Thyroid Dysfunction, Thyroid Hormone Replacement, and Colorectal Cancer Risk

Background: Current screening guidelines for colorectal cancer (CRC) do not consider thyroid dysfunction as a risk factor for disease development. We sought to determine the risk of developing CRC in patients with thyroid dysfunction, with and without thyroid hormone replacement (THR). Methods: We conducted a nested case-control study using a large population-based medical records database from the United Kingdom. Study case patients were defined as those with any medical code of CRC. Subjects with familial colorectal cancer syndromes or inflammatory bowel disease (IBD) were excluded. For every case patient, four eligible control patients matched on age, sex, practice site, and duration of follow-up before index date were selected using incidence density sampling. Exposure was THR therapy before index date. We further divided the THR unexposed group into patients with hypothyroidism (TSH > 4 mg/dl), patients with hyperthyroidism (TSH < 0.4 mg/dl), and subjects without documented thyroid abnormality. The adjusted odds ratios (ORs) and 95% confidence intervals (CIs) for CRC were estimated using conditional logistic regression. All statistical tests were two-sided. Results: We identified 20 990 CRC patients and 82 054 control patients. The adjusted odds ratio for CRC associated with THR was 0.88 (95% CI = 0.79 to 0.99, P =.03) and 0.68 (95% CI = 0.55 to 0.83, P <.001) for treatment initiated five to 10 years and more than 10 years before index date, respectively. This protective association increased with cumulative duration of therapy. In contrast, hyperthyroidism (adjusted OR = 1.21, 95% CI = 1.08 to 1.36, P =.001) or untreated hypothyroidism (adjusted OR = 1.16, 95% CI = 1.08 to 1.24, P <.001) were associated with increased risk of CRC. Conclusion: Long-term THR is associated with a decreased risk of CRC. Hyperthyroidism and untreated hypothyroidism are associated with modestly elevated risk of CRC.