MON-417 - A Rare Case of Intrathyroidal Parathyroid Carcinoma

Introduction:
Parathyroid carcinomas are rare, mostly extrathyroidal, and usually present with severe hypercalcemia . We are reporting here an unusual case of intrathyroidal parathyroid carcinoma in a patient presenting with mildly elevated calcium and PTH levels. The intrathyroidal location adds more challenge to the case, as it can easily mimic a benign thyroid nodule and mask the real diagnosis.

Clinical

Case:
A 67-year-old gentleman with past medical history of hypertension, hyperlipidemia was evaluated for a elevated serum calcium of 10.8 mg/dl. His PTH was 104.6 pg/mL, vitamin D 39 ng/mL, with a normal renal function, and a 24-hour urine calcium of 347 mg. DEXA scan demonstrated severe osteoporosis, T-score for the lumbar spine: -3.2; T-score for the femoral neck: -2.4
T-score for the total hip: -2.2 and T-score for the distal third of the radius: -4.6. Sestamibi parathyroid scan was negative.
Thyroid ultrasound identified well-circumscribed hypoechoic nodule with marked peripheral blood flow and a feeding vessel possibly representing intrathyroidal parathyroid adenoma versus just thyroid nodule measuring 0.51 x 0.65 x 1.23 cm in the posterior portion of the right lobe.
Given these findings, fine-needle aspiration cytology was performed and revealed a benign pathology. However, PTH washout was suggestive of parathyroid tissue. Given the lesion’s entirely intrathyroidal location, a right hemithyroidectomy was performed for definitive diagnosis. Pathology studies finally revealed a solid proliferation of parathyroid cells with infiltrative growth, thick fibrous bands, and tumor cells wrapping around large vessels and intermingling with nerve structures, findings diagnostic of parathyroid carcinoma. No lymphovascular invasion was identified, and intraoperative PTH levels decreased significantly, supporting good prognosis. Follow up calcium and PTH returned to normal levels.

Conclusion:
This report shows how an intrathyroidal lesion can mask a case of parathyroid carcinoma. Mildly elevated calcium and PTH levels, absence of a neck mass, the negative FNA cytology, and the negative parathyroid scan can be misleading. FNA PTH washout is essential in making the diagnosis in this case. Given a rarity of the tumor, there are no sufficient clinical trials for use of chemotherapy or radiation therapy.
Long-term surveillance with calcium and PTH is needed with possible need for neck imaging if calcium increases is essential.

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