Endocrine Abstracts

Objective: Globally, cancer is one of the leading causes of death. Therefore, identification of risk factors is of paramount interest. Whether thyroid dysfunction is a cancer risk factor is currently being debated. The aim of this study was to evaluate the risk of cancer in patients diagnosed with hypothyroidism.

Methods: This is a register-based nationwide cohort study of individuals diagnosed with hypothyroidism referred to Danish hospitals (followed from 1995-2017). Each hypothyroid individual was matched to four reference individuals from the general population according to age and sex. Using Fine & Gray competing risk regression models, we studied the association of hypothyroidism and all-cause cancer, adjusted for pre-existing morbidity. Sub-analyses were stratified for cancer site, (breast, prostate, lung, and colorectal respectively) and age when first diagnosed with hypothyroidism.

Results: The cohort consisted of 96,825 patients diagnosed with hypothyroidism (followed for a median of 11.4 years (range: 6.1-17.9)), 16,482 of whom received a cancer diagnosis. The reference population consisted of 387,300 individuals (followed for a median of 10.9 years (range: 5.7-17.5)), 56,737 of whom received a cancer diagnosis. Receiving a hypothyroidism diagnosis was associated with an increased risk of all-cause cancer (sub-distribution hazard ratio (SHR): 1.10; 95% confidence interval (CI): 1.08-1.12), as well as an increased risk of colorectal- (SHR: 1.13; 95% CI: 1.08-1.18) and breast cancer (SHR: 1.12; 95% CI 1.08-1.16). The risk of lung- (SHR: 0.81; 95% CI 0.77-0.85) and prostate cancer (SHR: 0.86; 95% CI 0.79-0.95) was lower in hypothyroid patients compared to the reference population. Sub-analyses stratified by age when first diagnosed with hypothyroidism yielded similar results.

Conclusion: In this register-based study, hypothyroid patients referred to Danish hospitals had an increased risk of all-cause cancer, as well as breast- and colorectal cancer, but a decreased risk of lung- and prostate cancer compared to the general population. Whether a causal link exists remains to be proven.