A pooled analysis of case-control studies of thyroid cancer - III. Oral contraceptives, menopausal replacement therapy and other female hormones

Objective: The relations between oral contraceptives (OC), hormone replacem ent therapy (HRT) for menopause, and other female hormone use and thyroid c ancer risk was analyzed using the original data from 13 studies from North America, Asia and Europe. Methods: Based on 2,132 cases and 3,301 controls, odds ratios (OR) and the corresponding 95% confidence intervals (CI) were obtained by conditional re gression models, conditioning on study and age at diagnosis, and adjusting for age, radiation exposure and parity. Results: Overall, 808 (38%) cases versus 1,290 (39%) controls had ever used OCs, corresponding to an OR of 1.2 (95% CI 1.0 to 1.4). There was no relat ion with duration of use, age at first use, or use before first birth. The OR was significantly increased for current OC users (OR = 1.5, 95% 1.0 to 2 .1), but declined with increasing time since stopping (OR = 1.1 for > 10 ye ars since stopping). The association was stronger for papillary cancers (OR = 1.6 for current users) than for other histologic types. No significant h eterogeneity was observed across studies or geographic areas. Eight studies had data on HRT, for a total of 1,305 cases and 2,300 controls: 110 (8%) c ases and 205 (9%) controls reported ever using HRT (OR = 0.8; 95% CI 0.6 to 1.1). The ORs were 1.6 (95% to 0.9 to 2.9) for use of fertility drugs, and 1.5 (95% CI 1.1 to 2.1) for lactation suppression treatment. Conclusions: The studies considered in these analyses include most of the e pidemiological data on the role of exogenous hormone use in the, etiology o f thyroid cancer, and they provide reassuring evidence on the absence of an association of practical relevance. The moderate excess risk in current OC users, if not due to increased surveillance for thyroid masses among OC us ers, is similar to that described for breast cancer, and would imply a role of female hormones on thyroid cancer promotion. There was no indication of increased thyroid cancer risk 10 or more years after discontinuing OC use.