Parathyroid Physiology

Question 1

what form is the majority of body calcium in

Answer 1

stored in bone as Ca-P complexes

Question 2

what the measurable form of calcium

Answer 2

extracellular

50% ionized, 50% protein or mineral bound

Question 3

functions of calcium

Answer 3

1. structural: stored in bone to provide integrity to skeleton and teeth, protect organs, house hematopoietic cells, and Ca reservoir
2. functional: cell signaling, muscle contraction, nerve conduction, hormone secretion, coagulation

Question 4

what form is the majority of body phosphorus in

Answer 4

bone

Question 5

functions of phosphorus

Answer 5

1. structural: skeletal integrity, nucleic acids, phospholipids/membranes
2. functional: enzyme cofactor, energy transfer (ATP to ADP), intracellular processes

Question 6

what hormones are responsible for Ca and P maintenance in the body

Answer 6

protein/peptides: PTH, calcitonin, FGF-23

steroids: vitamin D3

Question 7

what secretes PTH

Answer 7

chief cells in the parathyroid gland

Question 8

what stimulates PTH secretion

Answer 8

low ionized Ca

NOT under pituitary control

Question 9

effect of magnesium on PTH

Answer 9

cofactor - if Mg is low, PTH will not function as well leading to low Ca

Question 10

what inhibits PTH secretion

Answer 10

• high Ca
• high FGF-23
• high calcitriol

Question 11

how do chief cells detect Ca concentration

Answer 11

CaSR (Ca sensing receptor)

Ca binding to receptor will inhibit PTH secretion

Question 12

is the half life of PTH short or long

Answer 12

short

Question 13

net effects of PTH

Answer 13

increase Ca
decrease P

Question 14

direct effects of PTH

Answer 14

kidney: stimulate Ca reabsorption and P excretion in tubules

bone: stimulate Ca and P reabsorption from bone

Question 15

indirect effects of PTH

Answer 15

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

Question 16

effect of calcitriol

Answer 16

stimulates dietary Ca and P absorption from the GI lumen

Question 17

how is calcitriol produced in carnivores

Answer 17

dietary vitamin D3 (required in carnivores, do not produce in skin) –> liver –> gets converted to calcidiol –> kidneys –> gets converted to calcitriol

Question 18

what secretes calcitonin

Answer 18

parafollicular / C cells located between the follicles in the thyroid

Question 19

what stimulates calcitonin secretion

Answer 19

high ionized Ca

Question 20

effects of calcitonin

Answer 20

decreases Ca

bone: decreases osteoclast activity (dec. Ca/P reabsorption from bone)

kidneys: minor increase in Ca excretion

Question 21

what secretes FGF-23

Answer 21

osteocytes in bone

Question 22

what stimulates FGF-23 secretion

Answer 22

• high P
• high calcitriol
• high PTH

Question 23

effects of FGF-23

Answer 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

Question 24

what secretes PTHrp

Answer 24

certain neoplasms; leads to humoral hypercalcemia of malignancy (HHM)

Question 25

effects of PTHrp

Answer 25

increase Ca decrease P uses the same direct/indirect mechanisms as PTH

Question 26

ddx for hypercalcemia

Answer 26

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

Question 27

ddx for hypocalcemia

Answer 27

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

Question 28

what species is primary hyperparathyroidism common in

Answer 28

older dogs

Question 29

what clinical signs are associated with primary hyperparathyroidism

Answer 29

often incidental PU/PD, uroliths, lethargy, GI signs

Question 30

what is seen on lab work for primary hyperparathyroidism

Answer 30

- high Ca - low/normal phosphorus - high/inappropriately normal PTH (should be 0)

Question 31

treatment for primary hyperparathyroidism

Answer 31

parathyroidectomy heat albation

Question 32

what are the causes of hyperparathyroidism

Answer 32

1. primary 2. secondary nutritional 3. secondary renal 4. humoral hypercalcemia of malignancy

Question 33

primary hyperparathyroidism

Answer 33

functional parathyroid adenoma thats secretes excess PTH

Question 34

what lesion is associated with primary hyperparathyroidism

Answer 34

one enlarged PTG rest atrophied PTGs

Question 35

secondary renal hyperparathyroidism

Answer 35

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

Question 36

what lesion is associated with secondary renal hyperparathyroidism

Answer 36

all PTGs enlarged

Question 37

secondary nutritional hyperparathyroidism

Answer 37

low dietary Ca, vitamin D3, or high phosphorus leads to overstimulation of PTH

Question 38

what lesion is associated with secondary nutritional hyperparathyroidism

Answer 38

all PTGs enlarged

Question 39

primary hypoparathyroidism

Answer 39

low levels of PTH

Question 40

what species is primary hypoparathyroidism common in

Answer 40

middle aged dogs > cats esp poodles

Question 41

what causes primary hypoparathyroidism

Answer 41

parathyroid atrophy

Question 42

clinical signs of primary hypoparathyroidism

Answer 42

- muscle twitching (tetany, paresis) - weakness - facial pruritus - restlessness

Question 43

what labs are seen with primary hypoparathyroidism

Answer 43

- severely low Ca - high/normal P - low PTH

Question 44

treatment for primary hypoparathyroidism

Answer 44

calcitriol Ca supplementation

Question 45

metastatic mineralization

Answer 45

mineralization of the tissues (especially kidneys) when Ca/P is out of balance

Question 46

when does metastatic mineralization occur

Answer 46

when [Ca] x [P] > 60-80 mg/dL

Question 47

what parathyroid lesion spares tissue mineralization

Answer 47

primary hyperparathyroidism decreases phosphorus despite increasing calcium