A 35-year-old man is found to have serum calcium 7.2 mg/dL (normal 8.5–10.5), serum phosphate 5.8 mg/dL (normal 2.5–4.5), and PTH 12 pg/mL (normal 15–65) on routine blood work. He is asymptomatic. Which investigation is most appropriate to establish the underlying cause of hypocalcemia?

Explanation

## Diagnostic Approach to Hypocalcemia with Low PTH **Key Point:** Hypocalcemia with **inappropriately LOW PTH** + hyperphosphatemia is the classic triad of **hypoparathyroidism** — but before attributing this to intrinsic parathyroid failure, **hypomagnesemia must be excluded first**, as it is a common, reversible cause of functional hypoparathyroidism. ### Clinical Interpretation The patient presents with: - Hypocalcemia (7.2 mg/dL) - Hyperphosphatemia (5.8 mg/dL) — typical of reduced PTH activity - **Low PTH (12 pg/mL)** — inappropriate for the degree of hypocalcemia When PTH is low in the setting of hypocalcemia, the differential narrows to: 1. **Hypomagnesemia** — impairs both PTH secretion AND end-organ PTH action (most important reversible cause) 2. **Hypoparathyroidism** — intrinsic gland failure (surgical, autoimmune, infiltrative) 3. **Severe vitamin D deficiency** — rarely suppresses PTH absolutely; PTH is typically elevated in vitamin D deficiency **High-Yield:** Serum magnesium is the **first and most critical** investigation in this scenario. Hypomagnesemia (Mg²⁺ < 0.8 mEq/L) causes: - Impaired PTH secretion from chief cells - Peripheral resistance to PTH action at target organs - Refractory hypocalcemia that does NOT correct until magnesium is repleted This is emphasized in **Harrison's Principles of Internal Medicine** (21st ed., Ch. 410): "Hypomagnesemia is the most common reversible cause of hypoparathyroidism and must be excluded before other diagnoses are pursued." ### Why Serum Magnesium is First-Line | Investigation | Role | Priority | | --- | --- | --- | | **Serum magnesium** | Identifies reversible functional hypoparathyroidism | **FIRST** — common, treatable, must not be missed | | Serum 25-hydroxyvitamin D | Assesses vitamin D status | Secondary — vitamin D deficiency causes HIGH PTH (secondary hyperparathyroidism), not low PTH | | Parathyroid ultrasound | Structural imaging | Only after biochemical workup confirms hypoparathyroidism | | Urinary phosphate excretion | Renal phosphate handling | Not part of standard initial algorithm | ### Why Vitamin D Deficiency Is Less Likely Here In vitamin D deficiency, the expected pattern is: - Low calcium ✓ - **Elevated PTH** (secondary hyperparathyroidism) ✗ — this patient has LOW PTH - Low/normal phosphate (PTH drives phosphaturia) ✗ — this patient has HIGH phosphate The combination of low PTH + high phosphate argues strongly against vitamin D deficiency and toward a PTH-deficient state. Serum 25-OH vitamin D would not be the first discriminating test in this biochemical pattern. **Clinical Pearl:** Always check serum magnesium in any patient with unexplained hypocalcemia and low/inappropriately normal PTH. Magnesium repletion alone can normalize PTH secretion and resolve hypocalcemia in hypomagnesemic patients. (Harrison's 21st ed.; Endocrinology: Adult and Pediatric, 7th ed.)