67 - Calcium, magnesium and phosphate homeostasis

What is calcium physiologically important for?

- Blood clotting

- Muscle contraction

- Neuronal excitation

- Enzyme activity

What is calcium structurally important for?

Hydroxyapatite - predominant mineral in bone

What is phosphate physiologically important for?

- ATP

- Intracellular signalling

- Glycolysis

What is phosphate structurally important for?

- DNA backbone

- Hydroxyapatite - mineral in bone

- Phospholipid membrane

What is the normal range for calcium in an adult?

2.2 - 2.6 mmol/L

What does calcium and phosphate homeostasis rely on a balance of?

- GI absorption

- Bone storage/resorption

- Renal excretion/reabsorption

What are 2 key regulators of calcium and phosphate homeostasis?

1) PTH

2) Vitamin D

What are the 2 direct actions of PTH due to hypocalcaemia?

1) Bone resorption of calcium and phosphate

2) Kidney to reabsorb calcium but excrete phosphate

What is the indirect actions of PTH due to hypocalcaemia?

Increased conversion of vitamin D to calcitriol in the kidneys which increases calcium and phosphate absorption in the gut

What is the net effect of PTH?

- Increase serum calcium

- Decrease serum phosphate

What factors affect your vitamin D level?

- Season

- Climate

- Clothing

- Sunscreen use

- Skin tone

- Sun exposure

- Age

- Diet

- BMI

- Liver and kidney disease

Which regulators inhibit bone resorption?

- Calcitonin

- Oestrogen

Which regulators promote bone resorption?

- PTH

What are the causes of hypocalcaemia?

- Vitamin D deficiency

- CKD

- Malnutrition

- Hypoparathyroidism

- Hyperphosphataemia

- Acute pancreatitis (binds to lipids)

- Hypoalbunaemia

- Spurious results

What are the clinical features of hypocalcaemia?

- Tetany

- Paraesthesia

- Cramps

- Convulsions

- Psychosis

What is tetany?

Muscle spasms

What is Chvostek's sign?

Push the cheek and it spasms

(low calcium)

What is Trosseau sign?

Migratory widespread thrombi (swelling, erythema, tenderness in extremities); indicative of pancreatic carcinoma

What are the causes of hypercalcaemia?

- Hyperparathyroidism

- Malignancy

- Medications (thiazides, lithium)

- Vitamin D excess

- Hyperthyroidism

- Bone disease (OP)

What are the clinical features of hypercalcaemia?

- Nephrolithiasis

- Bone pain

- Lethargy

- Depression

- Abdominal pain

- Constipation

- Nausea

- Vomiting

- Dehydration

What are the causes of hypophosphataemia?

- Malnutrion/malabsorption

- Hyperparathyroidism

- Diarrhoea

- Refeeding syndrome

What is refeeding syndrome?

Hypokalaemia, hypomagnesemia, and hypophosphatemia after refeeding a starved patient

What are the clinical features of hypophosphataemia?

- Haemolysis

- Thrombocytopenia

- Respiratory muscle failure

- Rhabdomyolysis

- Convulsions

- Osteomalacia/rickets

What are the causes of hyperphosphataemia?

- Renal failure

- Hypoparathyroidism

What is magnesium physiologically important for?

- Cofactor for ATP reactions

- Neuromuscular excitability

- Enzyme function

- Reugulation of ion channels

When is hypomagnesemia most commonly seen?

In hospitalised patients

What are the causes of hypomagnesaemia?

- Malnutrition/ malabsorption

- Drugs

- Diarrhoea

- Refeeding syndrome