Objectives: Primary hyperparathyroidism (pHPT) is a common endocrine disease. The aim of this study was to assess the effect of the presence of thyroid nodules, adenoma weight, and ectopic localization on the sensitivity of different imaging studies in patients with hyperparathyroidism. Study Design: A series of 125 patients with pHPT who underwent low-frequency ultrasonography, high-frequency ultrasonography, Thallium-Technetium scintigraphy, sestamibi scintigraphy, and combined technique was reviewed retrospectively. Patients were divided in two groups depending on the presence or absence of thyroid nodules. Results: The overall sensitivity of low-frequency ultrasonography, high-frequency ultrasonography, Thallium-Technetium scintigraphy, sestamibi scintigraphy, and combined technique was 69%, 89%, 71%, 86%, and 98%, respectively. The sensitivity of these imaging studies was 94%, 100%, 94%, 96%, and 100%, respectively, in our patients with no thyroid nodules but decreased to 54%, 84%,54%, 81%, and 79%, respectively, in the presence of thyroid nodules. The parathyroid adenoma weight in true-positive imaging studies was significantly higher than those in false-positive and false-negative imaging studies. The numbers of ectopic parathyroid adenomas were found to be higher in the group of parathyroid adenomas undetected with ultrasonography when, compared with ultrasonographically detected adenomas. Conclusion: In patients with parathyroid adenoma, the sensitivity of imaging studies correlates with the presence of thyroid nodules, adenoma weight, and ectopic localization.