Endocrinology - Calcium and phosphate regulation Flashcards
What are the basics involved in calcium homeostasis
- Calcium homeostasis is the interaction between PTH and vitamin D
- Increased retention of calcium in kidneys
- Causes bones to release calcium
- Regulates the activation of inactive vitamin D which increases dietary absorption of calcium
How is phosphate regulated
- Phosphate is pumped from the urine into proximal convoluted tubule cells
- The pump is inhibited by PTH and fibroblast growth factor 23 (from osteocytes)
- FGF23 also inhibits calcitriol which decreases phosphate reabsorption from the gut
How is PTH secretion regulated
- Calcium sensor receptors are on the surface of parathyroid cells
- High calcium concentration binds to receptor and inhibits PTH release
- If calcium is low then there is no binding so PTH is released
How is calcitriol synthesised
- The skin contains 7-dehydrocholesterol
- UVB converts it to vitamin D3 (cholecalciferol)
- The liver converts it to 25 OH-D3
- Renal 1a hydroxylase stimulated by PTH converts it to 1,25 (OH)2-D3 (Calcitriol)
What does calcitriol do
- Causes increased maintenance of calcium in bone
- Causes increased absorption of calcium in the gut
- Causes increased calcium reabsorption in the kidneys
- Has negative feedback on PTH
Causes of vitamin D deficiency
- Malabsorption or dietary insufficiency
- Lack of sunlight
- Liver disease
- Renal disease
- Vitamin D receptor defects
How does extra cellular calcium affect nerve and skeletal muscle excitability
- To generate an AP Na+ needs to pass across the cell membrane
- Hypercalcaemia blocks Na+ influx, so there is less membrane excitability
- Hypocalcaemia enables greater Na+ influx, which causes more membrane excitability
What is the normal range of calcium and what are the signs and symptoms of hypocalcaemia
Normal range is 2.2-2.6 mmol/L Signs and symptoms - parasthesia (hands, mouth, feet, lips) - Convulsions - Arrhythmia - Tetany
What is Chvosteks sign
- Tap the facial nerve just below the zygomatic arch
- Positive response = facial muscles twitching
- Indicates neuromuscular irritability due to hypocalcaemia
What is Trousseaus sign
Inflation of the BP cuff for several minutes induces carpopedal spasm = neuromuscular irritability due to hypocalcaemia
What are the causes of Hypocalcaemia
- Vitamin D deficiency
- Low PTH levels = hypoparathyroidism
- Surgical - neck surgery
- Autoimmune
- Magnesium deficiency
- PTH resistance e.g. pseudoparathyroidism
- Renal failure (impaired 1a hydroxylation)
Signs and symptoms of hypercalcaemia
Stones, moans and psychic groans
Stones - renal effects
- Polyuria & thirst
- Nephrocalcinosis, renal colic, chronic renal failure
Abdominal moans - GI effects
- Anorexia, nausea, dyspepsia, constipation, pancreatitis
Psychic groans - CNS effects
- Fatigue, depression, impaired concentration, altered mentation, coma (at >3mmol/L)
Causes of hypercalcaemia
- Primary hyperparathyroidism
- Malignancy - tumours/metastases often secrete a PTH like peptide
- Conditions with high bone turnover
- Vitamin D excess
- Secondary hyperparathyroidism - PTH increases to try to normalise serum
- Hyperparathyroidism caused by vitamin D deficiency
What would you expect to see in a patient with a vitamin D deficiency
- Plasma 25(OH)D3 usually low
- Plasma Calcium low
- Plasma Phosphate low
- Plasma PTH high
How would you treat a patient with vitamin D deficiency
In patients with normal renal function give 25(OH)D
In patients with renal failure give 1a hydroxycholecalciferol
