Hypocalcemia

Hypocalcaemia

Written by – Dr Ashly Liu

Calcium

Serum calcium is normally maintained in a narrow range that’s required for optimal activity of extra and intracellular processes.

Where is it in our body?

Calcium is typically transported partly bound to plasma proteins (i.e. albumin), to small anions (phosphate and citrate) and partly in the ‘free’ or ionised state. Of note, only the ionised state is metabolically active.

What controls it?

Directly by hormonal control

• Parathyroid hormone causes increase in serum Ca by:
  • Increasing reabsorption of calcium in kidneys and GIT
  • Releasing Ca and Phosphate from bones
• Vitamin D (calcitriol) causes increase in serum Ca by:
  • Increasing calcium and phosphate absorption in the GIT
  • Increasing calcium and phosphate reabsorption in the kidneys
• Calcitonin causes decrease in serum Ca by:
  • Opposing PTH and inhibits bone resorption

Indirectly by

• Magnesium: Hypomagnesemia reduces PTH secretion or causes PTH resistance and therefore indirectly leads to hypocalcaemia.
• Phosphate: Hyperphosphataemia leads to hypocalcaemia as phosphate binds to serum calcium

Directly impacting ratio of protein bound calcium to ionised calcium

• Serum protein levels (e.g. albumin)
• Acid base status (e.g. low pH 

Approach

1. Confirm true hypocalcaemia 
2. Screen for symptoms and monitor for haemodynamic instability
3. If low and symptomatic or severely low, start management immediately

Differentials

Pseudohypocalcaemia

• Hypoalbuminaemia
• Gadolinium contrast 
• Acid-base disturbance 

Hypocalcaemia with low PTH 

• Hypoparathyroidism 
  • Surgical removal of the PTH glands intentionally or non-intentionally
  • Autoimmune destruction of the PTH glands
  • Loss of PTH glands from surgical/autoimmune/development)

Hypocalcaemia without high PTH 

• Vitamin D deficiency
• Chronic kidney disease
• Parathyroid hormone resistance
• Extravascular deposition – pancreatitis

Other

• Medications (loop diuretics, bisphosphonates, denosumab)
• Hypomagnesemia 
• Multiple blood transfusions/haemolysis
• Osteoblastic metastases
• Hungry bone syndrome

Management

Mild-Moderate Hypocalcaemia (asymptomatic and >1.9mmol/L corrected Ca)

• 1 tablet of Caltrate once or twice a day

Severe Hypocalcaemia (symptomatic or <1.9mmol/L corrected Ca)

• *IV 10mL of 10% calcium gluconate in 5% dextrose over 10 minutes
• Followed by a slow infusion of 100mL of 10% calcium gluconate in 1L in normal saline/dextrose 5% at 50mL/h
• Monitor every 304 hours until normocalcaemia is achieved
• Change to oral calcium

If there is associated hypomagnesaemia

• IV 10% Magnesium sulfate infused over 10-20 min

*Remember to monitor for extravasation of calcium, which may lead to skin necrosis