Intrathyroidal parathyroid carcinoma masquerading as a cystic thyroid nodule: a case report.

Background: Parathyroid carcinomas (PCs) are uncommon malignancies, accounting for an estimated 0.005% of all cancers. Intrathyroidal PCs are exceedingly rare, with fewer than 20 reported cases. Severe hypercalcemia and high parathyroid hormone (PTH) levels at presentation are risk factors for PC. The majority of cases are diagnosed postoperatively. Preoperative localization of PC can be challenging, especially in cystic lesions, where the lack of oxyphil cell predominance may reduce the sensitivity of technetium-99m (^99mTc)-methoxyisobutylisonitrile (MIBI) scans.

Case description: We report a female patient diagnosed with primary hyperparathyroidism, presenting with a PTH level of 192 pmol/L (1.8-8 pmol/L) and severe hypercalcemia. Preoperative ^99mTc-MIBI imaging localized a left inferior hyperfunctioning parathyroid gland. Excision of the left parathyroid gland did not yield the expected fall in intraoperative parathyroid hormone (IOPTH) levels. However, an earlier ultrasound scan detected a concomitant right thyroid cyst. The cyst fluid was aspirated and sent for IOPTH assay, which revealed a high PTH level (1,060 pmol/L). The patient subsequently underwent a right hemithyroidectomy. IOPTH levels showed a 78% reduction post-surgery. Histology of the right lobe confirmed an intrathyroidal right PC amid benign thyroid follicles. The serum PTH level normalized to 1.1 pmol/L postoperatively. The patient was discharged in good condition and is undergoing annual ultrasound and PTH level surveillance.

Conclusions: This case highlights an unusual presentation of PC masquerading as a cystic thyroid nodule. Clinicians should consider PC in the differential diagnosis of cystic neck lesions. Aspiration of cystic fluid for PTH assay may be a valuable adjunct in diagnosis, though further evidence is needed.