Increased Prevalence of Autoimmune Rheumatologic Diseases in Patients With Primary Hyperparathyroidism.

Background Parathyroid hormone (PTH) and Dickkopf-related protein 1 (DKK-1) have been mentioned together at the intersection of autoimmune rheumatologic diseases (ARDs) and osteoimmunology. However, few studies have evaluated the association between primary hyperparathyroidism (PHPT) and ARDs. Methodology This retrospective study included 225 PHPT patients and 386 patients with thyroid nodules as a control group. The electronic hospital records of all patients were screened going back nine years for the presence of ARDs. Patients who were diagnosed at least three months ago, had complete serologic tests, and were continuing with rheumatologic follow-up were included. Results The prevalence of ARDs in the PHPT group was 9.77% (22/225), while the prevalence of ARDs in the CG was 1.04% (4/386, p < 0.001). The prevalence of rheumatoid arthritis in the PHPT group was 4.4% (10/225), ankylosing spondylitis 3.1% (7/225), systemic lupus erythematosus 0.88% (2/225), Behçet's disease 0.88% (2/225), and mixed connective tissue disease 0.44% (1/225). Of the 22 patients with ARDs, 21 (95.45%) were diagnosed before they were diagnosed with PHPT, and the median time from diagnosis with ARD to the onset of PHPT was 36 months (interquartile range = 61.5). Logistic regression analysis showed a positive correlation between the duration of PHPT and ARDs (odds ratio (OR) = 1.06; 95% confidence interval (CI) = 1.02-1.09, p < 0.001) and a negative correlation between ARDs and calcium levels (OR = 0.26; 95% CI = 0.09-0.79, p = 0.018). Conclusions The prevalence of ARDs increased in PHPT patients and PHPT accompanying ARDs developed after rheumatologic disease. ARDs with PHPT are cases with a prolonged duration of PHPT and mildly elevated calcium, probably preceded by parathyroid hyperplasia. Therefore, the factors that cause ARDs may trigger a process that leads to mild PHPT.