Calcium and Phosphate Metabolism Flashcards
(57 cards)
Which mechanisms aid Calcium and phosphate balance?
Homeostasis of serum calcium and phosphate maintained via:
- Bone turnover
- Parathyroid hormone (PTH)
- Vitamin D (1,25-dihydroxy D3)
- Calcitonin
- FGF-23
How are calcium stores maintained in the short term?
Bone remodelling releases minerals esp. calcium into circulation, therefore can be controlled in short-term in
calcium homeostasis
Where is Ca found in the body?
99% of body calcium is in bone
Remaining 1% is mainly intracellular
Describe the significance of the hormonal control of calcium
Hormonal control of the tiny (<0.1%) extracellular fraction is what maintains Ca balance
Outline the normal extracellular calcium level
Extracellular: plasma Ca 2.2-2.6 mmol L-1
About half is free [Ca2+] (physiologically active), half protein bound (mainly albumin)
Where is phosphate found in the body?
85% of body phosphorus is in bone
Remainder is mainly intracellular
What are the normal extracellular phosphate levels?
Extracellular H2PO4-, HPO42-
- 5-4.5 mg dL-1
(0. 75-1.45 mmol L-1)
May fluctuate more than Ca
What is hypercalcaemia?
Excessive Ca2+
Outline the clinical features of hypercalcaemia
- 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 are the most common causes of hypercalcaemia?
In ambulatory patients
- primary hyperparathyroidism
In hospitalized patients
- malignancy
What are the less common hypercalcaemia causes?
Hyperthyroidism
Excessive intake of vitamin D
Where is PTH found in the body?
PTH synthesized and secreted by parathyroid glands (4 small glands located behind the thyroid)
What is hyperparathyroidism?
Hyperparathyroidism is typically a benign tumour of one (or more) of the 4 glands, causing increased PTH levels ⇒ increased serum Ca2+
How is the serum biochemistry altered in hypercalcaemia?
Calcium - modest / marked increase
Phosphate - low / low normal
Alkaline phosphatase raised in ~ 20% of cases
Creatinine may be elevated in long standing disease (kidney damage)
[PTH] should be interpreted in relation to calcium - inverse relationship
What is alkaline phosphatase?
Enzyme found in the liver, bone and many other tissues
What causes increased levels of alkaline phosphatase?
In presence of bone disease may be elevated due to increased bone turnover
What is hypercalcaemia of malignancy?
(HCM) - condition in cancer patients defined when serum calcium level (corrected for albumin) is > 2.6 mmol/L
Describe humoral hypercalcaemia of malignancy
Humoral
e.g., lung carcinoma secreting PTHrP
PTHrP secreted by tumor cells and binds to PTHr ⇒ hyperparathyroidism
Can cause bone lesions in hospitalised patients
How does metastatic hypercalcaemia of malignancy occur?
Tumour cells release cytokines promoting osteoclast differentiation = bone resorption
What causes haematological hypercalcaemia of malignancy?
Myeloma of plasma cells secreting osteoclastogenic cytokines e,g, RANKL
What are the most common causes of hypocalcaemia?
Vitamin D deficiency
Renal failure
What are the less common causes of hypocalcaemia?
Hypoparathyroidism
What are rickets / osteomalacia?
Bone disease associated with vitamin D deficiency
What is rickets?
Rickets - in children, failure of bone mineralisation and disordered cartilage formation
