Congenital thyroid abnormalities are rarely seen. They mostly include hemiagenesis with or without involving the isthmus. In this report, we present a case of bilateral lobe agenesis with hypertrophied isthmus and high calcium and elevated PTH levels which were detected during routine examinations of a 49-year-old female patient. Some findings consistent with parathyroid pathology on the right side were determined in parathyroid scintigraphy. At thyroid scan and neck ultrasonography there was no sign of bilateral thyroid tissue except a mass localized isthmus. The right lower parathyroidectomy and thyroidectomy for isthmus were performed; the pathology report was shown as parathyroid adenoma and nodular colloidal goiter. This case of bilobar agenesis with incidental primary hyperparathyroidism due to single parathyroid adenoma is the first case reported in literature.