Risk of thyroid cancer after exposure to fertility drugs: results from a large Danish cohort study.

BACKGROUND Findings from the few epidemiological studies that have investigated thyroid cancer risk after fertility drugs have been inconclusive. Using data from the largest cohort of infertile women to date, we examined the effects of fertility drugs on thyroid cancer risk. METHODS A cohort of 54 362 women with infertility problems referred to Danish fertility clinics in the period 1963-1998 was established. A detailed data collection including information about type and amount of treatment was conducted. Using case-cohort techniques, we calculated rate ratios (RRs) of thyroid cancer associated with different fertility drugs after adjustment for age at first live birth. RESULTS A total of 29 thyroid cancers were identified during follow-up through 2000. Use of clomiphene [RR = 2.28; 95% confidence interval (CI): 1.08-4.82] or progesterone (RR = 10.14; 95% CI: 1.93-53.33) was associated with an increased thyroid cancer risk, although the latter estimate was based on few cases. When stratifying for parity status, the risk was primarily associated with clomiphene (RR = 3.09; 95% CI: 1.21-7.88) in parous women. No significantly increased risk was found after use of gonadotrophins, hCG or GnRH. We observed no association with number of cycles of use or years since first use (latency). CONCLUSIONS Clomiphene and possibly progesterone may increase thyroid cancer risk, particularly among parous women. Longer follow-up is needed to confirm our findings.