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Flashcards in Calcium and Phosphate Metabolism Deck (31)
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1

Why is bone turnover important? 

For homeostasis of serum calcium and phosphate

 

2

What affects the homeostasis of serum calcium and phosphate? 

  • Parathyroid Hormone (PTH) = increases Ca2+ 
  • Vitamin D = Increases Ca2+ 
  • Calcitonin = Decreases Ca2+ 
  • FGF-23 

3

What is the distribution of body calcium? 

99% in the bone 

1% intracellular 

<0.1 extracellular (this maintains Ca2+ balance) 

4

What maintains Ca balance? 

Hormonal control of small extracellular fraction (<0.1%) of calcium 

5

What is the composition of calcium in extracellular fluid? 

Half is free (Ca2+) and physiologically active 

Other half is protein bound (mainly albumin) 

6

What is the distribution of phosphate? 

85% of phosphate is in bone 

Remainder is intracellular 

Extracellular = H2PO4-, HPO4-

7

What are the clinical features of hypercalcaemia? 

PAINFUL BONES, RENAL STONES, ABDOMINAL GROANS, PSYCHIC MOANS 

  • Depression, fatigue, anorexia, nausea, vomiting 
  • Abdominal pain, constipation 
  • Renal calcification (kidney stones) 
  • Bone pain 

8

What are clinical features of severe hypercalcaemia? 

  • Cardiac arrthymias 
  • Cardiac arrest 

9

What are the common causes of hypercalcaemia in ambulatory patients? 

Primary hyperparathyroidism 

  • e.g tumour within the thyroid gland 

10

What is the most common cause of hypercalcaemia in hospitalised patients? 

Hypercalcaemia of malignancy 

11

What are less common causes of malignancy? 

Hyperthyroidism/ excessive intake of vitamin D 

12

What would the serum biochemistry look like in someone with hypercalcaemia? 

  • Serum calcium = modest to marked increase 
  • Serum phosphate = low to low normal 
  • Serum alkaline phosphatase = 20% of cases due to increased bone turnover 
  • Serum creatinine = May be elevated in long standing disease 
  • Serum PTH = interpreted in relation to calcium 

13

What does high alkaline phosphatase indicate? 

Increase in bone remodelling 

14

What is the function of PTH? 

Secreted when blood calcium levels drop to prevent hypocalcaemi

15

What are the main actions of PTH? 

  •  Promote release of Ca and Pi from bone 
  • Increases renal Ca reabsorption from DCT 
  • Decreases renal Pi reabsorption fromo DCT (increases excretion) 
  • Upregulates 1 alpha hydroxylase activity activating vitamin D 

 

16

What are the main actions of PTH on bone? 

  • Will promote bone remodelling 
  • PTH receptors are present on osteoblasts and pre-osteclasts 
  • PTH will bind to osteoblast receptors activating pre-osteoblasts via RANKL and OPG inhibition (normally inhibits RANK) 
  • Osteoclasts will absorb bone and release more calcium and phosphate

17

How does PTH regulate itself? 

As free Ca2+ levels are low serum PTH levels rise, as free Ca2+ levels are high serum PTH levels decrease.

18

What is hypercalcaemia of malignancy? 

Most common cause of hypercalcaemia in HOSPITALISED PATIENTS 

  • humoural e.g lung carcinoma secreting PTHrP (PTH related peptide) 
  • Metastatic 
  • Haematological (e.g myeloma) 

19

What is the most common cause of hypocalcaemia? 

  • Vitamin D deficiency 
  • Renal failure 

20

What is a less common cause of hypocalcaemia? 

Hypoparathyroidism 

21

What can vitamin D deficiency result in? 

Rickets (in children) 

  • failure in bone mineralisation and disordered cartilage formation 

Osteomalacia (in adults) 

  • Impaired bone mineralisation (soft bones) 

22

What are features of osteomalacia? 

  • Bone pain 
  • Waddling gait, muscle weakness 
  • On X-ray, stress fractures 

23

What is the serum biochemistry of osteomalacia? 

  • Low/normal calcium 
  • Hypophosphataemia 
  • Raised alkaline phosphatase
  • Secondary hyperparathyroidism (the low calcium would stimulate PTH release) 

24

What is osteoporosis? 

Loss of bone mass/density due to the thinning of both cortical bone and trabecular mesh 

25

What are some causes of osteoporosis? 

  • Endocrine 
  • Malignancy 
  • Drug-induced 
  • Renal disease 
  • Nutritional 
  • Age 

26

What is the difference between osteoporosis and osteomalacia? 

Osteoporosis 

  • Loss of bone mass/ density 

Osteomalacia 

  • Loss of bone mineralisation 

27

First sign of osteoporosis 

Sustaining a fracture, usually: 

  • Wrist 
  • Neck of the femur 
  • Intervertebral 

28

How do we diagnose osteoporosis? 

Dual Energy X-Ray Absorptiometry Scan (DEXA) 

  • Measures bone mass density 

29

How does DEXA work? 

  • Bone mineral density is at its peak in a young adult (25 years old) 
    • after it will slowly decline 
  • With DEXA we will achieve a T score by taking the average bone density for a young adult and then interpreting your bone mineral density score in relation 
    • T score = Number of standard deviations below average bone mineral density for a young adult at peak bone density (- 25yo) 

30

Endocrine causes of osteoporosis 

  • Hypogonadism = notably any cause oestrogen deficiency 
  • Excess gluocorticoids - endogenous or exogenous 
  • Hyperparathyroidism 
  • Hyperthyroidism 

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