Background: Measuring thyroid-stimulating hormone (TSH) is essential for diagnosing and monitoring thyroid diseases. The aim of this study was to evaluate the effect of sex, age, sampling time and season on TSH in a large Chinese population and to determine which factor had the greatest impact on TSH measurement results. Methods: Data were obtained from the laboratory information system from September 1, 2013 to August 31, 2016. A total of 80150 TSH measurements of outpatients were enrolled in this study. TSH was measured using a Siemens ADVIA Centaur XP automatic chemiluminescence immunoassay analyzer. Linear regression models were used to assess the association between log-transformed TSH concentrations and sex, age, sampling time and season. Results: The serum TSH concentrations in women were significantly higher than in men. In all subjects, serum TSH concentrations increased by 0.005 mu lU/mL for each year of age. TSH concentrations showed circannual variation during the 3 consecutive years of data collection and decreased during the summer while increased during the winter. The serum TSH concentrations decreased from 7 a.m. to 1 p.m. while increased from 1 p.m. to 4 p.m. The same trend was observed in TSH concentrations for sampling time stratified by sex. Linear regression revealed that sampling time might be the major factor affecting serum TSH concentrations. Conclusion: Sex, age, season, and sampling time significantly affected serum TSH concentrations. Age-related alteration in serum TSH concentrations was observed in this study. Sampling time was the major factor affecting serum TSH concentrations. (C) 2018 Elsevier Inc. All rights reserved.