Calcium and phosphate metabolism Flashcards
What does bone turnover serve and in conjunction with what?
• Bone turnover serves homeostasis of serum calcium and phosphate, in conjunction with:
○ Parathyroid hormone (PTH)
○ Vitamin D (1,25-dihydroxy D3)
○ Calcitonin
○ FGF-23What does FGF-23 lower?
Lowers serum phosphate levels
What does bone remodelling release in the short term?
In the short term, bone remodelling releases minerals, notably calcium, into the circulation
What percentage of body calcium is in bone?
• 99% of body calcium is in bone
Where is 1% of calcium mainly found?
Mainly found intracellularly
What is normal plasma Ca levels?
plasma Ca 2.2-2.6 mmol L-1
How is extracellular calcium found?
• About half of extracellular is free [Ca2+] (physiologically active), half protein bound (mainly albumin)
What does PTH increase?
Increases bone remodelling
How is PTH at low doses?
○ At low doses, PTH is anabolic: promotes formation of new bone
What happens when there is prolonged higher levels of PTH and what does this result in?
Prolonged higher level = excess of resorption over formation
Results in rise in extracellular calcium
What does a persistent raised level of PTH result in and why?
§ Persistent raise
Results in hypercalcaemia because it promotes/increases resorption of Ca from kidney tubule
What does PTH lower?
Lowers phosphate
What does PTH increase expression of what is it responsible for?
○ Increases expression α hydroxylase enzyme that is responsible for converting precursor of vitamin D to 1,25 dihydroxy D3
What percentage of body phosphorous is found in the bone and in what form?
• 85% of body phosphorus is in bone
-Mainly in the form of hydroxyapatite
What is normal levels of phosphate extracellularly?
2.5-4.5 mg dL-1
Why do phosphate levels fluctuate more than calcium levels?
Less tightly controlled
What are clinical features of hypercalcaemia?
• May be asymptomatic for a while
• Depression, fatigue, anorexia, nausea, vomiting,
• Abdominal pain, constipation
• Renal calcification (kidney stones)
• Bone pain
○ “painful bones, renal stones, abdominal groans, and psychic moans”
• Severe: cardiac arrhythmias, cardiac arrest
What is the most common cause of hypercalcaemia in ambulatory patients?
primary hyperparathyroidism
§ Involves benign tumour of one or more of the parathyroid glands
§ parathyroid tumour = excess PTH = excess resorption = high calcium
What is the cause of hypercalcaemia in hospitalised patients?
Malignancy
What are the less common causes of hypercalcaemia?
○ Hyperthyroidism
○ Excessive intake of vitamin D
Serum biochemistry in hypercalcaemia
- Serum calcium - modest to marked increase
- Serum phosphate - low or low normal
- Serum alkaline phosphatase raised in ~ 20% of cases
- Serum creatinine may be elevated in longstanding disease (kidney damage)
- Serum PTH concentration should be interpreted in relation to calcium
What is the most common cause of hypercalcaemia in hospitalised patients?
• Most common cause of hypercalcaemia in hospitalized patients
○ Humoral, e.g., lung carcinoma secreting PTHrP
○ Metastatic
○ Haematological
§ Myeloma
What is the most common cause of hypocalcemia?
- Vitamin D deficiency
- Renal failure
What are the less common causes of hypocalcemia?
Hypoparathyroidism
