calcium metabolism

Question 1

role of parathryoid hormone (PTH)

Answer 1

provides minute to minute regulation of ECF calcium
- major regulator of PTH hormone (negative feedback loop)

described as the main defender of the serum calcium

Question 2

components of ECF calcium

Answer 2

50% in ionised calcium form, bioactive

45-50% protein bound, albumin & globulins

5-10% complexed with anions e.g. HCO3

Question 3

how does PTH restore calcium levels to ‘normal’

Answer 3

1. stimulates osteoclastic bone resorption
2. stimulates renal tubular reabsorption of calcium
3. stimulates renal 1-hydroxylation of 25(OH)D

restores serum calcium by acting on all effector organs
bone and kidney directly; intestine indirectly

Question 4

PTH release is regulated by

Answer 4

1. serum ionized calcium
2. serum phosphate
3. serum 1,25 dihydroxyvitamin D (-)

Question 5

what is calcitonin not a regulator of

Answer 5

is NOT a physiological regulator of serum calcium

calcitonin is made in the thyroid

Question 6

parathyroid hormone-related peptide (PTHrP)

Answer 6

• important paracrine regulator of breast, skin and bone development
• not a physiological regulator of serum calcium
• produced in excess in some cancers, when in the bloodstream it mimics PTH
  
  • acts similarly to PTH signals via PTHR1 = cancer associated hypercalcemia

Question 7

causes of hypercalcemia

Answer 7

• PTH-dependent
  (high Ca2+, normal/high PTH)
  - primary hyperparathyroidism
  - familial benign hypercalcaemia

- PTH-independent
         (high Ca2+, suppressed PTH)
        -cancer
                  -PTHrP
                  -extensive bone reabsorption

    - vitamin D-dependent
              - sarcoidosis = endogenous
              - vitamin D intoxication = exogenous

Question 8

causes of hypocalcemia

Answer 8

• hypoparathyroidism
  - postsurgical, post neck irradiation, autoimmune
• parathyroid hormone resistance
• abnormalities of vitamin D metabolism
  - deficiency, renal failure

Question 9

what plays an important role in phosphate metabolism

Answer 9

• vitamin D (between intestines and ECF)
• PTH (from bone to ECF)
• phosphatonins (regulate in kidneys)
  e. g. fibroblast GF 23

Question 10

important organs involved in phosphate and calcium metabolism

Answer 10

• intestines
• bone
• kidneys

Question 11

causes of hyperphosphatemia

Answer 11

• increased input - IV phosphate e.g. burn, cell death e.g. tumour
• decreased excretion - *renal failure, PTH deficiency or resistance

Question 12

causes of hypophosphatemia

Answer 12

• inadequate GI absorption e.g. Vit D deficiency
• intracellular shift
  - resp alkalosis
• renal loss
  - increased PTH, high phosphatonins, alcoholism

Question 13

vitamin D metabolism

Answer 13

sunlight (UV) exposure causes inactivate precursor = 25 hydroxyvitamin D
the kidneys then hydroxylate this into the active form 1,25 dihydroxyvitmain D
(PTH regulates the enzyme that converts this)

Question 14

if a patient comes in with hypercalcemia what is the next best step

Answer 14

check PTH levels

Question 15

blood test results for PTH-dependent hypercalcemia vs PTH-independent hypercalcemia

Answer 15

PTH- dependent
= high sCa++
high/ normal PTH

PTH-independent
=high sCa++
low PTH (appropriate response)

Question 16

factors affecting ECF phosphate levels

Answer 16

• diet
• balance between ICF and ECF
• kidney, tissue responsible for hypophosphatemia

Question 17

phosphatonins and how it causes hypophosphatemia

Answer 17

FGF23 - derived from bone (osteocytes)
causes decreased phosphate absorption from proximal tubule and hypophosphatemia
- it does this by reducing 1,25 dihydroxyvitamin D by inhibiting 1 alphahydroxase