parathyroid adenoma
Summary
Parathyroid adenoma is a benign neoplasm of one parathyroid gland (usually a single gland) that autonomously secretes excess PTH, causing primary hyperparathyroidism. It is the most common cause of primary hyperparathyroidism (~85% of cases), presenting classically with hypercalcemia and low/normal phosphate.
Detail
Pathophysiology: Parathyroid adenomas arise from monoclonal proliferation of chief cells within a single parathyroid gland, often due to somatic mutations affecting cyclin D1 (PRAD1 gene) overexpression or, less commonly, MEN1 gene mutations. The adenoma autonomously secretes PTH independent of normal calcium feedback regulation, leading to primary hyperparathyroidism. This causes increased bone resorption (osteoclast activation via RANKL), increased renal calcium reabsorption and phosphate wasting, and increased renal 1-alpha-hydroxylase activity (boosting active vitamin D and intestinal calcium absorption).
Clinical features: Often asymptomatic, found incidentally on routine labs showing hypercalcemia. Classic mnemonic 'stones, bones, groans, and psychiatric overtones' - nephrolithiasis, bone pain/osteitis fibrosa cystica, abdominal pain/constipation/pancreatitis, and depression/fatigue/confusion. Labs show elevated calcium, elevated or inappropriately normal PTH, low phosphate, elevated urine calcium, and elevated alkaline phosphatase if bone involvement.
Associations: Can occur sporadically or as part of multiple endocrine neoplasia (MEN) syndromes - MEN1 (parathyroid, pituitary, pancreatic tumors) and MEN2A (parathyroid hyperplasia/adenoma, medullary thyroid carcinoma, pheochromocytoma). Prior neck irradiation is a risk factor.
Diagnosis: Confirmed biochemically (elevated calcium and PTH), localized preoperatively using sestamibi scan and/or neck ultrasound.
Treatment: Surgical parathyroidectomy is curative and indicated for symptomatic disease, or asymptomatic disease meeting criteria (calcium >1 mg/dL above normal, age <50, osteoporosis, renal impairment, or nephrolithiasis). Postoperative 'hungry bone syndrome' can cause transient hypocalcemia.
Differentiate from: Parathyroid hyperplasia (multiple gland involvement, seen in MEN syndromes), parathyroid carcinoma (rare, presents with very high calcium and palpable neck mass), and secondary/tertiary hyperparathyroidism (due to chronic kidney disease).
Sources
- First Aid for the USMLE Step 1
- Robbins and Cotran Pathologic Basis of Disease
- UpToDate: Primary Hyperparathyroidism
- Harrison's Principles of Internal Medicine
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