Functional parathyroid carcinoma is an exceptionally rare cause of primary hyperparathyroidism and typically presents with profound parathyroid hormone (PTH) elevation, severe hypercalcemia, and end-organ complications. Although 99mTc MIBI scintigraphy is a cornerstone for localizing hyperfunctioning parathyroid tissue, uptake intensity can vary and does not always reflect malignant potential.

A 24-year-old woman presented with progressive neck swelling, intermittent dyspnea, and severe bone pain. Laboratory evaluation revealed markedly elevated PTH (204 pg/mL) and hypercalcemia, while radiographs demonstrated multiple lytic lesions and pathological fractures consistent with metabolic bone disease. Dual-tracer scintigraphy using 99mTc pertechnetate and 99mTc MIBI identified a mildly MIBI-avid nodule at the posterior inferior right thyroid lobe. SPECT/CT confirmed a 2.2-cm lesion. Surgical excision was performed, and histopathology demonstrated parathyroid carcinoma with capsular and lymphovascular invasion and high mitotic activity.

This case highlights a notable biochemical–imaging discordance: despite severe biochemical hyperparathyroidism and a functional tumor, MIBI uptake was only modest. Potential explanations include mixed cellular composition, low oxyphil cell content, fibrosis, or increased expression of efflux transporters (P-glycoprotein/MRP) that reduce tracer retention. Such presentations may lead to underestimation of disease severity if imaging is interpreted without biochemical context.

Mild or moderate 99mTc MIBI uptake does not exclude parathyroid carcinoma in patients with markedly elevated PTH. Integrating functional imaging with biochemical severity is essential for accurate diagnosis and surgical planning. This case underscores the importance of multimodality assessment in detecting functional parathyroid carcinoma, especially in young patients.