Abstract: 【Abstract】 Objective To detect changes of thyroid stimulating hormone (TSH) with gender, age and levels of thyroid peroxidase antibodies (TPO-Ab) in patients with subclinical hypothyroidism, and to establish composite reference intervals for TSH and free thyroxine (FT4) in determination of subclinical hypothyroidism. Methods From Oct. 2011 to July 2012, 7 964 healthy people (males: 4789, females: 3175) undergoing medical examinations were recruited. Their serum levels of TSH and FT4 were determined. Of those participants, 794 were also tested for TPO-Ab. The serum TSH and FT4 data were transformed into normal distributions, with outliers being eliminated and a correction for skewness (0.909 and 0.384, respectively) and kurtosis (2.605 and 1.947, respectively). The composite reference intervals were established according to the Mahalonobis distance formula. Results Serum TSH increased with age and TPO-Ab. Using the conventional reference standards, 358 participants were identified with subclinical hypothyroidism, which included 230 at 41-70 years of age and 43 showing TPO-Ab positive. In contrast, using composite reference intervals, 301 participants were identified with subclinical hypothyroidism, which included 142 at 41-70 years of age and 25 showing TPO-Ab. Conclusion The conventional cutoff values for FT4 and TSH separately lead to overestimation of the prevalence of subclinical thyroid disease.