Design and setting
A population-based matched cohort study was designed to estimate the effect of HRT on the hazards of all-cause mortality using The Health Improvement Network (THIN) database. The database holds retrospectively collected information of anonymized patients from UK primary care dating back from the 1960s. THIN is representative of the UK general population in terms of demographics, prevalence of major medical conditions, and mortality rates when adjusted for demographics and deprivation.25 Currently, THIN database retains longitudinal records of 17 million patients from over 770 GP practices, of which 3.1 million are actively registered, covering 6.2% of the UK population.26, 27