MON-775 Primary Hyperparathyroidism Complicated by Deep Vein Thrombosis, unexplained Urticaria, and Diagnostic Challenges Due to Coexisting Thyroid Nodule: A Case Report

Abstract Disclosure: R. Reguram: None. S. Neti: None. D. Yerasuri: None. Background: Primary hyperparathyroidism (PHPT) is often asymptomatic at presentation and its association with thromboembolic events remains underexplored. Hypercalcemia can predispose patients to a hypercoagulable state, a risk that is often overlooked in the absence of other procoagulant conditions. This case illustrates a complex interplay of hyperparathyroidism, unexplained urticaria, and deep vein thrombosis (DVT) with diagnostic challenges from a coexisting thyroid nodule. Case Presentation: A 44-year-old female with a history of asymptomatic PHPT presented to the emergency department with acute onset pain and swelling of the left leg and knee. She denied trauma, prolonged immobilization, recent travel, or personal/family history of thrombophilia. Her medical history was notable for persistent hypercalcemia (10.7-12.3 mg/dL) and elevated PTH levels (93.6-157.4 pg/mL). She also had chronic vitamin D deficiency (16.9-25.3 ng/mL). On examination, the patient had swelling of the left lower extremity without erythema, warmth, or tenderness. D-dimer level was elevated at 367 ng/mL, prompting a venous duplex ultrasound, which confirmed the presence of a peroneal vein DVT. Coagulation studies, including prothrombin time (PT), activated partial thromboplastin time (aPTT), and fibrinogen levels, were within normal limits. Tests for Factor V Leiden, prothrombin gene mutation, and antiphospholipid syndrome, were negative. The patient also reported a two-year history of episodic urticaria and facial swelling. Workup for allergic etiologies, including IgE levels and chronic urticaria index, was unremarkable. Imaging studies included an ultrasound of the neck that revealed a 1.4 cm hypoechoic nodule in the left thyroid lobe, consistent with a benign thyroid lesion. A Tc99 sestamibi scan that was done also failed to identify her parathyroid adenoma due to tracer uptake from the coexisting thyroid nodule. Given the strong clinical suspicion despite discordance between biochemical evidence of PHPT and imaging findings, the patient underwent surgical exploration, revealing a hypercellular left superior parathyroid adenoma (1.2 x 1.1 x 0.9 cm) and a coexisting benign hyperplastic thyroid nodule. Outcome: Following parathyroidectomy, the patient’s calcium levels normalized to 8.8 mg/dL, and PTH decreased to 10.3 pg/mL. Anticoagulation therapy was resumed. Urticaria and facial swelling resolved completely within three weeks, suggesting a possible causal relationship with hypercalcemia. Conclusion: This case features the rare but worrisome risk of DVT due to hypercalcemia in PHPT, the challenges of diagnosing parathyroid pathology with concurrent thyroid disease, and the need for awareness of unusual hypercalcemic manifestations like urticaria. It explains the critical role of clinical decision-making even when imaging is inconclusive. Presentation: Monday, July 14, 2025