Introduction: MIBI scintigraphy is commonly being used for the preoperative localization of parathyroid adenomas. Multiple studies showed MIBI uptake in pituitary adenomas are likely due to higher metabolic activity. When hyperfunctioning pituitary adenomas were reported, both had CD [1,2]. We present the third case of increased pituitary uptake on a MIBI scan later confirmed as CD. CaseA 64-year-old Caucasian female s/p renal transplantation for RPGN who presented for evaluation of hypercalcemia. Evaluation confirmed primary hyperparathyroidism with persistently elevated PTH levels 74-108 pg/ml (11-68), serum calcium levels 10.0-10.4 mg/dl (8.4-10.3), albumin 4.1-4.3 g/dl (3.6-5.1), phosphorus 3.0-3.2 mg/dl (2.5-4.5), creatinine 0.94-1.07 mg/dl. 24-hour urine calcium 60 mg/day (35-250). Vitamin D-25 OH level was 37 ng/dL (30-100). A sestamibi scan showed uptake in the right lower parathyroid, the midsternal chest region and the pituitary gland. MRI of the pituitary revealed a 7mm cystic pituitary microadenoma in the right posterior pituitary. CD was confirmed by the findings of persistently elevated 8 AM serum cortisol levels of 28.4 and 24.2 mcg/dl (4-22), ACTH levels of 59 and 39 pg/ml (10-48), and an elevated plasma free cortisol of 1.43 mcg/dl (0.07-0.93). CT of the abdomen showed L adrenal thickening suggesting adrenal hyperplasia from CD. Plasma cortisol suppressed to 1.2 mg/dl following 1 mg of dexamethasone. 24 urine for free cortisol 26.7 mcg/day (4-50). The patient had no proximal muscle weakness, striae or Cushingoid facial features. She had no hyperglycemia or hypertension. Patient was diagnosed with an ACTH secreting pituitary microadenoma with mild CD and adrenal hyperplasia. Her DXA scan showed osteoporosis. Genetic testing for MEN1 mutation was negative. Patient did not wish surgery for either her hyperparathyroidism or her CD and is being evaluated for medical treatment of hypercortisolism. Conclusion: There are two prior case reports of an incidentally discovered pituitary adenoma on sestamibi scan later diagnosed as CD [1,2]. Corticotrophs may have a strong affinity for sestamibi. Our case is the first, to our knowledge, of pituitary MRI confirmation of the ACTH secreting pituitary incidentaloma initially suspected by pituitary uptake on a sestamibi scan in a patient with hyperparathyroidism. Reference1. Kuhadiya ND et al. Incidentally Discovered ACTH-Secreting Pituitary Adenoma on a Sestamibi Scan in a Patient With Hyperparathyroidism. AACE Clinical Case Reports. 2015;1(3):e152-5. 2. Gierach M et al. The case of Cushing’s disease imaging by SPECT examination without manifestation of pituitary adenoma in MRI examination. Nuclear Medicine Review. 2005;8(2):137-9.