OBJECTIVE: The association between thyroid dysfunction and gastrointestinal cancer is unclear.

DESIGN: We conducted a nationwide population-based cohort study to examine this potential association.

METHODS: We used Danish medical registries to assemble a nationwide population-based cohort of patients diagnosed with hyperthyroid or hypothyroid disease from 1978 through 2013. We computed standardized incidence ratios (SIRs) with corresponding 95% confidence intervals (CI) as measures of the relative risk of each cancer, comparing patients with thyroid dysfunction with that expected in the general population.

RESULTS: We included 163,972 patients, of which 92,783 had hyperthyroidism and 71,189 had hypothyroidism. In general, we found an increased risk of all gastrointestinal cancers within the first year after thyroid disease diagnosis. After more than five years of follow-up, patients with hyperthyroidism had a slightly increased risk of pancreatic and gallbladder and biliary tract cancer. Patients with hypothyroidism had a slightly increased risk of stomach, anal, liver, gallbladder and biliary tract, and pancreatic cancer after more than five years of follow-up, but the observed numbers of cancers were in general similar to the expected.

CONCLUSIONS: The increased risks of all gastrointestinal cancers in the first year following hyper- or hypothyroidism diagnosis are likely due to detection bias. After more than five years of follow-up, there does not seem to be a consistent causal association between thyroid disease and gastrointestinal cancer.