Linear reduction in thyroid cancer risk by oral contraceptive use: a dose-response meta-analysis of prospective cohort studies.

STUDY QUESTION: Is there an association between oral contraceptive (OC) use and thyroid cancer risk in females?

SUMMARY ANSWER: OC use is inversely associated with the risk of thyroid cancer in females.

WHAT IS KNOWN ALREADY: OC use may be relevant to the risk of thyroid cancer as suggested by some epidemiological studies. However, the findings are inconsistent regarding the effect direction and size.

STUDY DESIGN, SIZE, DURATION: This systematic review and meta-analysis included a total of 1906 patients from about 1.3 million individuals who had participated in 9 prospective cohort studies. The follow-up length ranged 7.5-15.9 years.

PARTICIPANTS/MATERIALS, SETTING,

METHODS

PubMed (MEDLINE) was searched through to January 2015 for eligible studies. References of relevant review articles were also manually screened. Prospective cohort studies that evaluated the association between OC use and thyroid cancer risk were included. Study characteristics including patients' characteristics, length of the follow-up and risk estimates were extracted. The quality of the studies was also assessed.

MAIN RESULTS AND THE ROLE OF CHANCE: The included studies were of high methodological quality according to the Newcastle-Ottawa Quality Assessment Scale. After pooling risk estimates from all the studies, there was a significant inverse association between the longest versus shortest duration of OC use and the risk of thyroid cancer [relative risk (RR) = 0.84, 95% confidence interval (CI) 0.73-0.97], with no considerable heterogeneity (I(2) = 26.1%). There was no significant publication bias. The significant association persisted in the subgroup of high-quality studies (RR = 0.84, 95% CI 0.72-0.97). By dose-response analysis, there was a linear relationship (P = 0.0001) between the duration of OC use and thyroid cancer risk. The summary RR for an increment of 1 year of OC use was 0.96 (95% CI 0.94-0.98), with no significant heterogeneity.

LIMITATIONS, REASONS FOR CAUTION: Individual patient data were unavailable for a more accurate estimation.

WIDER IMPLICATIONS OF THE FINDINGS: These results indicate that OC use may decrease the risk of thyroid cancer in females. This may have implications for women's decisions regarding the use of OC.

STUDY FUNDING/COMPETING INTERESTS: No funding was involved for this study. No conflicts of interest are declared.