A 52-year-old woman presents with hypercalcemia (serum calcium 11.5 mg/dL) and elevated PTH levels (120 pg/mL; normal <65). She has a 10-year history of nephrolithiasis and chronic kidney disease stage 3. What is the most common anatomical site of pathology in this clinical scenario?

Explanation

## Most Common Cause of Primary Hyperparathyroidism **Key Point:** Parathyroid adenoma accounts for approximately 80–85% of all cases of primary hyperparathyroidism, making it the single most common pathological entity in this disease. ### Epidemiology and Frequency | Pathology | Frequency (%) | Clinical Features | |-----------|---------------|-------------------| | Adenoma | 80–85 | Solitary lesion, usually <2 cm, unilateral | | Hyperplasia | 10–15 | Multiglandular involvement, bilateral | | Carcinoma | 1–2 | Rare, severe hypercalcemia, palpable mass | | Ectopic PTH | <1 | Very rare, associated with malignancy | **High-Yield:** In the given case, the patient has: - Elevated serum calcium with elevated PTH (diagnostic of primary hyperparathyroidism) - Chronic kidney disease and nephrolithiasis (classic complications of PTH excess) - No mention of malignancy or multiple gland involvement This clinical picture is **classic for parathyroid adenoma**, which is the overwhelming majority cause of primary hyperparathyroidism in non-syndromic patients. ### Why Adenoma Is Most Common 1. **Monoclonal origin:** Adenomas arise from a single clonal proliferation of parathyroid cells, often due to somatic mutations (e.g., CCND1, MEN1 inactivation). 2. **Age of onset:** Typically presents in the 5th–6th decade, matching the patient's age. 3. **Unilateral disease:** Single gland involvement is the rule; bilateral disease suggests hyperplasia. 4. **Biochemistry:** PTH is typically elevated but not massively so (as in carcinoma). **Clinical Pearl:** Parathyroid adenoma is the **"go-to" diagnosis** in primary hyperparathyroidism unless there is a specific clue pointing to hyperplasia (family history of MEN, multiple gland enlargement on imaging) or carcinoma (severe hypercalcemia >13 mg/dL, palpable neck mass, renal insufficiency, bone disease). ### Differential Reasoning - **Hyperplasia** occurs in ~10–15% of cases, often associated with MEN syndromes or familial hypocalciuric hypercalcemia (FHH)—not suggested here. - **Carcinoma** is rare (<2%) and presents with much higher calcium levels and more aggressive clinical course. - **Ectopic PTH** is extremely rare and requires an identifiable malignancy (lung, kidney, ovary)—not mentioned. [cite:Harrison 21e Ch 397]