Parity as a Risk Factor for Differentiated Thyroid Carcinoma: A Population-based Study

Context: Female sex is a risk factor for differentiated thyroid carcinoma (DTC), potentially due to reproductive influences. However, data on the association between parity and DTC risk remain inconsistent. Objective: To assess the association between parity and DTC risk in a high-multiparity population. Design: Population-based case-control study (1982-2022). Setting: A single tertiary medical center. Patients or other participants: The study included 300 female patients with DTC and 900 controls, matched by birth year and ethnicity. The primary exposure was the number of deliveries before DTC diagnosis. Main outcome measure(s): Association between parity and DTC risk evaluated by logistic regression, adjusted for socioeconomic status, TSH levels, oral contraceptive use, and autoimmune thyroid diseases. Results: The median age at DTC diagnosis was 39 years; 60% of participants were Jewish and 40% were Arab. Baseline characteristics were comparable, except for higher rates of autoimmune thyroid diseases in cases: Hashimoto thyroiditis (9.7% vs 1.8%, P < .001) and Graves' disease (6.3% vs 2.7%, P = .005). Parity was associated with increased DTC risk starting at 4 deliveries (OR = 1.70, 95% CI: 1.051-2.741, P = .030), with the highest risk at 6 or more (odds ratio = 1.89; 95% CI, 1.052-3.393; P = .033). This association was largely driven by Arab women, who had significantly higher grand multiparity rates (62.6% vs 13.4%; median 5 vs 3 deliveries; P < .001). Conclusion: High parity, primarily among Arab women, was associated with increased DTC risk, with significance observed at 4 or more deliveries.