Coexistence of Primary Hyperparathyroidism and Differentiated Thyroid Carcinoma: Is It a Coincidence?

Objective: Coexistence of primary hyperparathyroidism and differentiated thyroid carcinoma has been reported from time to time. However, the clinical features and risk factors of this association are unclear. In this study, we aimed to evaluate the relationship between primary hyperparathyroidism and differentiated thyroid carcinoma. Methods: The files of patients who were diagnosed with parathyroid adenoma and underwent parathyroid surgery in the endocrinology outpatient clinic between January 2015 and June 2021 were reviewed retrospectively. The clinical signs, biochemical abnormalities, and histological features of the patients were analyzed. Results: This study included 255 patients who were operated on with the diagnosis of parathyroid adenoma. When the ultrasonography reports of the patients were evaluated, thyroid nodules were detected in 100/255 (39.2%) patients. When the postoperative histology of the patients was evaluated, differentiated thyroid carcinoma was detected in 35/255 (13.7%) patients. Parathormone and calcium levels were found to be significantly lower in patients with differentiated thyroid carcinoma compared to patients with benign thyroid nodules (P <.05). In logistic regression analysis, there was a significant correlation between the presence of differentiated thyroid carcinoma and low serum calcium level (odds ratio: 0.031; 95% CI: 0.001-0.654; P =.035). In the receiver operating characteristic curve analysis, we found that serum calcium level <11.05 mg/dL in patients with parathyroid adenoma has good capacity to differentiate differentiated thyroid carcinoma from benign thyroid nodule (area under the curve: 0.648, P =.032, 71.4% sensitivity, and 72.7% specificity). Conclusion: We can say that the incidence of thyroid nodules increases in parathyroid adenoma compared to the general population and the probability of differentiated thyroid carcinoma increases as the calcium level decreases in these nodules.