A 58-year-old man from Bangalore is found to have serum calcium 7.8 mg/dL (normal 8.5–10.2), serum phosphate 5.2 mg/dL (normal 2.5–4.5), and intact PTH 8 pg/mL (normal 10–65) on routine screening. He is asymptomatic. Serum magnesium is 1.6 mg/dL (normal 1.7–2.2). Serum albumin is 4.0 g/dL. Renal function is normal (creatinine 0.9 mg/dL). Vitamin D level is 32 ng/mL (normal 30). What is the most likely diagnosis?

Question

A 58-year-old man from Bangalore is found to have serum calcium 7.8 mg/dL (normal 8.5–10.2), serum phosphate 5.2 mg/dL (normal 2.5–4.5), and intact PTH 8 pg/mL (normal 10–65) on routine screening. He is asymptomatic. Serum magnesium is 1.6 mg/dL (normal 1.7–2.2). Serum albumin is 4.0 g/dL. Renal function is normal (creatinine 0.9 mg/dL). Vitamin D level is 32 ng/mL (normal >30). What is the most likely diagnosis?

Accepted Answer

D. Hypomagnesemia-induced suppression of PTH secretion. ## Clinical Presentation Analysis

**Key Point:** This patient has **hypocalcemia + hyperphosphatemia + LOW PTH + hypomagnesemia**. This constellation is pathognomonic for **hypomagnesemia-induced PTH suppression**.

## Why Hypomagnesemia Suppresses PTH

### Mechanism of Magnesium in PTH Secretion

1. **Magnesium is a cofactor for the calcium-sensing receptor (CaSR)** on parathyroid cells
2. **Hypomagnesemia (Mg^2+^ < 1.7 mg/dL) impairs CaSR function**, making it less sensitive to extracellular calcium
3. **Result:** Parathyroid cells cannot "sense" hypocalcemia properly → **PTH secretion is suppressed** despite low serum calcium
4. **Without PTH action:**
   - Renal calcium reabsorption ↓ → hypercalciuria
   - Renal phosphate wasting ↓ → hyperphosphatemia
   - Intestinal calcium absorption ↓
   - Bone resorption ↓

### Why This Is Not the Other Options

**Option 0 (Vitamin D deficiency):** Vitamin D is NORMAL (32 ng/mL); moreover, vitamin D deficiency causes **secondary hyperparathyroidism** (elevated PTH), not suppressed PTH.

**Option 2 (Primary hypoparathyroidism):** While PTH is low, primary hypoparathyroidism is an intrinsic parathyroid gland disorder (genetic, autoimmune, or post-surgical). This patient's low PTH is **functional suppression due to hypomagnesemia**, not gland failure. Magnesium repletion will restore PTH secretion.

**Option 3 (Tertiary hyperparathyroidism):** Tertiary hyperparathyroidism occurs in chronic kidney disease and is characterized by **elevated PTH**, not suppressed PTH. This patient has normal renal function.

## Diagnostic Clue: The Magnesium Level

**High-Yield:** Always check serum magnesium in any patient with:
- Hypocalcemia + low/normal PTH (apparent hypoparathyroidism)
- Refractory hypocalcemia despite calcium + vitamin D supplementation
- Hypomagnesemia is often missed because it is not routinely ordered

| Feature | Vitamin D Deficiency | Hypomagnesemia-Induced PTH Suppression | Primary Hypoparathyroidism |
|---------|---------------------|----------------------------------------|---------------------------|
| Serum Calcium | ↓ | ↓ | ↓ |
| Serum Phosphate | ↑ | ↑ | ↑ |
| PTH | ↑↑ (secondary hyperparathyroidism) | ↓ or normal | ↓ |
| Vitamin D | ↓ | Normal or ↑ | Normal or ↑ |
| Magnesium | Normal | ↓ | Normal |
| Response to Mg repletion | No | Yes (PTH rises, Ca rises) | No |

## Clinical Pearl

**Clinical Pearl:** Hypomagnesemia suppresses PTH secretion; hypermagnesemia stimulates it. This is why patients on **loop and thiazide diuretics** (which cause urinary magnesium wasting) may develop refractory hypocalcemia despite normal vitamin D and renal function. The solution is **magnesium repletion**, not more calcium or vitamin D.

**Mnemonic:** **"Mg is the gatekeeper of the CaSR"** — Without adequate magnesium, the calcium-sensing receptor cannot function, and PTH secretion fails even in the face of hypocalcemia.

Answer

A. Chronic kidney disease with tertiary hyperparathyroidism

Answer

B. Primary hypoparathyroidism

Answer

C. Vitamin D deficiency secondary hyperparathyroidism

Explanation

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