Diana H Christensen, Katalin Veres, Anne G Ording, Jens Otto L Jørgensen, Suzanne C Cannegieter, Reimar W Thomsen, Henrik T Sørensen

Risk of venous thromboembolism (VTE) is increased in patients with hypo/hyperthyroidism. It is unknown whether VTE may be a presenting symptom of occult cancer in these patients. Nationwide population-based cohort study based on Danish medical registry data. We identified all patients diagnosed with VTE during 1978-2013 who had a previous or concurrent diagnosis of hypothyroidism (N=1481) or hyperthyroidism (N=1788). We followed them until a first-time cancer diagnosis, death, emigration, or study end, whichever came first. We calculated 1-year absolute cancer risk and standardized incidence ratios (SIRs) for cancer incidence in the study population compared with national cancer incidence in the general population. During the first year after a VTE diagnosis, the 1-year absolute cancer risk was 3.0% among patients with hypothyroidism and 3.9% among those with hyperthyroidism. During the first year of follow-up, SIRs for cancer in the study population compared with the general population were 1.96 (95% CI: 1.42-2.64) among patients with hypothyroidism and 2.67 (95% CI: 2.07-3.39) among those with hyperthyroidism. SIRs declined substantially after 1 year but remained increased during the remainder of the follow-up period (up to 36 years) (SIR for hypothyroidism=1.16 [95% CI: 0.97-1.39]; SIR for hyperthyroidism=1.26 [95% CI: 1.08-1.46]). VTE may be a marker of underlying occult cancer in patients with hypothyroidism or hyperthyroidism.