Calcium and Phosphorus Disorders: A Student Guide
Learning Objectives
- Understand the calcium-phosphorus-PTH-vitamin D axis
- Recognize normal values and severity classifications
- Interpret PTH levels in calcium disorders
- Understand the kidney’s central role in regulation
- Manage acute and chronic mineral disorders
Physiologic Overview
Normal Values
- Calcium: 8.5-10.5 mg/dL (ionized: 4.5-5.5 mg/dL)
- Phosphate: 2.5-4.5 mg/dL
- PTH: 10-65 pg/mL
- 1,25-vitamin D: 25-65 pg/mL
The Regulatory Axis
Three hormones control calcium-phosphorus balance:
- PTH (parathyroid hormone)
- Released when calcium drops
- Increases calcium, decreases phosphate
- Acts on kidneys and bone
- Vitamin D (1,25-dihydroxyvitamin D)
- Active form made in kidneys
- Increases intestinal calcium absorption
- Increases phosphate absorption
- Inhibited by PTH and FGF23
- FGF23 (fibroblast growth factor 23)
- Released when phosphate rises
- Promotes phosphate excretion
- Inhibits vitamin D activation
Hypocalcemia (Calcium <8.5 mg/dL)
Causes
Low PTH: - Surgical removal (thyroidectomy, parathyroidectomy) - Autoimmune destruction - Infiltrative disease
High PTH (secondary hyperparathyroidism): - Vitamin D deficiency - Chronic kidney disease - Vitamin D-dependent rickets
PTH Resistance: - Chronic kidney disease (most common) - Pseudohypoparathyroidism (genetic)
Clinical Features
- Paresthesias (lips, fingers)
- Muscle cramps, tetany
- Chvostek sign (tap cheek → twitch), Trousseau sign (BP cuff → spasm)
- Seizures (severe)
- Cardiac: QT prolongation, arrhythmias
Management
Acute symptomatic: - IV calcium gluconate 10% 1-2 ampules in 50-100mL saline over 10-20 minutes - Monitor for overcorrection, extravasation risk - Check magnesium first—hypomagnesemia prevents calcium correction
Chronic: - Calcium supplements 1-3g daily + active vitamin D - Vitamin D deficiency: 50,000 IU weekly × 8 weeks, then maintenance - Hypomagnesemia replacement (magnesium citrate 400-800mg daily)
Hypercalcemia (Calcium >10.5 mg/dL)
Two Main Causes
PTH-mediated (30%): - Primary hyperparathyroidism (85% from adenoma) - Malignancy-related hyperparathyroidism
Non-PTH-mediated (70%): - Malignancy (PTHrP secretion, osteolytic lesions) - Vitamin D intoxication - Granulomatous disease (sarcoidosis, TB) - Thyroid toxicosis
Clinical Features
- Neuropsychiatric: confusion, lethargy, coma
- GI: nausea, anorexia, constipation
- Renal: polyuria, nephrolithiasis
- Cardiac: shortened QT, arrhythmias
- “Stones, bones, groans, psychiatric overtones”
Diagnostic Approach
- Check PTH first
- High/normal PTH → primary hyperparathyroidism or lithium
- Suppressed PTH → check PTHrP, vitamin D levels
- PTHrP level (if PTH suppressed)
- Elevated → malignancy
- Vitamin D level (if PTH suppressed)
- Elevated 1,25-D → granulomatous disease
- Elevated 25-D → vitamin D intoxication
Management
Acute severe (>14 mg/dL or symptomatic): 1. IV hydration: 200-300 mL/hour normal saline (removes calcium in urine) 2. Bisphosphonate: - Zoledronic acid 4mg IV over 15 minutes (preferred) - Effect in 2-4 days, lasts 3-4 weeks 3. Calcitonin: 4-8 IU/kg every 6-12 hours (rapid but short-lived, for bridge) 4. Dialysis: For renal failure or unresponsive cases
Chronic (asymptomatic hyperparathyroidism): - Surgery (parathyroidectomy) = definitive treatment - Medical: bisphosphonates improve bone density; cinacalcet lowers calcium
Hyperphosphatemia (Phosphate >4.5 mg/dL)
Most Common Cause: Chronic Kidney Disease
In CKD: - Kidneys can’t excrete phosphate - FGF23 rises (tries to compensate) - PTH rises (secondary hyperparathyroidism) - Calcium drops (phosphate binds calcium)
Clinical Consequences
- Vascular calcification (reduces lifespan)
- Secondary hyperparathyroidism
- Renal osteodystrophy (bone disease)
- Cardiac events
Management in CKD
Dietary restriction: - Target <800-1000 mg/day - Avoid: seeds, nuts, processed cheese, dairy (high phosphate density)
Phosphate binders (with meals): - Calcium-based (calcium acetate): cheap, but avoid if hypercalcemic - Non-calcium-based: sevelamer, lanthanum, iron-based (newer) - Bind dietary phosphate → reduced absorption
Novel agents: - Tenapanor: blocks intestinal phosphate absorption (new) - Nicotinamide: reduces phosphate reabsorption
Hypophosphatemia (Phosphate <2.5 mg/dL)
Causes
Intracellular shift: - Refeeding syndrome (biggest acute risk) - Insulin administration - Alkalosis
Renal losses: - Diuretics, steroids - Primary hyperparathyroidism - FGF23-mediated (X-linked hypophosphatemia)
GI losses: - Malabsorption, diarrhea, short bowel - Phosphate binders overuse
Clinical Features
- Rhabdomyolysis (muscle breaks down)
- Hemolytic anemia (RBC dysfunction)
- Respiratory depression (weak respiratory muscles)
- Decreased WBC function (infection risk)
Management
Severe symptomatic (<1.0 mg/dL): - IV phosphate: 0.08-0.16 mmol/kg over 6 hours - Monitor for hypocalcemia (phosphate binds calcium) - Central line preferred (caustic)
Moderate (1.0-2.0): - Oral phosphate supplements: 250-500 mg TID-QID - Monitor calcium (may need to supplement)
Mild: - Dietary sources (meat, dairy, eggs) - Discontinue phosphate binders
Special Scenario: Refeeding Syndrome
What is it? Severe electrolyte shifts when malnourished patient receives nutrients
Phosphate specifically: - Cells take up phosphate for ATP synthesis - Serum phosphate DROPS precipitously - Can cause rhabdomyolysis, respiratory failure, death
Prevention: - Start nutrition slowly - Monitor phosphate, potassium, magnesium daily - Supplement phosphate, K, Mg pre-emptively
Practice Questions
Q1: A 65-year-old with recent thyroidectomy has ionized calcium 3.8 mg/dL and is tingling around lips. Magnesium is 1.2 (low). What’s your priority?
Answer
The patient needs magnesium FIRST before calcium will work. Give magnesium 2-4g IV, then reassess. If calcium doesn’t improve after magnesium correction, give IV calcium gluconate. Hypomagnesemia prevents PTH release and causes PTH resistance—you can’t correct calcium until Mg2+ is normal.Q2: A 58-year-old with lung cancer has calcium 13.2 and PTH is suppressed. What test distinguishes between PTHrP hypercalcemia and vitamin D intoxication?
Answer
Check PTHrP level (elevated in malignancy) and 1,25-vitamin D level (elevated in granulomatous disease or vitamin D intoxication). Lung cancer usually causes PTHrP-mediated hypercalcemia. Treatment: hydration, bisphosphonate, and treat underlying cancer.Q3: A 42-year-old with CKD stage 4 has phosphate 5.8 and PTH 280 (high). He won’t eat less dairy. What’s your management?
Answer
He needs phosphate binders taken with meals. Calcium-based acetate can help, but non-calcium alternatives (sevelamer, lanthanum) are preferable given his rising PTH. Ensure adequate vitamin D (may need calcitriol). Dietary counseling on high-phosphate foods remains important—aim for <1000 mg/day.Key Takeaways
- Always check PTH to distinguish primary from secondary hyperparathyroidism
- Magnesium must be corrected before calcium will normalize
- CKD causes secondary hyperparathyroidism via phosphate retention and vitamin D loss
- Vitamin D deficiency is common (>1 billion people affected)
- Refeeding syndrome = severe hypophosphatemia risk → start nutrition slowly
- Bisphosphonates take 2-4 days to work (need calcitonin for bridge therapy)
- PTHrP assay distinguishes malignancy from primary hyperparathyroidism
- Phosphate binders work best with meals → timing matters
Study tip: Draw the “calcium-phosphorus seesaw”—when one goes up, the other tends down. Remember PTH and vitamin D both push calcium UP and phosphate DOWN.
See Also
Clinical Content (01-Clinical-Medicine/Nephrology)
- Electrolyte Disorders Hub
- CKD Hub - Full Clinical Reference
- Essential Renal Laboratory Tests
Butler-COM Resources
- Butler COM - Nephrology Deep Dive
Clinical Resources
- Clinical Review: Calcium Phosphate Review — Comprehensive clinical review with PubMed references
- Clinical Review: Phosphorus Management Clinical Report — Comprehensive clinical review with PubMed references