According to various authors, thyroid disorders like Hashimoto's thyroiditis (HT), diffuse goiter or multinodular goiter, Graves' disease, medullary or papillary carcinoma could be found in a number of patients with primary hyperparathyroidism (PHPT). This association is more common in elderly women. Neck irradiation, lithium treatment and elevated TSH levels have been suggested as some of the possible causes of this co-existance. The aim of this study was to investigate and determine the prevalence of patients having both HT and PHPT, and the possible relation between these two diseases. We conducted a prospective study during three and a half years. This study included 45,231 patients, which were referred by their general practitioner or endocrinologist, under suspicion of having thyroid and/or parathyroid disease. In these patients we measured serum levels of the following parameters: anti-thyroid peroxidase antibodies (antiTPO-Ab), anti-thyroglobulin antibodies (Tg-Ab), anti-TSH-receptor antibodies (TSHR-Ab), thyroid-stimulating hormone (TSH), parathyroid hormone (PTH) and calcium (Ca). In 2,267 of these 45,231 patients (5.01%) we noticed elevated antiTPO-Ab (3542±3407IU/mL), with statistical significant difference from normal values (normal range 0-70IU/mL), P<0.05, and normal levels of other antithyroid antibodies (Tg-Ab, TSHR-Ab). All patients with elevated antiTPO-Ab were assumed to have HT. Within this group, 43 patients (1.89%) also had elevated serum levels of PTH (112.4±33.2pg/mL, normal range 8-76pg/mL) as well as elevated serum levels of calcium (2.92±0.06mmol/L, normal range 2.2-2.65mmol/L). These laboratory findings, accompanied with clinical symptoms, satisfied the criteria for PHPT. The mean age in this subgroup was 60.5±12.2 years. All 2,267 patients had normal or slightly elevated TSH levels. In conclusion, although the reported rate of prevalence of PHPT in the general population is about 0.3%, our results indicated a 1.89% occurrence of PHPT in 2267 patients with HT in central Serbia. This may be due to the autoimmune inflammatory process in HT supporting PHPT to PTH or calcium supporting HT or to common genetical predisposition of both entities.