A pooled analysis of case-control studies of thyroid cancer - II. Menstrual and reproductive factors

Objective: It has been suggested that female hormones, and hence menstrual and reproductive factors, play a role in thyroid cancer etiology. Epidemiological data, however, are limited and inconsistent, partly because of the small number of cases included in each study. To clarify the etiology of thyroid cancer, we conducted a pooled analysis of original data from 14 case-control studies, 4 from the United States, 2 from Asia, and 8 from Europe. Methods: This analysis included a total of 2,247 female cases of thyroid cancer (80% papillary) and 3,699 control women. Pooled odds ratios (OR) were estimated using logistic regression, conditioning on study and (i) matching sets for individually matched studies, or (ii) quinquennia of age for the other studies. Additional terms for age and history of radiation exposure were included in the regression equations. Results: The OR per year of later menarche was 1.04 (95% confidence interval (CI) 1.0-1.1). Compared to premenopausal women, the OR was 1.3 for women with natural menopause, and 1.8 for those with artificial menopause, but the studies were heterogeneous and the association may be due, at least in part, to diagnostic or ascertainment bias. Parity, spontaneous or induced abortions and history of infertility were not associated with thyroid cancer risk. The OR was above unity in women reporting later age at first birth (OR = 1.1, 95% CI 1.0-1.3 for 5-year delay) and higher in the first years after a birth. Conclusions: The associations of menstrual and reproductive factors with thyroid cancer risk were generally weak, but appeared stronger among women diagnosed with thyroid cancer at younger ages.