Supernumerary parathyroid tissue hidden by high uptake in the submandibular gland

A 63-year-old woman with chronic renal failure after subtotal parathyroidectomy was hospitalized for recurrent hyperparathyroidism, with a serum parathyroid hormone (PTH) level of 1,012 pg/ml (normal, 10 to 55 pg/ml). A presurgical Tc-99m sestamibi (925 MBq; 25 mCi) parathyroid scan revealed a left superior parathyroid remnant. After the remnant was surgically removed, the PTH level decreased to 164 pg/ml. Two days after surgery, however, the PTH level was elevated at 579 pg/ml. Another Tc-99m sestamibi parathyroid scan revealed a left supernumerary parathyroid gland. This finding was hidden on the previous study by overlying uptake by the left submandibular gland.