Calcium Homeostasis & Bone Metabolism – Clinical Scenarios with Lab Data
You will be presented with 8 clinical scenarios of children with suspected calcium, vitamin D, or parathyroid disorders.
For each, interpret the lab data and provide: 1) Diagnosis, 2) Any other test, 3) What to do next, 4) Follow-up plan.
Case 1
A 6-year-old boy presents with carpopedal spasm, perioral tingling, and a recent seizure. He has dysmorphic facies (micrognathia, low-set ears) .
Serum Calcium
5.8 mg/dL (low)
Serum Phosphorus
8.5 mg/dL (high)
PTH
8 pg/mL (low)
25-OH Vitamin D
28 ng/mL (normal)
Magnesium
1.8 mg/dL (normal)
1️⃣ Diagnosis: (Write your answer below)
2️⃣ Any other test: (Write your answer below)
3️⃣ What to do next: (Write your answer below)
4️⃣ Follow-up plan: (Write your answer below)
✅ Model Answer:
• Diagnosis: Hypoparathyroidism due to DiGeorge syndrome (22q11.2 deletion).
• Any other test: FISH or chromosomal microarray for 22q11.2 deletion; immune work-up (T-cell subsets); echocardiogram.
• What to do next: IV calcium gluconate 10% (1-2 mL/kg) for acute tetany/seizure; start calcitriol 0.01-0.1 mcg/kg/day and oral calcium 25-50 mg/kg/day.
• Follow-up plan: Monitor ionized calcium, phosphate, magnesium weekly initially; cardiology & immunology follow-up; avoid live vaccines if T-cell deficient. Lifelong calcium and calcitriol therapy.
Case 2
A 4-year-old girl, exclusively breastfed until 18 months, presents with bowed legs, wrist widening, and delayed walking.
Serum Calcium
7.2 mg/dL (low)
Serum Phosphorus
2.8 mg/dL (low)
Alkaline Phosphatase
950 U/L (elevated)
25-OH Vitamin D
12 ng/mL (low)
PTH
95 pg/mL (elevated)
1️⃣ Diagnosis: (Write your answer below)
2️⃣ Any other test: (Write your answer below)
3️⃣ What to do next: (Write your answer below)
4️⃣ Follow-up plan: (Write your answer below)
✅ Model Answer:
• Diagnosis: Nutritional vitamin D deficiency rickets (25-OH D ↓, secondary hyperparathyroidism).
• Any other test: Ionized calcium, 24-hour urine calcium/creatinine, renal function; wrist X-ray for rachitic changes; screen for celiac disease (TTG IgA) if malabsorption suspected.
• What to do next: Ergocalciferol (vitamin D₂) 2,000-5,000 IU/day + oral calcium 30-50 mg/kg/day elemental.
• Follow-up plan: Check 25-OH D, ALP, calcium, phosphorus at 8-12 weeks; repeat wrist X-ray for healing; maintain 400 IU/day maintenance. Monitor for hypercalcemia.
Case 3
A 10-year-old with short stature, round face, brachydactyly (short 4th/5th metacarpals).
Serum Calcium
6.5 mg/dL (low)
Serum Phosphorus
7.0 mg/dL (high)
PTH
120 pg/mL (elevated)
25-OH Vitamin D
30 ng/mL (normal)
Urine Calcium/Creatinine
Low
1️⃣ Diagnosis: (Write your answer below)
2️⃣ Any other test: (Write your answer below)
3️⃣ What to do next: (Write your answer below)
4️⃣ Follow-up plan: (Write your answer below)
✅ Model Answer:
• Diagnosis: Pseudohypoparathyroidism type 1a (Albright hereditary osteodystrophy) – PTH resistance, GNAS mutation.
• Any other test: GNAS gene sequencing; Ellsworth-Howard test (PTH infusion) – blunted urinary cAMP and phosphate response; check TSH (TSH resistance common).
• What to do next: Start oral calcium 25-50 mg/kg/day + calcitriol 0.01-0.1 mcg/kg/day. Goal low-normal calcium (8-8.5 mg/dL) to avoid hypercalciuria.
• Follow-up plan: Monitor serum calcium, phosphorus, PTH; urine calcium/creatinine ratio (goal <0.2-0.3); check thyroid function (TSH resistance); if hypothyroidism, start levothyroxine standard dose. Lifelong therapy.
Case 4
A 14-year-old boy with recurrent abdominal pain, polyuria, and constipation.
Serum Calcium
11.2 mg/dL (elevated)
Serum Phosphorus
2.6 mg/dL (low)
PTH
82 pg/mL (elevated)
25-OH Vitamin D
30 ng/mL (normal)
24h Urine Calcium
320 mg/day (elevated)
Ca/Cr Clearance Ratio
0.025 (normal)
1️⃣ Diagnosis: (Write your answer below)
2️⃣ Any other test: (Write your answer below)
3️⃣ What to do next: (Write your answer below)
4️⃣ Follow-up plan: (Write your answer below)
✅ Model Answer:
• Diagnosis: Primary hyperparathyroidism (elevated Ca, low P, elevated PTH, hypercalciuria).
• Any other test: CaSR gene (to differentiate from FHH); 1,25-(OH)₂D; MEN1 genetic testing; neck ultrasound or sestamibi scan for localization.
• What to do next: Bilateral neck exploration with subtotal (3-3.5 gland) parathyroidectomy.
• Follow-up plan: Monitor for hungry bone syndrome post-op – check Ca, P, Mg daily; give IV/oral calcium and calcitriol as needed. Long-term: monitor calcium, PTH annually. Screen for MEN1-associated tumors.
Case 5
A 7-year-old with recurrent gross hematuria, dysuria, and abdominal pain.
Serum Calcium
9.8 mg/dL (normal)
Serum Phosphorus
4.2 mg/dL (normal)
PTH
40 pg/mL (normal)
Spot Urine Ca/Cr
0.3 (elevated, >0.2)
Urinalysis
Microscopic hematuria, no proteinuria, no RBC casts
Renal Ultrasound
Normal
1️⃣ Diagnosis: (Write your answer below)
2️⃣ Any other test: (Write your answer below)
3️⃣ What to do next: (Write your answer below)
4️⃣ Follow-up plan: (Write your answer below)
✅ Model Answer:
• Diagnosis: Idiopathic hypercalciuria – excessive urinary calcium excretion with normal serum calcium.
• Any other test: 24-hour urine calcium (>4 mg/kg/day confirms). Screen for secondary causes: serum 25-OH D, PTH, electrolytes, renal function. Family history for hypercalciuria/nephrolithiasis.
• What to do next: High fluid intake (2-3 L/day, urine output >1.5 L/day), low sodium diet (<2 g/day), normal calcium intake (DO NOT restrict), low oxalate diet if stones.
• Follow-up plan: Monitor urine Ca/Cr every 3-6 months. If persistent symptoms or recurrent stones, start hydrochlorothiazide 1-2 mg/kg/day. Renal ultrasound annually for nephrocalcinosis.
Case 6
A 6-month-old infant with elfin facies, supravalvular aortic stenosis, and hypercalcemia.
Serum Calcium
12.5 mg/dL (elevated)
Serum Phosphorus
4.5 mg/dL (normal)
PTH
8 pg/mL (low/suppressed)
25-OH Vitamin D
40 ng/mL (normal)
1,25-(OH)₂ Vitamin D
Elevated
1️⃣ Diagnosis: (Write your answer below)
2️⃣ Any other test: (Write your answer below)
3️⃣ What to do next: (Write your answer below)
4️⃣ Follow-up plan: (Write your answer below)
✅ Model Answer:
• Diagnosis: Williams syndrome (7q11.23 deletion) – elfin facies, supravalvular aortic stenosis, transient hypercalcemia.
• Any other test: FISH or chromosomal microarray for 7q11.23 deletion; echocardiogram for SVAS and renal artery stenosis; ophthalmology for strabismus.
• What to do next: Low calcium diet (avoid vitamin D/calcium supplements), IV hydration if severe; hypercalcemia usually transient.
• Follow-up plan: Monitor serum calcium monthly until normal; repeat echocardiogram annually; monitor blood pressure; developmental assessment; avoid excessive calcium/vitamin D.
Case 7
A newborn presents with seizures. Mother has a history of hyperparathyroidism. Infant is otherwise well.
Serum Calcium
6.0 mg/dL (low)
Serum Phosphorus
7.0 mg/dL (high)
PTH
5 pg/mL (low)
25-OH Vitamin D
25 ng/mL (normal)
Magnesium
1.9 mg/dL (normal)
1️⃣ Diagnosis: (Write your answer below)
2️⃣ Any other test: (Write your answer below)
3️⃣ What to do next: (Write your answer below)
4️⃣ Follow-up plan: (Write your answer below)
✅ Model Answer:
• Diagnosis: Neonatal hypoparathyroidism secondary to maternal hyperparathyroidism – maternal hypercalcemia suppresses fetal PTH.
• Any other test: Check maternal calcium and PTH; ionized calcium in infant; 1,25-(OH)₂D; renal ultrasound for nephrocalcinosis.
• What to do next: IV calcium gluconate 10% (1-2 mL/kg) for seizures; then oral calcium 25-50 mg/kg/day + calcitriol 0.01-0.1 mcg/kg/day. Treat until PTH recovers (usually days to weeks).
• Follow-up plan: Monitor calcium and PTH weekly; wean calcium/calcitriol as PTH recovers. Most recover fully; maternal hyperparathyroidism needs treatment.
Case 8
A 2-year-old with hypercalcemia, suppressed PTH, and elevated 1,25-(OH)₂D. No granulomatous disease or vitamin D toxicity.
Serum Calcium
13.0 mg/dL (elevated)
Serum Phosphorus
4.0 mg/dL (normal)
PTH
5 pg/mL (low/suppressed)
25-OH Vitamin D
30 ng/mL (normal)
1,25-(OH)₂ Vitamin D
Elevated
Urine Calcium
High
1️⃣ Diagnosis: (Write your answer below)
2️⃣ Any other test: (Write your answer below)
3️⃣ What to do next: (Write your answer below)
4️⃣ Follow-up plan: (Write your answer below)
✅ Model Answer:
• Diagnosis: Idiopathic infantile hypercalcemia (CYP24A1 mutation) – impaired 1,25-(OH)₂D degradation.
• Any other test: CYP24A1 gene sequencing; 24-hour urine calcium; renal ultrasound for nephrocalcinosis; exclude granulomatous disease (sarcoid, TB) with chest X-ray, ACE levels.
• What to do next: Low calcium and low vitamin D diet; IV fluids for hydration; pamidronate 1 mg/kg IV if severe hypercalcemia.
• Follow-up plan: Monitor calcium, phosphorus, 1,25-(OH)₂D, urine calcium; avoid vitamin D supplements; renal ultrasound annually; genetic counseling. May improve with age.
⚠️ Key Concept: Calcium-PTH-Vitamin D Axis
• Hypoparathyroidism: ↓Ca, ↑P, ↓PTH (DiGeorge, APS-1, post-surgical)
• Pseudohypoparathyroidism: ↓Ca, ↑P, ↑PTH (PTH resistance, GNAS mutation)
• Vitamin D deficiency: ↓Ca, ↓P, ↑PTH, ↓25-OH D (rickets)
• Primary hyperparathyroidism: ↑Ca, ↓P, ↑PTH (adenoma, MEN1)
• FHH: ↑Ca, normal P, low urine calcium (CaSR mutation)
• Williams syndrome: ↑Ca, ↓PTH, 7q11.23 deletion
• Idiopathic hypercalciuria: ↑urine Ca, normal serum Ca, Ca/Cr >0.2
🎯 Examiner Scoring Checklist
• Correctly identifies hypoparathyroidism vs pseudohypoparathyroidism
• Recognizes vitamin D deficiency rickets and secondary hyperparathyroidism
• Identifies primary hyperparathyroidism and distinguishes from FHH
• Recognizes Williams syndrome and idiopathic hypercalciuria
• Understands acute management of hypocalcemia and hypercalcemia
• Knows genetic testing and long-term follow-up protocols
📌 Key Lab Interpretation:
• Hypoparathyroidism: ↓Ca, ↑P, ↓PTH → treat with calcium + calcitriol
• Pseudohypoparathyroidism: ↓Ca, ↑P, ↑PTH → PTH resistance, GNAS mutation
• Vitamin D deficiency: ↓Ca, ↓P, ↑PTH, ↓25-OH D → treat with vitamin D + calcium
• Primary hyperparathyroidism: ↑Ca, ↓P, ↑PTH → parathyroidectomy
• FHH: ↑Ca, low urine calcium → benign, no treatment
• Williams syndrome: ↑Ca, ↓PTH, 7q11.23 deletion → low calcium diet
• Idiopathic hypercalciuria: ↑urine Ca, normal serum Ca → hydration, low sodium, thiazide
📋 Mock Test Feedback
💡 Examiner's note: Compare your answers with model answers. In real TOACS, you would discuss these with the examiner.