, instead of only one thyroid hormone indicator, we found that lower-normal thyroid function (TSH > 2 mIU/L and FT4 ≤ 16 pmol/L) is associated with a higher risk of MetS.