Objective: Exercise has been hypothesized to influence cancer risk through a variety of mechanisms including hormonal, metabolic and immunologic effects, yet its relation with the risk of thyroid cancer has not been examined. We conducted a population-based case–control study in women aged 18–64 in three counties of western Washington State to assess the relation of recreational physical activity with risk of papillary thyroid cancer. Methods: Of 558 women with thyroid cancer of the follicular epithelium diagnosed during 1988–1994 who were identified as eligible, 468 (83.9%) were interviewed; this analysis was restricted to women with papillary histology (n = 410). Controls (n = 574) were identified by random digit dialing, with a response proportion of 73.6%. Logistic regression was used to calculate odds ratios (OR) and associated confidence intervals (CI) estimating the relative risk of papillary thyroid cancer associated with various aspects of recreational exercise. Results: Risk of thyroid cancer was reduced among women who reported that they engaged in regular recreational exercise during the 2 years before diagnosis relative to women who did not report exercise during that time period (OR = 0.76, 95% CI 0.59–0.98). A similar risk reduction was noted among women who reported having exercised regularly between ages 12 and 21 (OR = 0.83, 95% CI 0.64–1.1). However, no clear associations with aspects of recreational activity, including average hours exercised per week or weekly energy expenditure, were observed. Conclusions: These results provide some initial support for the hypothesis that physical activity may reduce risk of thyroid cancer.
