A newly reported case in Sage Open Pathology highlights a rare but diagnostically important scenario in endocrine pathology: the coexistence of an intrathyroidal parathyroid adenoma and papillary thyroid carcinoma within the same thyroid lobe, with direct invasion of carcinoma into the parathyroid lesion. According to the authors, this is the first documented example of papillary thyroid carcinoma infiltrating an intrathyroidal parathyroid adenoma, underscoring challenges in preoperative localization and histopathologic interpretation.
Intrathyroidal parathyroid adenomas are an uncommon cause of primary hyperparathyroidism, accounting for less than 6% of cases. True intrathyroidal lesions, which are completely embedded within thyroid parenchyma, are even rarer. Because these lesions can closely resemble benign thyroid nodules on imaging and cytology, they are frequently misidentified or overlooked before surgery. The simultaneous presence of papillary thyroid carcinoma further complicates diagnosis, with only a small number of similar cases reported in the literature.
The case involved a 63-year-old woman evaluated for hypercalcemia and osteopenia. Laboratory testing showed elevated parathyroid hormone levels consistent with primary hyperparathyroidism. Localization studies, including technetium-99m sestamibi scintigraphy and 4D computed tomography, identified a small lesion in the right thyroid lobe suspected to be an intrathyroidal parathyroid adenoma. No definitive thyroid malignancy was identified preoperatively.
Following right hemithyroidectomy, histopathologic examination revealed a hypercellular parathyroid adenoma embedded within thyroid tissue. Careful microscopic evaluation also identified multifocal papillary thyroid carcinoma with classic nuclear features. Notably, carcinoma cells were seen directly invading the parathyroid adenoma, a finding not previously described. Surgical margins were negative, and subsequent completion thyroidectomy identified an additional microscopic papillary thyroid carcinoma focus.
This case underscores several diagnostic considerations. First, intrathyroidal parathyroid tissue should remain in the differential diagnosis when evaluating thyroid nodules in patients with biochemical evidence of hyperparathyroidism. Second, fine-needle aspiration biopsy may be misleading in this setting, as parathyroid adenomas can mimic follicular or papillary thyroid lesions cytologically. Prior studies cited by the authors indicate that parathyroid hormone washout from needle aspirates may improve diagnostic accuracy, although this technique is not routinely used.
Most importantly, the case demonstrates why thorough histologic examination of thyroidectomy specimens performed for presumed parathyroid disease is essential. The presence of a parathyroid adenoma does not exclude concurrent thyroid malignancy, and rare patterns such as direct tumor invasion may only be detected on permanent sections.
By drawing attention to this unusual coexistence, the authors highlight a diagnostic pitfall that has direct implications for surgical pathology, endocrine workups, and patient management, reinforcing the need for close clinicopathologic correlation in complex endocrine cases.
The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Source: Sage Open Pathology
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