## Histopathology of Parathyroid Adenoma **Key Point:** The histological hallmark of a parathyroid adenoma is a **solitary enlarged gland with a compressed rim of normal parathyroid tissue at the periphery**, distinguishing it from hyperplasia and carcinoma. ### Histological Features of Parathyroid Lesions | Feature | Adenoma | Hyperplasia | Carcinoma | | --- | --- | --- | --- | | **Number of glands involved** | Single (80–85%) | All four (or 3.5) | Single (usually) | | **Rim of normal tissue** | Present (compressed) | Absent | Absent; infiltrative | | **Fat content** | Preserved in contralateral glands | Reduced in all glands | Variable; may have invasion | | **Cellularity** | Monoclonal; uniform | Polyclonal; mixed | Atypical; mitotic figures | | **Fibrous capsule** | Well-defined | Absent | Incomplete; breached | | **Weight** | Usually > 1 g | Usually < 1 g per gland | Often > 1 g | **High-Yield:** The **compressed rim of normal parathyroid tissue** at the edge of an adenoma is a critical diagnostic feature. This rim is absent in hyperplasia (where all glands are affected) and in carcinoma (where there is infiltration and loss of capsule integrity). **Clinical Pearl:** In adenoma, the contralateral three glands are typically **suppressed and atrophic** due to negative feedback from elevated PTH. This is why intraoperative PTH monitoring (drop ≥50% after adenoma removal) confirms successful surgery. **Mnemonic:** **"Adenoma = Alone + Atrophic"** — A single enlarged gland (adenoma) with atrophic, fat-preserved contralateral glands is the hallmark of adenoma. ### Distinction from Hyperplasia In **parathyroid hyperplasia**, all four glands are enlarged and hypercellular with loss of fat in all glands. There is no compressed rim of normal tissue because the entire gland is hyperfunctional. This is seen in familial hyperparathyroidism, MEN syndromes, and secondary/tertiary hyperparathyroidism.
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