Primary hypoparathyroidism.
- Hypoparathyroidism can be hereditary or acquired due to gland failure or an acute but reversible dysfunction.
- The most common cause used to be the prior loss of parathyroid tissue during thyroidectomy, but now it is surgery to correct hyperparathyroidism.
- Other causes include autoimmune destruction or DiGeorge syndrome.
- The initiating factor is inadequate secretion of PTH by the parathyroid glands.
- The decrease in plasma calcium is accompanied by an increased plasma phosphate. Despite less phosphate being reabsorbed from bone, plasma phosphate increases because the normal action of PTH is to inhibit phosphate reabsorption and increase excretion by the kidney.
- Symptoms center on the hypocalcemia-induced increased excitability of motor neurons, leading to muscular spasms and tetany.
- This sort of neuromuscular hyperexcitability becomes clinically apparent when serum calcium levels drop below 7.0 mg/dl.
- Chvostek’s sign is induced by tapping the facial nerve just anterior to the ear lobe.
- The jaw reflex is a variant of Chvostek's sign, which is a facial muscle contraction elicited by tapping on the facial nerve just anterior to the ear.
- Trousseau's sign is elicited by inflating a pressure cuff on the upper arm, with a positive response being carpal spasm.
- Hypomagnesemia prevents PTH secretion and induces temporary hypocalcemia. This condition responds immediately to an infusion of magnesium.
Pseudohypoparathyroidism (Albright hereditary osteodystrophy).