Background/aim: Primary hyperparathyroidism (PHPT) is characterized by increased calcium (Ca) and parathyroid hormone (PTH) levels. Surgical removal of the culprit hyperfunctioning parathyroid gland is the preferred treatment. In this study, we aimed to determine whether PTH-washout or cytological examination of suspicious lesions was superior in MIBI-negative patients diagnosed with PHPT.Materials and methods: We retrospectively evaluated the medical records of 98 patients diagnosed with PHPT. Seventy-six patients who had positive parathyroid scintigraphy and who did not undergo parathyroidectomy in our center due to various reasons were excluded. We evaluated the remaining 22 patients with negative scintigraphy. Medical records including PTH levels in serum and washout fluid of fine-needle aspiration biopsy (FNAB), biochemical data, cytological results of FNAB, and histologic reports were reviewed.Results: The patients had a mean age of 50 +- 9 (31-72) years, serum Ca of 10.9 +- 0.5 (10.3-12.7) mg/dL, serum PTH of 285 +- 156 (107.2-679) pg/mL, and PTH of washout fluid of 19,523 +- 38,632 (1410-166,000) pg/mL. Cytological evaluation revealed insufficient material in 9 patients and cells of indeterminate origin in 4 patients.Conclusion: Our results showed that when evaluating ambiguous lesions on neck ultrasound, measuring the PTH level in washout fluid of FNAB is a reliable and effective method for diagnosis of parathyroid lesions and is superior to FNAB for localization.