Low level of serum PTH
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Low Serum PTH (Hypoparathyroidism)
A low or inappropriately low serum PTH in the setting of hypocalcemia defines hypoparathyroidism - the parathyroid glands fail to secrete adequate PTH despite falling calcium levels. This is the key diagnostic distinction: PTH should be elevated (secondary hyperparathyroidism) when calcium is low, so a "normal" or low PTH in that context is always pathological.
Classification of Hypocalcemia by PTH Level
| PTH Low/Normal | PTH High (compensatory) |
|---|---|
| Hypoparathyroidism | Vitamin D deficiency/resistance |
| Chronic kidney disease | |
| Intestinal malabsorption |
Causes of Low PTH (Hypoparathyroidism)
The causes fall into four broad categories (Frameworks for Internal Medicine, p.130):
1. Iatrogenic (Most Common in Adults)
- Surgery - inadvertent removal of or damage to the parathyroid glands or their blood supply during thyroidectomy, parathyroidectomy, or radical neck dissection. Most post-surgical cases are transient ("stunning"); persistent hypoparathyroidism >6 months is relatively rare.
- Radiation - external beam radiation for head and neck cancers, or rarely radioactive iodine therapy for hyperthyroidism.
- Medications - cinacalcet (a calcimimetic) activates calcium-sensing receptors (CaSR) on parathyroid cells, suppressing PTH secretion.
2. Autoimmune (Second Most Common in Adults)
- Immune-mediated parathyroid destruction - can be isolated or part of Autoimmune Polyglandular Syndrome type 1 (APS-1), caused by mutations in the AIRE gene. APS-1 features hypoparathyroidism, chronic mucocutaneous candidiasis, and adrenal insufficiency.
- Activating antibodies to CaSR - these mimic high calcium, persistently suppressing PTH secretion.
- Even late-onset idiopathic adult hypoparathyroidism frequently involves anti-CaSR and anti-NALP5 antibodies.
3. Infiltrative Disease
- Sarcoidosis, IgG4-related disease
- Iron overload (hemochromatosis, transfusion-dependent thalassemia)
- Copper overload (Wilson's disease)
4. Other / Genetic
- DiGeorge syndrome (chromosome 22q11.2 deletion) - congenital absence of parathyroids with thymic aplasia and cardiac defects.
- Inactivating mutations in GCM2, GATA3, or the PTH gene itself.
- Activating mutations in CaSR or GNA11 (autosomal dominant hypocalcemia).
- Hypomagnesemia - impairs intracellular PTH synthesis and release. Causes include alcohol abuse, cisplatin, anti-EGFR antibodies, proton pump inhibitors.
- Severe hypermagnesemia (e.g., obstetric Mg infusions) - can suppress PTH via CaSR activation.
- COVID-19 infection caused transient hypoparathyroidism by unknown mechanism.
Lab Pattern
| Serum Calcium | Serum PTH | Serum Phosphorus | Urinary Calcium |
|---|---|---|---|
| Low | Low or low-normal | High (PTH normally promotes phosphaturia) | Low |
The phosphorus is elevated because PTH normally promotes phosphate excretion in the kidneys; without PTH, phosphate is retained.
- Tietz Textbook of Laboratory Medicine, p.986; Frameworks for Internal Medicine, p.130
Clinical Manifestations (Due to Hypocalcemia)
Acute:
- Paresthesias (fingers, toes, perioral)
- Chvostek's sign - facial twitch on tapping facial nerve (also present in ~10% of normals)
- Trousseau's sign - carpal spasm on BP cuff inflation 20 mmHg above systolic for 3 min
- Tetany, carpopedal spasm
- Bronchospasm, laryngospasm
- Seizures
- QTc prolongation → ventricular tachyarrhythmia (Torsade de Pointes), sudden death
Chronic:
- Cataracts
- Calcification of basal ganglia
- Dental abnormalities
- Harrison's Principles of Internal Medicine 22e, p.409; Robbins & Kumar Basic Pathology, p.742
Diagnosis
A suppressed or inappropriately low PTH in the setting of hypocalcemia establishes hypoparathyroidism. Work-up should also include:
- Serum albumin (to calculate corrected calcium)
- Serum phosphorus, magnesium
- 25(OH)D levels
- 24-hour urinary calcium
- History of neck surgery, radiation, family history
- Harrison's Principles of Internal Medicine 22e, p.409
Treatment
Acute symptomatic hypocalcemia:
- IV calcium gluconate - 10 mL of 10% solution (90 mg elemental Ca) diluted in 50 mL dextrose/saline, given over 5-10 min with telemetry, repeated as needed.
- May require continuous IV calcium infusion (~1 mg/min elemental calcium) titrated to symptoms, ECG, and blood levels.
- Correct hypomagnesemia first if present - PTH cannot be secreted adequately with low Mg.
Chronic hypoparathyroidism:
- Oral calcium supplements + active vitamin D (calcitriol) - since PTH is absent, 1α-hydroxylation of 25(OH)D in the kidney is impaired, so the active form must be supplemented directly.
- Monitor urinary calcium to avoid hypercalciuria (without PTH, kidneys don't retain calcium normally).
- Recombinant PTH 1-84 (natpara) - approved for chronic hypoparathyroidism uncontrolled by supplements.
- Harrison's Principles of Internal Medicine 22e, p.409-410; Katzung's Basic and Clinical Pharmacology 16e
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