Hypocalcemia is a calcium concentration in the extracellular fluid below the reference range. Only the free ionised fraction is active, and about 40% of the calcium in serum is bound to albumin, so the measured total may not reflect the ionised level.
The level can fall acutely or drift down over months, and it can cause nothing at all or produce tetany, a state of sustained involuntary muscle contraction: the speed and the size of the fall decide how it presents. What drives the fall, though, is a limited set of failures — the response of the parathyroid glands, which release parathyroid hormone (PTH), missing or ineffective, vitamin D deficient or unable to act, or calcium drawn into bone or damaged tissue faster than it is replaced — and the cause decides both what the patient feels and how the level is put right.
- Etiologies of hypocalcemia — the main causes and the mechanism behind each: vitamin D deficiency, hypoparathyroidism after neck surgery and in its syndromic forms, hungry bone syndrome, and the wider list of miscellaneous causes.
- Pseudohypoparathyroidism: PTH resistance and AHO — the form in which the parathyroid gland is normal but the tissues do not answer PTH, and how the GNAS mutation and its imprinting produce pseudohypoparathyroidism (PHP) 1a, PHP 1b, PHP 2 and pseudopseudohypoparathyroidism (PPHP).
- Clinical manifestations of hypocalcemia — what a falling calcium level does to nerves, muscle and the heart, why the rate of the fall matters, and the bedside signs that bring out latent neuromuscular irritability.
- Diagnosis of hypocalcemia — confirming that the calcium is truly low, using PTH to sort the causes into those inside and outside the parathyroid gland, and confirming PTH resistance.
- Treatment of hypocalcemia — intravenous calcium for the emergency, oral calcium with activated vitamin D for chronic hypocalcemia, and the magnesium and hypercalciuria problems that complicate both.
The etiologies are the place to start, because every later question — how the patient presents, what to test, how to correct the level — has a different answer depending on the cause.