Hypocalcemia is an extracellular fluid calcium concentration below the reference range.
Which calcium counts?
Only the free ionised fraction of calcium is active. About 40% of serum calcium is bound to albumin, so the measured total calcium may not reflect the ionised level. The diagnosis note returns to this.
What holds the level up?
The fall is driven by a limited set of failures. The parathyroid glands, which release PTH, are missing or ineffective. Vitamin D is deficient or unable to act. Or calcium is drawn into bone or damaged tissue faster than it is replaced. Calcium control rests on these three levers, and the etiologies note teaches how they work.
Does the cause change the picture?
Yes. The cause decides what the patient feels, and it decides how the level is put right. The satellite notes each take one side of that.
How does the fall show up?
A falling calcium level affects nerves, muscle and heart. The level can fall acutely or drift down over months, and the rate of the fall matters. The speed and size of the fall decide how hypocalcemia presents. It can cause nothing at all, or it can produce tetany: a state of sustained involuntary muscle contraction. Bedside signs bring out latent neuromuscular irritability.
What causes it?
The main etiologies are vitamin D deficiency, hypoparathyroidism after neck surgery and in syndromic forms, hungry bone syndrome, and a group of miscellaneous causes. One cause deserves its own note. In pseudohypoparathyroidism the parathyroid gland is normal, but the tissues do not respond to PTH. A GNAS mutation and its imprinting produce PHP 1a, PHP 1b, PHP 2 and pseudopseudohypoparathyroidism (PPHP).
How is it confirmed and corrected?
Diagnosis first confirms that the calcium is truly low. PTH then sorts the causes into those inside and those outside the parathyroid gland, and further testing confirms PTH resistance. In an emergency, treatment is intravenous calcium. Chronic hypocalcemia is treated with oral calcium and activated vitamin D. Magnesium and hypercalciuria problems complicate both treatments.
Where to start?
Etiology is the place to start, because presentation, tests and correction of the level each have different answers depending on the cause. Read the notes in this order:
- Etiologies of hypocalcemia
- Pseudohypoparathyroidism: PTH resistance and AHO
- Clinical manifestations of hypocalcemia
- Diagnosis of hypocalcemia
- Treatment of hypocalcemia
Which of these failures is behind this patient’s low calcium?