Hypoparathyroidism and PTH resistance
Effect · Ca low, PO4 high PTH low
Too little PTH, or a receptor that cannot hear it. Calcium falls and phosphate rises because the phosphaturic drive has gone, while alkaline phosphatase stays normal: the pattern that separates it from vitamin D deficiency.
Traced from the start
Detail
- Biochemistry
- Calcium low, phosphate HIGH, PTH low or inappropriately normal, alkaline phosphatase normal; always measure magnesium as well
- Causes
- Anterior neck surgery is by far the commonest; then autoimmune disease (isolated, or autoimmune polyendocrine syndrome type 1), 22q11.2 deletion, infiltration by iron or copper, radiation, and severe hypomagnesaemia
- Pseudohypoparathyroidism type 1a
- GNAS loss of function causing PTH resistance: low calcium and high phosphate with a HIGH PTH, plus Albright hereditary osteodystrophy with short stature, a round face and shortened fourth and fifth metacarpals
- Treatment
- Calcium with calcitriol or alfacalcidol, since PTH-driven activation of vitamin D is what has been lost; watch for hypercalciuria and nephrocalcinosis, because renal calcium reabsorption is no longer being supported
When it goes wrong
Day one to three after total thyroidectomy
Perioral paraesthesia progressing to carpopedal spasm, with a positive Chvostek sign (facial twitch on tapping over the facial nerve) and Trousseau sign (carpal spasm within three minutes of cuff inflation above systolic pressure), a prolonged QT interval, laryngospasm and seizures
Chronic untreated hypoparathyroidism
Cataracts, basal ganglia calcification with extrapyramidal features, and dental enamel hypoplasia in children
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1 question in the bank tagged Parathyroid.