## Most Common Endocrine Tumor in MEN-1 **Key Point:** Parathyroid adenoma (or hyperplasia) is the most common manifestation of MEN-1, occurring in 90–95% of affected individuals, often presenting as the earliest clinical sign. ### Frequency of MEN-1 Tumors | Tumor Type | Frequency (%) | Age of Onset | |---|---|---| | Parathyroid adenoma/hyperplasia | 90–95 | 20–25 years | | Pancreatic NET (gastrinoma, insulinoma) | 60–70 | 30–40 years | | Pituitary adenoma (prolactinoma) | 30–40 | 30–40 years | | Adrenocortical tumor | 10–40 | Variable | ### Clinical Presentation **High-Yield:** Parathyroid involvement in MEN-1 is typically: - Multiglandular (hyperplasia of all four glands) - Results in primary hyperparathyroidism - Presents with hypercalcemia, nephrolithiasis, and osteoporosis - Often the first manifestation detected (20s–30s) ### MEN-1 Pathophysiology **Key Point:** MEN-1 is caused by mutations in the *MEN1* gene (chromosome 11q13), which encodes menin, a tumor suppressor protein. Loss of menin function leads to uncontrolled proliferation of neuroendocrine cells. ### The "3 P's" of MEN-1 **Mnemonic:** **PPP** = **P**arathyroid, **P**ancreas, **P**ituitary - These are the three major organ systems involved - Parathyroid is most common and earliest - Pancreatic NETs (gastrinomas most common) occur in 60–70% - Pituitary adenomas (prolactinomas predominate) in 30–40% **Clinical Pearl:** Screening for MEN-1 should begin with serum calcium and PTH levels, as parathyroid disease is the earliest and most frequent manifestation. Genetic testing is indicated if MEN-1 is suspected. [cite:Robbins 10e Ch 24]
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