SUN-499 An Unusual Case of a Misleading Elevated Parathyroid Hormone Level

Background: Subtotal parathyroidectomy with reimplantation of parathyroid tissue into the forearm might be necessary in less common cases of primary hyperparathyroidism, due to multiple adenomas, hyperplasia or ectopic adenoma. An elevated PTH level after surgery is concerning for persistent or recurrent disease but could also be due to blood sampling. We present a case of erroneously elevated PTH due to lab draws near implanted parathyroid tissue. Clinical Case: A 63 y.o. female presented for follow up in 2017. She had a history of hypercalcemia due to primary hyperparathyroidism and had resection of 2 parathyroid glands together with a thyroidectomy in 2007. She had persistent hypercalcemia afterwards, and was suspected to have an ectopic parathyroid adenoma in the mediastinum based on Sestamibi and contrast-enhanced CT scans. She had a transcervical thymectomy with resection of an anterior mediastinal parathyroid adenoma and reimplantation of parathyroid tissue into the left forearm in 2012. PTH and calcium levels normalized afterwards. Her medical history included hypertension, GERD, vitamin D deficiency, osteopenia and postsurgical hypothyroidism. She had no history of renal disease and no family history of parathyroid disorders. She took levothyroxine 100 mcg daily, and intermittently took ergocalciferol 50,000 IU weekly. On exam, she had a well healed thyroidectomy scar and no cervical lymphadenopathy. Her thyroid gland was not palpable. Labs showed an elevated PTH level of 1389 pg/mL (17-66), a normal calcium level of 9.2 mg/dL (8.4-10.5), and a low/normal 25-OH vitamin D level of 30 ng/mL (25-80). She had normal creatinine and TSH levels. There was concern for validity of the results, but repeat labs again revealed a significantly elevated PTH level of 4643 pg/mL. Calcium level was normal at 9.0 mg/dL, and 25-OH vitamin D was 22 ng/mL. Given concern for skewed lab results from implanted parathyroid tissue in the left arm, the PTH level was redrawn from both arms. The PTH level in the right arm was 25 pg/mL, and in the left arm was 2865 pg/mL, indicating variability based on location. PTH level was normal at subsequent visits when drawn from the right arm. Conclusion: Aberrant PTH levels can be seen if labs are drawn upstream from the graft site. Limited case report data showed a difference in PTH levels between the grafted forearm and the remote site ranged from 328 to 3643 ng/l. The frequency of this phenomenon has been estimated to be around 4% (1). If a clinician acts on spurious levels, the patient could be subjected to unnecessary testing or surgery. PTH levels should be drawn distant from the graft site, preferably in the other arm, to make sure that true systemic levels are obtained. Reference: 1) Khalil D, Kerr P. PTH monitoring after total parathyroidectomy with forearm autotransplantation: potential for spuriously high levels from grafted forearm. J Otolaryngol-Head N. 2017; 46:49.