Calcium homeostasis Flashcards
What does the parathyroid do?
the parathyroid hormone is very important in correcting calcium levels when serum calcium is low. parathryoid hormone causes increased bone resorption, and increased calcium resorption form the kidney.
parathyroid hormone also promotes activation of vitamin D by the kidney to 1,25 dihydroxyvitamin D. This increases calcium absorption in the gut.
What is important to know about anatomy of parathyroid glands
secreted by 4 glands adjacent to the thyroid
this is an anatomical, not a functional, relationship
variable location, and some people have a 5th parathyroid gland
When should parathyroid hormone be high?
when calcium is low
if calcium low and PTH is “normal,” you have a PTH deficiency
What cells secrete PTH? How do they sense calcium levels and how is PTH secreted?
chief cells secrete PTH
there is a calcium sensor on the chief cells: it is a G-protein coupled receptor. Increased intracellular Ca inhibits secretion of PREFORMED PTH
What are PTH effects on bone? What is one surprising clinical effect of this?
activate and expand the osteoblasts, which activate osteoclasts and increase bone turnover and bone resorption
therefore, intermitten PTH can have an ANABOLIC (building) function on bone and may be useful for osteoporosis
What are PTH’s effects on the kidney?
reduce renal calcium excretion by enhancing Ca resoprtion from the glomerular filtrate
increases phosphate excretion (helpful because phsophate can bind calcium and blood and effectively lower concentration)
increases activation of vitamin D
How does the kidney handle calcium?
filtered in the glomerulus
reabsorbed by PCT via passive resorption
reabsorbed in the thick ascending loop of henle with PTH action
in distal kidney, reabsorbed transcellularly.
also reabsorbed through the cells. In DCT this depends on PTH.
Effects of PTH on the gut
regulates formation of active vitamin D (hydroxylation rxn)
activated vitamin D increases calcium absorption from the intestine
vitamin D is often a limiting factor- you need lots of sun, or lots of weird foods (cod liver oil), to get sufficient vitamin D without supplementation
What happens in patients with a vitamin D deficiency?
rickets or osteomalacia- poor bone formation
can cause low bone density and/or fractures
What is PTHrP?
a hormone that usually acts locally rather than systemically
behaves just like PTH- binds the same receptors
does not respond to systemic calcium levels, so can cause hypercalcemia if inappropriately in systemic circulation
clinical manifestations of hypocalcemia
determined by Ca level and by chronicity
may see:
1. neuromuscular manifestions: paresthesias or tetany. can cause lethal laryngospasm
2. cardiac: prolonged QT
3. Opthoalmologic: subcapsular cataract
4. dermatologic: dry skin, brittle nails, dermatitis
DDx of hypocalcemia: organ based
parathyroid, kidney, intestine, bone, intravascular binding (resp. alkalosis), extravascular space (pancreaitis, rhabdomyolysis, tumor lysis syndrome)
(within that:
parathyroid: post-surgical, DiGeorge, hypomagnesemia, autoimmune
Renal failure: decr. Ca resorption, decr. phostphate excretion, not enough active vit D)
Hypocalcemia: molecular DDx
- calcium sensor: activating mutation
- parathyroid hormone mutations
- Parathyroid hormone receptor mutations
- Vitamin D problems
Treatment of hypocalcemia in acute setting
asymptomatic: oral replacement supplement or dietary change
symptomatic: intervene right away with calcium infusions, slow infusions, active vitamin D
Etiologies of hypoparathyroidism
surgical (may be permanent or transient), familial, autoimmune, DiGeorge, deposition, hypomagnesemia
