Calcium questions are deceptively simple on Step 1: the stem looks like “tingling,” “kidney stones,” or an “incidental lab abnormality,” but the real test is whether you can rapidly map Ca$^{2+}$ physiology → PTH/vitamin D axes → EKG changes → next best step. This post is your high-yield, Step-ready deep dive into hypocalcemia vs hypercalcemia—with the classic associations and “what do I do next?” management.

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Why calcium matters (and why Step loves it)

About half of serum calcium is albumin-bound, and the other half is ionized (active). Many “low calcium” labs are really low albumin.

Albumin correction (high-yield)

Use this when total calcium is low and albumin is abnormal:

$\text{Corrected Ca (mg/dL)} = \text{Measured Ca} + 0.8 \times (4.0 - \text{Albumin})$

• If albumin is low, total Ca looks low but ionized Ca may be normal.
• If you’re clinically worried (tetany, seizures), check ionized calcium.

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First Aid cross-reference: Endocrine/Parathyroid physiology (PTH/Vit D), and Renal/Electrolytes (Ca$^{2+}$/phosphate handling).

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The core physiology you need (PTH, vitamin D, phosphate)

PTH: the “raise Ca, dump phosphate” hormone

Net effects of PTH:

• Bone: increases resorption (via osteoblast RANKL → osteoclast activation) → ↑ Ca and ↑ phosphate released
• Kidney:
  • ↑ Ca reabsorption (distal tubule)
  • ↓ phosphate reabsorption (proximal tubule) → phosphaturia
  • ↑ 1$\alpha$-hydroxylase → ↑ calcitriol (1,25-(OH)$_2$-D)

Vitamin D (calcitriol): increases absorption

• Gut: ↑ absorption of Ca and phosphate
• Bone: supports mineralization (but in high doses can promote resorption)

Quick “pattern recognition” table

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First Aid cross-reference: “Hyperparathyroidism,” “Hypoparathyroidism,” “Pseudohypoparathyroidism,” “Vitamin D deficiency,” and renal failure mineral bone disease.

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Hypocalcemia

Definition

• Total Ca typically < 8.5 mg/dL or
• Ionized Ca low (more clinically meaningful)

The “hidden” hypocalcemia traps (Step favorites)

• Low albumin → low total Ca but normal ionized Ca (no symptoms)
• Alkalosis increases Ca binding to albumin → ↓ ionized Ca
  • Example: anxious patient hyperventilating → respiratory alkalosis → paresthesias/carpopedal spasm

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Pathophysiology (big buckets)

1) Low/ineffective PTH

• Hypoparathyroidism
  • Post-thyroidectomy (accidental removal/ischemia)
  • Autoimmune destruction (can be part of autoimmune polyglandular syndromes)
• Pseudohypoparathyroidism
  • End-organ resistance to PTH (Gs protein defect)
  • Albright hereditary osteodystrophy: short 4th/5th metacarpals, short stature, round face
  • Labs mimic hypoparathyroidism, but PTH is high

2) Vitamin D deficiency or impaired activation

• Dietary deficiency, lack of sunlight
• Fat malabsorption (celiac, pancreatic insufficiency, cholestasis)
• Liver disease (↓ 25-hydroxylation) or CKD (↓ 1$\alpha$-hydroxylation)

3) “Calcium gets consumed/chelated”

• Acute pancreatitis (saponification in fat necrosis)
• Massive blood transfusion (citrate binds Ca)
• Tumor lysis / rhabdomyolysis: phosphate release → Ca precipitates with phosphate

4) Hypomagnesemia (don’t miss)

• Low Mg can cause functional hypoparathyroidism (impaired PTH release and action)
• Refractory hypocalcemia won’t correct until Mg is repleted

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Clinical presentation (how it shows up on exams)

Neuromuscular irritability

• Perioral numbness, tingling, muscle cramps
• Carpopedal spasm, tetany
• Seizures (severe)

Classic signs (testable)

• Chvostek sign: facial twitch with tapping facial nerve
• Trousseau sign: carpal spasm with BP cuff inflation

EKG

• Prolonged QT interval (from prolonged ST segment)
• Severe cases: torsades risk

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High-yield Step move: tingling + prolonged QT + recent thyroid surgery = think hypocalcemia from hypoparathyroidism.

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Diagnosis (what to order and how to interpret)

Start with:

• Ionized calcium (if symptomatic or albumin abnormal)
• Serum magnesium
• Phosphate
• PTH
• 25-OH vitamin D (best for vitamin D status)
• Consider renal function (BUN/Cr)

Interpreting common patterns

• Low Ca + high phosphate + low PTH → hypoparathyroidism
• Low Ca + high phosphate + high PTH → pseudohypoparathyroidism
• Low Ca + low phosphate + high PTH → vitamin D deficiency (or malabsorption)
• Low/normal Ca + high phosphate + high PTH + CKD → secondary hyperparathyroidism

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Treatment (Step-style, severity-based)

If symptomatic or severe (tetany, seizures, arrhythmia)

• IV calcium gluconate
  • Safer peripherally than calcium chloride
• Place on telemetry (QT issues)
• Correct magnesium if low (key for refractory cases)

If mild/asymptomatic or chronic

• Oral calcium supplementation
• Vitamin D supplementation
  • If CKD: use calcitriol (1,25-(OH)$_2$-D) or vitamin D analogs (activation impaired)

Cause-specific pearls

• Post-thyroidectomy: treat acute symptoms with IV calcium, then oral calcium/vit D as needed
• Hypomagnesemia: Mg first (or alongside), otherwise Ca may not budge

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Hypercalcemia

Definition

• Typically total Ca > 10.5 mg/dL or elevated ionized Ca

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Pathophysiology (high-yield causes)

“Most common causes” (board-worthy)

• Primary hyperparathyroidism
• Malignancy

Primary hyperparathyroidism (PTH-mediated)

• Parathyroid adenoma or hyperplasia
• Labs: ↑ Ca, ↓ phosphate, ↑ PTH
• Increased risk: kidney stones; bone pain; GI symptoms

Malignancy (PTH-independent, usually)

1. PTHrP secretion (humoral hypercalcemia)
   • Squamous cell carcinoma (lung), RCC, bladder, ovarian, etc.
   • Labs: ↑ Ca, ↓ phosphate, ↓ PTH
2. Osteolytic metastases
   • Multiple myeloma, breast cancer
   • Labs: ↑ Ca, variable phosphate, ↓ PTH
3. Increased calcitriol (granulomatous disease, some lymphomas)
   • Sarcoidosis, TB (macrophages express 1$\alpha$-hydroxylase)
   • Labs: ↑ Ca, ↑ phosphate, ↓ PTH, ↑ 1,25-(OH)$_2$-D

Other classic causes (remember these)

• Thiazide diuretics (increase Ca reabsorption)
• Lithium (shifts PTH set point)
• Vitamin D intoxication
• Milk-alkali syndrome (excess calcium carbonate: hypercalcemia + metabolic alkalosis + AKI)
• Immobilization (especially high bone turnover states)

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First Aid cross-reference: Malignancy-associated hypercalcemia (PTHrP), granulomatous disease, diuretics (thiazides), and hyperparathyroidism.

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Clinical presentation (“stones, bones, groans, psychiatric overtones”)

Think decreased excitability (opposite of hypocalcemia).

• Renal: polyuria, nephrolithiasis (calcium oxalate/phosphate stones), dehydration
• GI: constipation, nausea, abdominal pain
• Neuropsychiatric: fatigue, confusion, depression
• MSK: bone pain (especially malignancy), weakness

EKG

• Shortened QT interval
• Severe: arrhythmias can occur (but Step often tests QT shortening)

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Diagnosis (approach that scores points)

Stepwise approach to hypercalcemia

1. Confirm (repeat, correct for albumin, consider ionized Ca)
2. Check PTH
   • High or inappropriately normal PTH → PTH-mediated (primary/tertiary hyperPTH, lithium, FHH)
   • Low PTH → PTH-independent (malignancy, vitamin D, granulomatous disease, thyrotoxicosis, etc.)
3. If PTH is high:
   • Check urinary calcium
     • Familial hypocalciuric hypercalcemia (FHH): high Ca, low urine Ca, normal/mildly elevated PTH
       • Due to inactivating CaSR mutation
       • Important because it doesn’t need parathyroidectomy
4. If PTH is low:
   • Consider PTHrP, 25-OH vit D, 1,25-(OH)$_2$-D, malignancy workup

Key distinguishing patterns

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Treatment (acute vs chronic; what Step expects)

Acute symptomatic hypercalcemia (or very high Ca)

Goal: restore volume, increase calciuresis, reduce bone resorption.

1. IV normal saline (first-line)
   • Hypercalcemia causes nephrogenic DI-like polyuria → volume depletion
2. Add calcitonin for rapid, short-term effect
3. Add IV bisphosphonate (pamidronate, zoledronic acid) for sustained effect, especially malignancy-related
4. Consider loop diuretics after rehydration if volume overload risk (Step often emphasizes: don’t give loops before fluids)
5. Glucocorticoids if due to:
   • Granulomatous disease (sarcoid/TB)
   • Vitamin D-mediated hypercalcemia (decrease 1$\alpha$-hydroxylase activity and intestinal absorption)
6. Dialysis if:
   • Severe hypercalcemia with renal failure
   • Refractory cases or inability to tolerate fluids

Chronic management (cause-directed)

• Primary hyperPTH: parathyroidectomy for symptomatic patients or those meeting criteria; medical therapy includes hydration, avoid thiazides; sometimes cinacalcet
• Malignancy: treat underlying cancer + bisphosphonates/denosumab
• FHH: usually reassurance (avoid unnecessary surgery)

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Rapid-fire high-yield associations (Step 1 gold)

Hypocalcemia associations

• Post-thyroidectomy → hypoparathyroidism → ↓ Ca, ↑ phosphate, ↓ PTH
• Respiratory alkalosis (panic attack) → ↓ ionized Ca → tingling/tetany
• Pancreatitis → fat saponification → hypocalcemia
• Massive transfusion → citrate chelation → hypocalcemia
• Hypomagnesemia → impaired PTH → refractory hypocalcemia until Mg corrected
• QT prolongation

Hypercalcemia associations

• Squamous cell carcinoma → PTHrP → hypercalcemia with low PTH
• Sarcoidosis/TB → ↑ 1,25-(OH)$_2$-D → hypercalcemia + hyperphosphatemia
• Thiazides → hypercalcemia
• Milk-alkali (Ca carbonate) → hypercalcemia + metabolic alkalosis + AKI
• QT shortening

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Mini question stems (practice your pattern recognition)

• “Tingling, carpopedal spasm, recent thyroid surgery, prolonged QT”
  → Hypocalcemia from hypoparathyroidism → treat with IV calcium gluconate if symptomatic

• “Smoker, weight loss, confusion, constipation, Ca 13.2, low PTH, low phosphate”
  → PTHrP malignancy → fluids + calcitonin + bisphosphonate

• “Short 4th metacarpal, round face, low Ca, high phosphate, high PTH”
  → Pseudohypoparathyroidism (PTH resistance)

• “Hypercalcemia + low PTH + high 1,25-(OH)$_2$-D”
  → Sarcoidosis/lymphoma → steroids are helpful

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Step 1 “Do not miss” checklist

• Always ask: Is it true hypocalcemia (ionized)?
• Tie calcium problems to PTH and phosphate—the pattern often gives the diagnosis.
• Memorize EKG: hypocalcemia = long QT, hypercalcemia = short QT.
• Mg is the sneaky one: low Mg can mimic hypoparathyroidism and make calcium “unfixable.”
• Hypercalcemia acute management starts with normal saline—not furosemide first.