The association between hormonal/reproductive factors and the risk of developing rheumatoid arthritis

Rheumatoid arthritis (RA) is a chronic inflammatory disease which leads to joint damage and bone destruction, with a complex interplay of genetic and environmental factors involved in its etiology. RA is more common among women than men at all ages, but the gender difference seems to be highest before menopause. It has been hypothesized that changes in female hormonal levels might have a role in RA pathogenesis. The overall aim of this thesis was to study the association between hormonal/reproductive factors and the risk of RA and to determine whether these factors were differently associated with serological phenotypes of the disease (according to the presence/absence of anti-citrullinated peptides antibodies (ACPA) and rheumatoid factor (RF)). This thesis is based on information from two large studies. Three articles were based on the Swedish Epidemiological Investigation of Rheumatoid Arthritis (EIRA), a population-based case- control study comprising incident RA cases. The study population were people aged 18 and above, living in diverse geographical parts of Sweden from 1996. Controls were randomly selected from the population register and matched to the cases by age, sex and residential area. Cases and controls completed an extensive questionnaire, collecting information about life- style/environmental exposures. One article was based on the Nurses’ Health Study (NHS), which consists of two prospective cohorts of female nurses in the USA. Data collection started in 1976 (women aged 30-55 years) and 1989 (women aged 25-42 years). Both cohorts of the NHS were followed via biennial questionnaires about diseases, lifestyle and health practices. According to our results, parous women had an increased risk of ACPA-negative RA compared with nulliparous women, aged 18-44 years. The increased risk was attributable to an elevated risk during the postpartum period, and to a young age at first birth. Older age at first birth seemed to be associated with a decreased risk of ACPA-positive RA. Parous women who breastfed for more than a year had a decreased risk of ACPA-positive RA compared with parous women who breastfed for up to 6 months. This decreased risk was non-significant after adjustment for smoking. Ever oral contraceptive use was significantly associated with a decreased risk of ACPA- positive RA, while a longer duration of use was significantly associated with a decreased risk for both RA subsets. Postmenopausal women had an increased risk of seronegative RA, but they had no association with the onset of seropositive RA. Women with a long duration of postmenopausal hormone therapy (PMH) had an increased risk of seropositive RA in the NHS. Finally, in the EIRA study, postmenopausal women who were currently using PMH at onset of their disease had a decreased risk of ACPA-positive RA. This decreased risk was mainly observed among women aged 50-59 years, with a short duration of use (