One Lobe, Two Pathologies: First Report of an Intrathyroidal Parathyroid Adenoma Invaded by Papillary Thyroid Cancer and Review of the Literature.

Intrathyroidal parathyroid adenomas (ITPAs) are rare causes of primary hyperparathyroidism, with type III variants, being the rarest. Coexistence of ITPA and papillary thyroid carcinoma (PTC) in the same lobe is exceedingly rare. The pathogenesis of this rare coexistence is unclear, possibly related to embryologic displacement or local microenvironmental factors. No prior reports have documented direct invasion of ITPA by PTC. We report a 63-year-old woman with hypercalcemia and osteopenia. Imaging suggested a right lobe ITPA. Hemithyroidectomy revealed a type II ITPA and multifocal PTC, with histopathology confirming carcinoma invasion into the adenoma. Postoperatively, calcium and PTH normalized. Completion thyroidectomy revealed a contralateral PTC focus, and adjuvant radioactive iodine was planned. Including this case, only 7 reports exist of concurrent ITPA and PTC, primarily in women aged 50 to 70 with right-lobe involvement. Diagnosis is difficult due to imaging overlap with benign nodules; although 99mTc-sestamibi and 4D-CT improve localization, fine-needle aspiration remains limited. This case is the first to document direct PTC invasion of an ITPA and highlights the importance of surgical excision in suspected ITPA, both for definitive treatment and to uncover occult malignancy. This paper aims to present the first reported case of concurrent ITPA and PTC within the same lobe, with histologically confirmed direct carcinoma invasion into the adenoma, review the existing literature on this rare coexistence and highlight the associated diagnostic challenges and surgical considerations.