Metabolic Bone Disease – Biochemistry Flashcards
When is peak bone mass reached?
Around 25 years
When does bone mass begin to decline?
Around 40 years
NOTE: in women, the decline in bone mass accelerates after menopause
How are microfractures repaired?
Bone remodelling
What are the two main targets of PTH?
Kidneys
Bone
Describe the effects of PTH in:
a. Bone
b. Kidneys
a. Bone
Acute release of available calcium (not stored in hydroxyapatite crystal form)
More chronically, increased osteoclast activity
b. Kidneys
Increased calcium reabsorption
Increased phosphate excretion
Increased stimulation of 1-alpha hydroxylase (thus increasing calcitriol production)
Where does the PTH-mediated increase in calcium reabsorption take place in the nephron?
DISTAL convoluted tubule
Where does the PTH-mediated increase in phosphate excretion take place in the nephron?
PROXIMAL convoluted tubule
What does the parathyroid gland use to monitor serum calcium?
Calcium-sensing receptors
Other than PTH, what else can cause increased phosphate excretion?
FGF23
Which cells produce FGF23?
Osteoblast lineage cells
Other than Vitamin D deficiency, what else can cause Rickets/Osteomalacia?
Phosphate deficiency
How does oestrogen deficiency lead to a decrease in bone mineral density?
It increases the number of bone remodelling units
It causes an imbalance in bone remodelling with increased bone resorption compared to bone formation
What is the single best predictor of fracture risk?
BMD
What is used to measure BMD?
DEXA scans
Which bones are used when measuring BMD and why?
Vertebral bodies
Commonest fracture
Good measure of cancellous bone
It is a highly metabolically active bone so it is quick to respond to treatment
Hip – second commonest fracture
NOTE: fracture risk assessment tool (FRAX) uses hip BMD
What is ALP a marker of
Bone turnover
Osteoporosis vs osteopenia
Osteoporosis a lack of bone quantity but osteopaenia a lack of mineralisation so microstructure only affected it osteopaenia
What happens in osteitis fibrosa cystica
Calcified supporting structures removed and replaced by fibrous tissue leading to loss of bone mass
How does renal failure lead to secondary hyperparathyroidism
Failure to produce calcitriol so serum calcium drops leading to secondary hyperparathyroidism
How does secondary hyperparathyroidism lead to tertiary hyperparathyroidism
The secondary hyperparathyroidism leads to parathyroid hyperplasia so eventually the cells stop responding to Ca2+ and become autonomous
- tertiary hyperparathyroidism
