Parathyroid Physiology Flashcards
what form is the majority of body calcium in
stored in bone as Ca-P complexes
what the measurable form of calcium
extracellular
50% ionized, 50% protein or mineral bound
functions of calcium
- structural: stored in bone to provide integrity to skeleton and teeth, protect organs, house hematopoietic cells, and Ca reservoir
- functional: cell signaling, muscle contraction, nerve conduction, hormone secretion, coagulation
what form is the majority of body phosphorus in
bone
functions of phosphorus
- structural: skeletal integrity, nucleic acids, phospholipids/membranes
- functional: enzyme cofactor, energy transfer (ATP to ADP), intracellular processes
what hormones are responsible for Ca and P maintenance in the body
protein/peptides: PTH, calcitonin, FGF-23
steroids: vitamin D3
what secretes PTH
chief cells in the parathyroid gland
what stimulates PTH secretion
low ionized Ca
NOT under pituitary control
effect of magnesium on PTH
cofactor - if Mg is low, PTH will not function as well leading to low Ca
what inhibits PTH secretion
- high Ca
- high FGF-23
- high calcitriol
how do chief cells detect Ca concentration
CaSR (Ca sensing receptor)
Ca binding to receptor will inhibit PTH secretion
is the half life of PTH short or long
short
net effects of PTH
increase Ca
decrease P
direct effects of PTH
kidney: stimulate Ca reabsorption and P excretion in tubules
bone: stimulate Ca and P reabsorption from bone
indirect effects of PTH
indirect effect on the gut
stimulates 1-a-hydroxylase in the kidneys to convert inactive vitamin D3 –> calcitriol to increase gut absorption of Ca and P
effect of calcitriol
stimulates dietary Ca and P absorption from the GI lumen
how is calcitriol produced in carnivores
dietary vitamin D3 (required in carnivores, do not produce in skin) –> liver –> gets converted to calcidiol –> kidneys –> gets converted to calcitriol
what secretes calcitonin
parafollicular / C cells located between the follicles in the thyroid
what stimulates calcitonin secretion
high ionized Ca
effects of calcitonin
decreases Ca
bone: decreases osteoclast activity (dec. Ca/P reabsorption from bone)
kidneys: minor increase in Ca excretion
what secretes FGF-23
osteocytes in bone
what stimulates FGF-23 secretion
- high P
- high calcitriol
- high PTH
effects of FGF-23
decrease phosphorus
kidneys: increases excretion of P
GI: inhibits 1-a-hydroxylase in the kidneys to decrease calcitriol –> dec. Ca/P absorption from gut
what secretes PTHrp
certain neoplasms; leads to humoral hypercalcemia of malignancy (HHM)
effects of PTHrp
increase Ca
decrease P
uses the same direct/indirect mechanisms as PTH
ddx for hypercalcemia
H: hyperparathyroidism
O: osteolytic
G: granulomatous (fungal)
S: spurious - lab error
I: idiopathic
N: neoplastic & nutritional
Y: young
A: Addison’s disease
R: renal disease - increases total calcium NOT ionized
D: vitamin D toxicity
ddx for hypocalcemia
P: primary hypoparathyroidism
E: eclampsia/“Milk Fever”
P: pancreatitis
E: ethylene glycol toxicity
R: renal disease - decreases ionized calcium NOT total
N: nutritional/GI malabsorption
I: iatrogenic - diuretics
S: spurious
H: hypoalbuminemia - decreases total calcium NOT ionized
what species is primary hyperparathyroidism common in
older dogs
what clinical signs are associated with primary hyperparathyroidism
often incidental
PU/PD, uroliths, lethargy, GI signs
what is seen on lab work for primary hyperparathyroidism
- high Ca
- low/normal phosphorus
- high/inappropriately normal PTH (should be 0)
treatment for primary hyperparathyroidism
parathyroidectomy
heat albation
what are the causes of hyperparathyroidism
- primary
- secondary nutritional
- secondary renal
- humoral hypercalcemia of malignancy
primary hyperparathyroidism
functional parathyroid adenoma thats secretes excess PTH
what lesion is associated with primary hyperparathyroidism
one enlarged PTG
rest atrophied PTGs
secondary renal hyperparathyroidism
chronic kidney disease –> decreased P excretion and vitamin D3 activation –> high P and low calcitriol –> hypocalcemia –> stimulates PTH
PTH gets stimulated by low Ca levels, but renal failure prevents PTH from increasing calcium, so PTH continues to get secreted
what lesion is associated with secondary renal hyperparathyroidism
all PTGs enlarged
secondary nutritional hyperparathyroidism
low dietary Ca, vitamin D3, or high phosphorus leads to overstimulation of PTH
what lesion is associated with secondary nutritional hyperparathyroidism
all PTGs enlarged
primary hypoparathyroidism
low levels of PTH
what species is primary hypoparathyroidism common in
middle aged dogs > cats
esp poodles
what causes primary hypoparathyroidism
parathyroid atrophy
clinical signs of primary hypoparathyroidism
- muscle twitching (tetany, paresis)
- weakness
- facial pruritus
- restlessness
what labs are seen with primary hypoparathyroidism
- severely low Ca
- high/normal P
- low PTH
treatment for primary hypoparathyroidism
calcitriol
Ca supplementation
metastatic mineralization
mineralization of the tissues (especially kidneys) when Ca/P is out of balance
when does metastatic mineralization occur
when [Ca] x [P] > 60-80 mg/dL
what parathyroid lesion spares tissue mineralization
primary hyperparathyroidism
decreases phosphorus despite increasing calcium
