Draw-it-out method: Primary hyperparathyroidism

Primary hyperparathyroidism (PHPT) is one of those USMLE “free points” topics if you can draw the lab pattern fast: high calcium + high PTH + low phosphate—then connect it to stones, bones, groans, and psychiatric overtones. Let’s lock it in with a quick sketchable visual and the exact high-yield pivots Step loves.

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The Draw-it-out Method (30-second sketch)

Grab a scrap paper and draw 3 boxes: Ca, PTH, Phos.

Step 1: Draw the lab triad

• Ca: ↑
• PTH: ↑
• Phosphate (PO$_4^{3-}$): ↓

Step 2: Add the mechanism arrows

Under PTH, draw 3 arrows:

1. Bone: ↑ resorption → Ca ↑
2. Kidney:
   • ↑ Ca reabsorption → Ca ↑
   • ↓ phosphate reabsorption (proximal tubule) → Phos ↓
   • ↑ 1$\alpha$-hydroxylase → ↑ 1,25-(OH)$_2$ vitamin D
3. Gut (via vitamin D): ↑ Ca absorption

That’s the whole disease in one doodle.

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One-liner mnemonic (shareable)

“Primary hyperPTH: the gland is the problem—PTH is high on purpose, Ca follows, phosphate gets dumped.”

Or even tighter:

“PHPT = PTH ↑ → Ca ↑, PO$_4$ ↓.”

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High-yield causes (what Step expects)

Most common etiologies

• Parathyroid adenoma (single benign tumor) — most common
• Parathyroid hyperplasia (often all glands)
• Parathyroid carcinoma — rare but classic for very high Ca + palpable neck mass

Syndrome association

• MEN1: “3 P’s”
  Pituitary, Pancreas, Parathyroid
• MEN2A can have hyperparathyroidism too (less emphasized than MEN1)

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Symptoms: translate the labs into clinical clues

Think hypercalcemia symptoms first:

• Stones: calcium oxalate/phosphate kidney stones
• Bones: bone pain, fractures (increased resorption)
• Groans: constipation, abdominal pain, pancreatitis
• Psychiatric overtones: fatigue, depression, confusion

Extra Step buzzwords:

• Polyuria/polydipsia (hypercalcemia causes nephrogenic DI-like picture)
• Short QT interval on EKG

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The “bones” pathology Step likes

Osteitis fibrosa cystica (classic but less common)

Due to excess osteoclast activity (driven by osteoblast RANKL signaling under PTH influence).

Key findings

• Subperiosteal bone resorption
• Bone pain, fractures
• “Brown tumors” (hemorrhage + hemosiderin + fibrous tissue)

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Lab pattern table (differentiate like a pro)

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The classic Step trap: PHPT vs FHH

Both can have high calcium with non-low PTH, so the differentiator is urine calcium.

Draw this mini-box:

• PHPT → high urine calcium
• FHH → low urine calcium

Test to know:

• 24-hour urine calcium
• Calcium/creatinine clearance ratio (CaCrCR) is low in FHH (classically $<0.01$)

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Imaging + management (Step 2 flavor, but Step 1 relevant)

Diagnosis basics

• Confirm labs (repeat Ca and PTH)
• Check:
  • Phosphate
  • 25-OH vitamin D (deficiency can coexist)
  • Creatinine/eGFR
  • DEXA scan (bone density)
  • 24-hour urine calcium (esp. if considering FHH)

Localization (after biochemical diagnosis)

• Sestamibi scan and/or neck ultrasound for adenoma localization
  (Don’t “diagnose” solely by imaging—USMLE likes that sequence.)

Treatment

• Definitive: parathyroidectomy (esp symptomatic, kidney stones, osteoporosis, very high Ca)
• If not surgical candidate:
  • Cinacalcet (calcimimetic → activates CaSR → lowers PTH)
  • Ensure hydration; manage bone density as indicated

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Quick-hit memory anchors (what to recall in 5 seconds)

• Labs: Ca ↑, PTH ↑, PO$_4$ ↓
• Most common cause: parathyroid adenoma
• EKG: short QT
• Differentiate from FHH: urine Ca high in PHPT, low in FHH
• Bone finding: subperiosteal resorption / osteitis fibrosa cystica

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Mini self-check (1 question)

A patient has recurrent kidney stones, constipation, fatigue. Labs: Ca 11.6 (high), PTH high, phosphate low. What else is likely?

• Answer you should auto-say: Increased urine calcium and short QT.