BackgroundTo investigate the relationship between primary hyperparathyroidism (pHPT) and papillary thyroid cancer (PTC).MethodsThe perioperative findings of 275 patients with pHPT who underwent surgery between January 2014 and December 2017 were retrospectively reviewed. Thirty-one patients were diagnosed with pHPT and PTC concurrently. Pathology results and demographic findings of these patients were compared with 186 patients who underwent thyroidectomy and diagnosed with PTC at the same time interval.ResultsThe co-occurrence of pHPT and PTC was 11.3% (31/275). The median ages of the pHPT, pHPT+PTC, and PTC groups were 55, 57, and 50years old, respectively (p<0.001). The diameter of tumor was smaller in the pHPT+PTC group [median 7mm (range 0.5-25mm) vs. 15mm (range 1-100mm)], with higher rates of microcarcinomas (p<0.001), than the patients in the PTC group. Examination of tumor morphology showed higher rates of tumor capsule invasion and multicentricity in the pHPT+PTC group than those in the isolated PTC group (p=0.02, p=0.04, respectively).ConclusionThe pHPT+PTC group had significantly smaller tumor diameter than the PTC group. This result may support the idea that pHPT leads to overdiagnosis of PTC. However, observation of high rates of tumor capsule invasion and multicentricity in the pHPT+PTC group may suggest an associative etiology with more aggressive PTC.