PSAT213 Intrathyroidal Parathyroid Adenoma Diagnosed on Thyroseq v2

BACKGROUND: The incidence of intrathyroidal parathyroid adenoma varies from <1% to 6% and can be a diagnostic dilemma. Intrathyroidal parathyroid adenomas can have suspicious sonographic features that prompt fine needle aspiration (FNA) to evaluate for thyroid cancer. Differentiation between thyroid and parathyroid tissue on cytology can be difficult, and these lesions may be diagnosed as indeterminate thyroid nodules. Molecular testing is a useful adjunct that allows for identification of parathyroid adenomas in this setting. CLINICAL CASE: A 59-year-old female with osteopenia and a recent history of traumatic vertebral fracture presented for workup of asymptomatic hypercalcemia. Labs showed intact parathyroid hormone (iPTH) 168 pg/mL (reference 11-51 pg/mL), serum total calcium 12.2 mg/dL (reference range 8.6-10.4 mg/dL), albumin 4.7 g/dL (reference 4.1-5.3 g/dL), phosphorus 2.1 mg/dL (reference 2.3-4.4 mg/dL), creatinine 0.7 (reference 0.6-1.3 mg/dL), and 25-OH vitamin of 28 ng/mL (reference 20-50 ng/mL). Cervical ultrasound showed a 2.1 cm hypoechoic left thyroid nodule. Subsequent 99mTc-Sesatmibi scan showed increased tracer retention in the left posterior thyroid lobe. FNA of the left thyroid nodule was performed with cytology showing follicular lesion of undetermined significance (FLUS). Molecular testing with Thyroseq v2 showed parathyroid tissue consistent with intrathyroidal parathyroid adenoma. The patient underwent successful left thyroid lobectomy with removal of an intrathyroidal parathyroid gland, with subsequent decrease of iPTH to 44 pg/mL (reference 11-51 pg/mL) and total serum calcium to 9.5 mg/dL (reference 8.6-10.4 mg/dL). Surgical pathology confirmed parathyroid adenoma. DISCUSSION: Due to the ubiquity of thyroid FNAs to rule out malignancy, there has been a rise in the number of incidental parathyroid gland aspirations. Thyroid lesions with diagnosis of FLUS or oncocytic features are prone to misdiagnosis due to similarities in morphologic overlap and anatomic proximity. Molecular testing such as Thyroseq v2 routinely tests for PTH and can identify parathyroid tissue in lesions initially diagnosed as indeterminate thyroid nodules. Newer molecular test versions including ThyroSeq v3 similarly have high analytical accuracy in detecting thyroid cancer and parathyroid lesions. Presentation: Saturday, June 11, 2022 1:00 p.m. - 3:00 p.m.