A 52-year-old woman presents with bone pain, polyuria, and constipation. Serum calcium is 11.5 mg/dL (normal 8.5–10.5), serum phosphate is 2.8 mg/dL (normal 2.5–4.5), and alkaline phosphatase is elevated. PTH level is 185 pg/mL (normal 15–65). Which investigation is most appropriate to confirm the diagnosis and localize the pathology?

Explanation

## Clinical Scenario Analysis The patient presents with classic features of **primary hyperparathyroidism**: hypercalcemia, hypophosphatemia, elevated alkaline phosphatase, and markedly elevated PTH. The diagnosis is biochemically confirmed; the next step is **localization** of the parathyroid lesion (adenoma, hyperplasia, or carcinoma) to guide surgical intervention. ## Investigation of Choice: Technetium-99m Sestamibi Scintigraphy **Key Point:** Technetium-99m sestamibi (Tc-99m MIBI) parathyroid scintigraphy is the **gold-standard localization study** for primary hyperparathyroidism. ### Why Tc-99m MIBI? | Feature | Tc-99m MIBI | Alternative | |---------|-----------|-------------| | **Sensitivity** | 85–95% for adenoma | CT: 50–70% | | **Specificity** | High; minimal thyroid uptake | Ultrasound: operator-dependent | | **Detects** | Single adenoma, multiple glands, ectopic glands | CT better for anatomy, not function | | **Timing** | Dual-phase (early + delayed) imaging | — | | **Cost** | Moderate | — | **High-Yield:** Tc-99m MIBI is retained in mitochondria-rich cells (parathyroid adenomas have high metabolic activity). Early images show both thyroid and parathyroid uptake; delayed images show thyroid washout but **persistent parathyroid uptake** — the diagnostic hallmark. ### Clinical Pearl Tc-99m MIBI is superior to ultrasound for detecting **ectopic parathyroid glands** (mediastinal, intrathyroidal) and **multiglandular disease**, making it essential for surgical planning. ## Why Other Options Are Incorrect 1. **Serum 1,25-dihydroxyvitamin D**: Elevated in primary hyperparathyroidism (PTH stimulates 1α-hydroxylase), but this **confirms the diagnosis, not localizes** the lesion. Already biochemically diagnosed. 2. **High-resolution CT neck**: Provides **anatomical detail** but lacks **functional specificity**. Sensitivity only 50–70%; used as **adjunct** after positive MIBI, not as first-line localization. 3. **Urinary cyclic AMP and phosphate excretion**: Reflects **renal response to PTH** (confirms PTH excess), not lesion localization. Diagnostic, not localizing. ## Diagnostic Algorithm ```mermaid flowchart TD A["Hypercalcemia + elevated PTH"]:::outcome --> B{"Diagnosis confirmed?"}:::decision B -->|"Yes"| C["Localize the lesion"]:::action C --> D{"Imaging modality?"}:::decision D -->|"First-line"| E["Tc-99m MIBI scintigraphy"]:::action D -->|"Adjunct/Ectopic?"| F["High-res CT or MRI"]:::action E --> G["Positive → Surgery"]:::outcome E --> H["Negative → Repeat or advanced imaging"]:::decision ``` **Citation:** Harrison 21e Ch 424; Endocrinology Board Review (Endocrine Society)