2.1 Study population and design
We performed a cross-sectional study among communities in Guangzhou, China, from March to July, 2015. This study population was obtained from the research project of the National Health and Family Planning Commission of China, titled-”National Survey of Thyroid Diseases and Diabetes”. This project has been set up as a multicenter prospective observational study aiming to evaluate chronic diseases in the Chinese population. Inclusion criteria: 1) 18-79 years old; 2) The Han ethnicity of the Chinese population; 3) Permanent residents, who lived in those regions ≥5 years. Exclusion criteria: 1) Pregnant women; 2) Those who suffer from severe diseases such as hepatic cirrhosis, chronic renal failure or evident cardiac insufficiency; 3) individuals who received medicines influencing thyroid function or hormones, such as iodine, amiodarone, somatostatin and glucocorticoids, within the past three months. A multistage stratified cluster random sampling method was used to select a representative sample. In the first stage, the districts of Guangzhou city were divided into rural areas and urban areas; In the second stage, one rural area and one urban area were randomly selected; In the third stage, one community was sampled from both rural and urban groups using a random selection; Finally, one adult resident was randomly selected from each household of the selected communities. During the recruitment phase, a total of 2767 residents were invited to participate via examination notices or home visits. In total, 2720 subjects signed the consent form and agreed to participate in the survey. 1567 females and 155 male with missing information regarding ED or osteoporosis were excluded. Finally, 998 eligible individuals were included in the data analyses. The flow chart for data analysis is shown in Figure 1 . All participants provided informed consent before being recruited into the study. The study protocol was in accordance with the principles of the Helsinki Declaration II and our study was approved by the Ethics Committee of the Sun Yat-sen Memorial Hospital (2014[3]).