Calcium dysregulation Flashcards
What increases Calcium?
Vitamin D
Synthesised in skin or intake via diet
Parathyroid hormone (PTH) (secreted by parathyroid glands)
Main regulators of calcium & phosphate homeostasis via actions on kidney, bone and gut
What decreases Calcium?
Calcitonin (secreted by thyroid parafollicular cells)
Can reduce calcium acutely, but no negative effect if parafollicular cells are removed eg thyroidectomy
How do we replenish vitamin D?
UVB light shines on our skin, cholesterol precursor, Vit D3 precursor taken to liver and first hydroxylation step takes place via 25-hydroxylase, making inactive vitD (25(OH)cholecalciferol). Second hydroxylation gives active form of Vit D (1,25(OH)2 cholecalciferol) via 1 alpha-hydroxylase.
What is the active form of Vitamin D known as?
Calcitriol
How is Calcitriol regulated?
1,25(OH)2 vitamin D (calcitriol) regulates its own synthesis by decreasing transcription of 1 alpha hydroxylase
What enzyme catalyses the first step of Vitamin D3 metabolism?
25-hydroxylase
What enzyme catalyses the second step of Vitamin D3 metabolism?
1 alpha-hydroxylase
What are the effects of Calcitriol?
Increased reabsorption of Calcium and phosphate from the kidney and gut. Increased osteoblast activity.
What are the effects of PTH?
Negative effects on bone, bone resorption due to increased osteoclast activity, increased plasma Calcium. Increases Calcium reabsorption in the kidney and Phosphate secretion. Stimulates 1-alphahydroxylase activity, so you have increased calcitriol synthesis, increase in calcium and phosphate absorption in the gut. Phosphate balances out.
Very high PTH can block the Na+/PO43- co-transporter.
How does FGF23 regulate serum Phosphate?
Phosphate usually reabsorbed in the PCT cells via Na+/PO43- co-transporter. FGF23 inhibits co-transporter.
It also inhibits Calcitriol Formation, so less Phosphate absorption from the gut.
What are the signs and symptoms of Hypocalcaemia?
Sensitises excitable tissues; muscle cramps, tetany tingling. CAT
Convulsions
Arrhythmias
Tetany
What is Trousseu’s sign
carpopedal spasm
What is Chvosteks’ sign?
facial paraesthesia
What are the causes of Hypocalcaemia?
Low PTH levels = hypoparathyroidism - Surgical – neck surgery - Auto-immune - Magnesium deficiency - Congenital (agenesis, rare) Low vitamin D levels - Deficiency – diet, UV light, malabsorption, impaired production (renal failure
What are the signs and symptoms of Hypercalcaemia?
SAP
Stones – renal effects
Nephrocalcinosis – kidney stones, renal colic
Abdominal moans - GI effects
Anorexia, nausea, dyspepsia, constipation, pancreatitis
Psychic groans - CNS effects
Fatigue, depression, impaired concentration, altered
mentation, coma (usually >3mmol/L)
What are the causes of hypercalcaemia?
Primary hyperparathyroidism
- Too much PTH
- Usually due to a parathyroid gland adenoma
- No negative feedback - high PTH, but high calcium
Malignancy
- Bony metastases produce local factors to activate
osteoclasts
- Certain cancers (eg squamous cell carcinomas) secrete
PTH-related peptide that acts at PTH receptors
Vitamin D excess (rare)
What is the relationship between PTH and calcium?
Negative feedback
What happens in Primary Hyperparathyroidism?
Parathyroid adenoma producing too much PTH
Calcium increases, but no negative feedback to PTH due to autonomous PTH secretion from parathyroid adenoma
What is the biochemistry for Primary hyperparathyroidism?
High calcium
Low phosphate – increased renal phosphate excretion (inhibition of Na+/PO43- transporter in kidney)
High PTH (not suppressed by hypercalcaemia
What is the treatment for Primary Hyperparathyroidism?
Parathyroidectomy is treatment of choice for primary hyperparathyroidism
What are the risks associated with untreated hyperparathyroidism?
Osteoporosis Renal calculi (stones) Psychological impact of hypercalcaemia – mental function, mood
What happens in Secondary hyperparathyroidism?
Secondary hyperparathyroidism is a normal physiological response to hypocalcaemia
Calcium will be low or low/normal
PTH will be high (hyperparathyroidism) secondary to the low calcium
This is different from primary hyperparathyroidism where calcium is high
What are the causes of secondary hyperparathyroidism?
Most common cause of secondary hyperparathyroidism is vitamin D deficiency
Commonly - diet, reduced sunlight
Less common, but important = renal failure – can’t make calcitriol in renal failure
What is the treatment for secondary hyperparathyoirdism?
Vitamin D replacement
In patients with normal renal function
Give 25 hydroxy vitamin D
Patient converts this to 1,25 dihydroxy vitamin D via 1a hydroxylase
Ergocalciferol - 25 hydroxy vitamin D2
Cholecalciferol - 25 hydroxy vitamin D3
In patients with renal failure - inadequate 1a hydroxylation, so can’t activate 25 hydroxy vitamin D preparation
Give Alfacalcidol - 1a hydroxycholecalciferol
What happens in Tertiary hyperparathyroidism?
RARE
Occurs in patients with impaired renal function.
Occurs in chronic renal failure
Can’t make calcitriol
PTH increases (hyperparathyroidism)
Parathyroid glands enlarge (hyperplasia)
Autonomous PTH secretion causes hypercalcaemia Initially calcium falls and PTH rises (secondary hyperparathyroidism), but over a long period high PTH drive by enlarged parathyroid glands increases calcium
What is the treatment for Tertiary hyperparathyroidism?
Parathyroidectomy
What should you do if you have a patient with hypercalcaemia?
Look at PTH level, Normal response to hypercalcaemia is for PTH to fall. Hypercalcaemia due to malignancy PTH is switched off.
What would happen in Vitamin D deficiency?
Calcium will be low or low/normal
PTH will be high (hyperparathyroidism) secondary to the low calcium
Vitamin D is measured as 25 (OH) vitamin D
Calcitriol (1,25 dihydroxy vitamin D) is very difficult to measure.
