Total vs. Ionized Calcium

Clinical Pearl: When albumin is low, measured total calcium falls 0.8 mg/dL per 1 g/dL albumin decrease, but ionized calcium (and symptoms) may be normal.

Corrected Calcium Formula [2]

Corrected Ca = Measured Ca + 0.8 × (4.0 – measured albumin [g/dL])

Example: Patient with measured Ca 7.2 mg/dL and albumin 2.0 g/dL - Corrected Ca = 7.2 + 0.8 × (4.0 – 2.0) = 7.2 + 1.6 = 8.8 mg/dL (likely normal ionized Ca)

The Negative Feedback Loop

Step 1: Serum calcium decreases (< 8.5 mg/dL) - Calcium-sensing receptor (CaSR) on parathyroid cells detects low Ca - PTH secretion increases immediately

Step 2: PTH acts on three target sites: - Kidney (proximal tubule): Activates 1α-hydroxylase → increases 1,25-(OH)2D (calcitriol) - Kidney (distal tubule): Increases calcium reabsorption via TRPV5 channels - Bone: Stimulates osteoclasts → releases Ca and phosphate

Step 3: Calcitriol increases intestinal calcium absorption (duodenum/jejunum)

Step 4: Serum calcium normalizes → CaSR senses high Ca → PTH suppressed

FGF-23 and Mineral Homeostasis

In chronic kidney disease, the axis becomes dysregulated: - Early CKD: Phosphate retention stimulates FGF-23 production - FGF-23 suppresses: 1α-hydroxylase (less calcitriol) and PTH (via Klotho co-receptor) - Later CKD: Secondary hyperparathyroidism develops as GFR < 45 and 1,25-D falls critically

PTH-Dependent Hypercalcemia (5–10% of cases)

FHH Pearl: Always check urine calcium; FHH presents with hypercalcemia + hypocalciuria (opposite of 1° hyperparathyroidism). PTH may be inappropriately high for calcium level.

PTH-Independent Hypercalcemia (90% of cases)

Diagnostic Algorithm for Hypercalcemia

Hypercalcemia (Ca > 10.5 mg/dL)
    ↓
[Measure PTH]
    ├─→ PTH High or Normal-High → 1° Hyperparathyroidism or FHH
    │   └─→ Check urine Ca
    │       ├─→ Urine Ca low (< 150/24h) → FHH
    │       └─→ Urine Ca high → 1° Hyperparathyroidism
    │
    └─→ PTH Low (< 15) → PTH-Independent
        ├─→ [Check 1,25-Dihydroxyvitamin D and PTHrP]
        ├─→ 1,25-D High + PTHrP High → PTHrP-secreting malignancy or granulomatous disease
        ├─→ 1,25-D High + PTHrP Low → Granulomatous disease (sarcoidosis, TB) or lymphoma
        └─→ 1,25-D Normal/Low + PTHrP Low → Osteolytic hypercalcemia or vitamin D intoxication

PTH Response in Hypocalcemia

Common Causes of Hypocalcemia [4]

Symptoms and Emergency Management

Acute IV Calcium Therapy: - Calcium gluconate 10%: 10 mL (1 g Ca) IV slowly over 2–5 min (can give peripherally; safer for extravasation) - Calcium chloride 10%: 10 mL (270 mg Ca) IV slowly (central line preferred; tissue necrosis risk if peripheral) - Repeat every 10–20 min as needed to stop tetany; monitor QT interval

The Vicious Cycle in CKD

GFR < 60 (CKD Stage 3b): - Phosphate retention → FGF-23 rises (first abnormality) - FGF-23 suppresses 1α-hydroxylase

GFR < 45 (CKD Stage 4): - Calcitriol falls → hypocalcemia develops - Hypocalcemia stimulates PTH (secondary hyperparathyroidism) - High PTH + high phosphate → vascular calcification

GFR < 15 (CKD Stage 5): - Severe hyperparathyroidism, bone disease, vascular calcification - Risk of cardiac arrhythmias, fracture, cardiovascular death

CKD-MBD Management Targets (KDIGO 2017)

Calcium-Phosphorus Product (CaxP)

CaxP = Serum Ca × Serum PO4 (both in mg/dL)

Traditional target: < 55 mg²/dL² (though some newer guidelines are more liberal)

Clinical significance: Products > 70 associated with increased vascular and soft-tissue calcification

Drug-Specific Interventions

Asymptomatic Hypocalcemia (> 7.0 mg/dL, no symptoms)

• Oral supplements: Calcium citrate 2–4 g/day divided TID (with meals for absorption)
• Vitamin D: 1,25-dihydroxyvitamin D (calcitriol) if deficient or in CKD
• Monitor: Ionized Ca, ECG, renal function

Symptomatic Hypocalcemia (< 7.0 mg/dL or paresthesias/tetany)

• IV calcium gluconate: 10 mL (1 g) in 50 mL saline over 2–5 min; repeat q10–20 min
• After initial stabilization: IV infusion (500 mg Ca in 250 mL saline over 2–4 h) or switch to oral
• Magnesium: Repleted to > 2.0 mg/dL (otherwise PTH secretion blocked)

Hypoparathyroidism Management

• Active vitamin D (calcitriol): 0.5–1 mcg TID–BID
• Thiazide diuretic: Paradoxically decreases urine Ca reabsorption (used to reduce hypercalciuria side effect)
• Low phosphate diet: Reduces PTH stimulus

CKD Hypocalcemia Management

• Vitamin D analogues: Cholecalciferol (vitamin D3) or calcitriol
• Phosphate binders: Non-calcium-based (sevelamer, lanthanum) preferred in later CKD
• Monitor PTH: Serial measurements guide intensity of therapy

Vignette 1: Asymptomatic Hypercalcemia

A 68-year-old woman is found to have Ca 11.2 mg/dL on routine labs. PTH 8 pg/mL (low), 1,25-D 22 pg/mL (low), albumin normal. 15-year history of sarcoidosis on chronic prednisone.

Diagnosis: Granulomatous (sarcoidosis) hypercalcemia Key clues: Low PTH + low 1,25-D + history of sarcoid = unregulated 1α-hydroxylase from granuloma macrophages Management: Increase prednisone dose; monitor Ca q1–2 weeks; hydroxychloroquine if refractory

Vignette 2: Severe Symptomatic Hypocalcemia

A 56-year-old man post-total thyroidectomy develops severe tetany (Trousseau sign positive). Ca 6.2 mg/dL (measured), Mg 1.2 mg/dL, PTH 2 pg/mL.

Diagnosis: Hypoparathyroidism (surgical) + hypomagnesemia Key clues: Undetectable PTH in setting of hypocalcemia = post-surgical hypoparathyroidism Management: 1. IV magnesium sulfate to Mg > 2.0 (allows PTH secretion) 2. IV calcium gluconate 1 g over 3–5 min, repeat q10–20 min until tetany stops 3. Start calcitriol 0.5 mcg BID + oral calcium 2 g TID 4. Serial ionized Ca, Mg, Ca-P product

Vignette 3: Tumor Lysis Syndrome

A 44-year-old with newly diagnosed Burkitt lymphoma (pre-treatment labs): Ca 6.8 mg/dL, PO4 8.2 mg/dL, uric acid 12.5 mg/dL, Cr 1.9 (baseline 0.9), K 6.8 mEq/L.

Diagnosis: Acute tumor lysis syndrome with incipient acute kidney injury Key clues: Hypocalcemia + hyperphosphatemia + hyperkalemia + hyperuricemia = massive cell lysis Management: 1. Aggressive IV hydration (target urine output > 200 mL/h) 2. Rasburicase 0.2 mg/kg IV to lower uric acid 3. Allopurinol 600 mg PO daily (or febuxostat) 4. Monitor Ca, K, PO4, Cr q4–6h initially 5. Loop diuretics if fluid overload develops 6. Prepare for possible dialysis

Vignette 4: CKD-MBD in Stage 4 CKD

A 72-year-old with CKD Stage 4 (GFR 18): Ca 8.2 mg/dL, PO4 5.1 mg/dL, Albumin 3.8 g/dL, PTH 412 pg/mL (markedly elevated), 1,25-D 18 pg/mL (low).

Diagnosis: Secondary hyperparathyroidism from CKD-MBD Key clues: Low 1,25-D (loss of 1α-hydroxylase) + phosphate retention (despite low GFR) = FGF-23 suppression of calcitriol Management: 1. Phosphate binder: Start sevelamer (non-calcium) or lanthanum given hypocalcemia already present 2. Vitamin D: Calcitriol 0.5 mcg BID to suppress PTH 3. Dietary: Restrict phosphate to < 1 g/day 4. Monitor: Repeat Ca, PO4, PTH q4–6 weeks; goal PTH 35–70 for Stage 4 5. Goals: Ca-P product < 55, maintain Ca 8.5–10.5, reduce PTH surge