Thymoma and parathyroid adenoma: false-positive imaging and intriguing laboratory test results.

IMPORTANCE Parathyroid hormone (PTH)-secreting thymomas are an exceedingly rare entity. A PTH-secreting thymoma was discovered in the workup of a patient with primary hyperparathyroidism. A concomitant parathyroid adenoma was removed from the same patient. We present the intriguing clinical course and review the literature on this rare entity. In addition, we discuss the use of scanning with technetium Tc 99m sestamibi, the PTH assay, and cervical ultrasonography in the workup of a parathyroid adenoma. OBSERVATIONS Scanning with technetium Tc 99m sestamibi demonstrated false-positive uptake of the mediastinal thymoma and false-negative uptake of the true cervical parathyroid adenoma. After removal of the thymoma, the parathyroid adenoma demonstrated appropriate uptake on a follow-up scan. After removal of the parathyroid adenoma, the hyperparathyroidism was cured. CONCLUSIONS AND RELEVANCE Given the extremely rare incidence of a PTH-secreting thymoma with a concurrent parathyroid adenoma, we do not recommend alterations in the diagnostic algorithm for primary hyperparathyroidism. However, in this case, the need for 2 separate operations may have been avoided by obtaining an ultrasonogram to further explore the findings on the technetium Tc 99m sestamibi scan. We recommend that both studies be considered in unclear cases of primary hyperparathyroidism.