Primary hyperparathyroidism (PHPT) secondary to parathyroid gland adenoma is mildly symptomatic and thus often incidentally diagnosed. In this report, we present a case of a 46-year-old man who was treated for hypertension and other nonspecific complaints. An elevated level of blood calcium led to the suspicion of parathyroid adenoma. Technetium-99m-methoxyisobutylisonitrile (99mTc-MIBI) planar and single-photon emission computed tomographic scintigraphy (SPECT/CT) confirmed the presence of a 10-mm-wide adenoma behind the sternal handle in the anterior mediastinum. The tumor was excised by Kocher's cervical access along with the right and left upper horns of the thymus gland and was histologically confirmed as being of parathyroid nature. However, blood parathyroid hormone and calcium remained persistently elevated. Repeated scintigraphy imaging revealed the presence of another retrosternal tracer focus at the level of Ludwig's angle. The patient was reoperated with the longitudinal sternotomy access, and thymus remnants, parathymic nodule, and fragments of mediastinal fat and right parietal pleura were removed. On the second postoperative day, parathyroid hormone and calcium reverted to normal values, but the patient remained hypertensive. Despite the successful surgical treatment, the patient remained hypertensive suggesting that the underlying reason was a familial hypertensive disease rather than parathyroid adenoma. In conclusion, this report underscores the need for diagnostic vigilance in the case of persisting hypercalcemia with hypertension and diagnostic and surgical difficulties in the management of ectopic PHPT secondary to parathyroid gland adenomas.