Intrathyroidal Fifth Parathyroid Adenoma: A Rare Cause of Primary Hyperparathyroidism

A 53-year-old female patient was incidentally found to have asymptomatic hypercalcemia, later diagnosed due to primary hyperparathyroidism (PHPT): ionized calcium 6.48 mg/dL (SI: 1.62 mmol/L) (reference range, 4.48-5.28 mg/dL [SI: 1.12-1.32 mmol/L]); total calcium 12.08 mg/dL (SI: 3.02 mmol/L) (reference range, 8.8-10.4 mg/dL [SI: 2.20-2.60 mmol/L]); and parathyroid hormone (PTH) 184.8 pg/mL (SI: 19.6 pmol/L) (reference range, 15-85 pg/mL [SI: 1.6-9.0 pmol/L]). Preoperatively, standard imaging modalities, including ultrasound (US), four-dimensional computed tomography (4DCT) and dual radiolabeled technetium-99 pertechnetate and sesta-methoxyisobutylisonitrile with single photon emission computed tomography (^99mTc-MIBI SPECT/CT), failed to localize a parathyroid adenoma. The patient underwent cervical exploration and parathyroidectomy where 4 orthotopic glands were identified, removing 2 mildly enlarged right-sided parathyroid glands and marking the 2 left-sided parathyroids with clip and suture; however, postoperative hypercalcemia persisted. Subsequent ¹⁸F-fluorocholine positron emission tomography/computed tomography (¹⁸F-FCH PET/CT) localized an intrathyroidal parathyroid adenoma. Fine needle aspiration (FNA) confirmed parathyroid tissue, and the patient underwent a right hemithyroidectomy, with biochemical cure. This case highlights the diagnostic and management challenges of an intrathyroidal fifth parathyroid adenoma causing PHPT, underscores potential pitfalls localizing parathyroid adenomas, and discusses the usefulness of ¹⁸F-FCH PET/CT imaging in challenging cases.