Hypocalcaemia

What it is

Hypocalcemia is a low serum calcium. Total calcium and ionized calcium can move apart, so the reading is weighed against the acid-base state: in respiratory or metabolic alkalosis the ionized calcium is decreased, because hydrogen ions dissociate from albumin and allow more calcium to bind to it, and the opposite occurs with acidosis. In alkalosis, then, total calcium remains normal while ionized calcium decreases, and this may cause symptoms of functional hypocalcemia.

Causes and risk

Hypoparathyroidism after thyroid surgery is a named cause, and hypocalcemia after surgery is one of the risks explained to the patient before thyroidectomy. A low magnesium level gives a low PTH and so a low calcium, and it also gives renal K wasting and so a low K; the magnesium level is therefore checked when hypocalcemia or hypokalemia is found, and magnesium is always corrected in refractory hypokalemia or hypocalcemia. In DiGeorge syndrome the 22q11.2 deletion causes defective development of the third and fourth pharyngeal pouches, so thymus and parathyroids are absent, PTH is low and calcium is low. Alkalosis leaves the total calcium normal while the ionized calcium falls.

Read by the PTH, the causes fall into two groups. A low or normal PTH means hypoparathyroidism, which is primary, from aplasia or dysplasia of the gland or an autoimmune process, or secondary, that is iatrogenic after radiation or surgery. A high PTH points away from the gland: vitamin D disorders with rickets, renal disease, consumptive causes such as pancreatitis, hyperphosphatemia and osteoblastic bone mets, and drugs such as bisphosphonate.

Thyroid surgery carries it as a numbered risk: permanent voice change in 1%, transient hypocalcemia in 30% for less than 1 year, and permanent hypocalcemia in 3% because of hypo-PTH, along with thyroxine for life, bleeding and infection. Hypocalcemia is also one of the causes of hoarseness of voice after that operation, beside recurrent laryngeal nerve injury, hematoma and edema, vocal cord injury by intubation and tracheomalacia.

The chain is written out link by link: a low Mg gives a low PTH release, which gives a low calcium, so hypocalcemia and severe hypomagnesium are seen together. The same low magnesium is the likely underlying cause of refractory hypokalemia in an alcoholic patient, and hypokalemia will not correct until the low Mg2+ is corrected. Beside it, a rise in insulin, an agonist drug, or alkalosis each give hypokalemia, and the RTA types split the other way: type 1 and type 2 cause hypokalemia while type 4 causes hyperkalemia.