Falsely Positive Parathyroid Scan in a Patient With Medullary Thyroid Cancer

Background: Technitium 99m sestamibi is a very useful method for preoperative localization in hyperparathyroidism. It have some limitations including false positivity due to thyroid pathology. We present a patient with positive Technitium 99m sestamibi uptake by a medullary thyroid cancer Case Presentation: A 67-year-old female referred to the endocrine clinic for evaluation of medullary thyroid cancer diagnosed by fine needle aspiration of left side thyroid nodule. TSH level was normal with high normal serum calcium and high PTH levels. No family history of medullary thyroid cancer, hyperparathyroidism or pheochromocytoma. Her plasma metanephrines were normal. Preoperative evaluation included a parathyroid scan with oral I-123 and Intravenous Technitium 99m sestamibi. I-123 scan showed a cold nodule in the left thyroid lobe and the subtraction images demonstrated persistent uptake of Technitium 99M sestamibi which was reported by the radiologist as a parathyroid adenoma. Repeated serum calcium was normal and intraoperative evaluation of parathyroid glands demonstrated no abnormal enlargement confirming that parathyroid scan was falsely positive secondary to medullary thyroid cancer. Conclusion: This case highlights the importance of careful interpretation of nuclear parathyroid scan results. False positive results have been reported with both benign and malignant thyroid disease including medullary thyroid cancer.