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

Abstract

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.

Keywords: Primary hyperparathyroidism; case report; parathyroid carcinoma (PC); parathyroid hormone (PTH); parathyroidectomy.

Figure 1

USG of thyroid shows two well defined TIRADS 1 thyroid nodules measuring 1.7 cm × 2.3 cm × 1.6 cm (A) and 2.3 cm × 3.2 cm × 3.1 cm (B), with no worrisome features. TIRADS, thyroid imaging, reporting and data system; USG, ultrasound.

Figure 2

Parathyroid scintigraphy showed right thyroid nodule/colloid cyst, no evidence of parathyroid adenoma.

Figure 3

SPECT/CT of the neck showed increased uptake of tracer at the left upper pole of thyroid gland, measuring 0.8 cm × 0.7 cm. SPECT/CT, single-photon emission computed tomography and computed tomography.

Figure 4

Gross specimen of right hemithyroidectomy, weighing 6.5 g, measuring 3.5 cm × 3.5 cm.

Figure 5

Hematoxylin and eosin stain demonstrates invasion of neoplastic cells into adjacent thyroid follicles (10× magnification).

Figure 6

Hematoxylin and eosin stain shows monomorphic neoplastic chief cells arranged in trabecular and solid patterns, interspersed with thin, incomplete fibrous septa (40× magnification).