Background: Cyclooxygenase (COX-2) has been associated with tumor growth and metastasis in several cancers, including thyroid cancer. For this reason, several investigators have studied COX-2 inhibitors in preclinical models of thyroid cancer and found antineoplastic effects. Thus, the primary aim of this study was to assess if the use of nonsteroidal anti-inflammatory drugs (NSAIDs) is associated with a reduced incidence of thyroid cancer. A second aim of the study was to determine additional risk or protective factors for thyroid cancer. Methods: Three large prospective population-based studies (the NIH-AARP Diet and Health Study; the Prostate, Lung, Colorectal and Ovarian Cancer Screening Trial; and the U.S. Radiologic Technologists Study) were pooled to investigate the association between self-reported frequency of aspirin and nonaspirin NSAID use one year prior to baseline (no use, ≤2/week, >2-6/week, and ≥7/week) and subsequent risk of thyroid cancer. A Cox regression proportional hazard model was used to estimate aggregated hazard ratios (HR) adjusted for cohort, sex, race/ethnicity, weight, smoking status, and alcohol intake. Results: There were 388,577 participants in the pooled cohort, with 481 cases of thyroid cancer. No significant risk reduction was observed with regular use of nonaspirin NSAIDs (HR = 1.14 [confidence interval (CI) 0.84-1.55]), and/or regular use of aspirin (HR = 1.06 [CI 0.82-1.39]). The multivariate regression analysis confirmed as previously reported in the literature that female sex, obesity class I (body mass index [BMI] = 30-34.99 kg/m2), and obesity class II (BMI = 35-35.99 kg/m2) were independently associated with an increased thyroid cancer risk. Current smoking status and moderate and excessive alcohol use were also confirmed as independent risk factors associated with a reduced thyroid cancer risk. Conclusions: Neither nonaspirin NSAIDs nor aspirin use is associated with a reduced risk of thyroid cancer. Women and obesity are associated with an increased risk of thyroid cancer, whereas smoking and alcohol use are associated with decreased risk of thyroid cancer.