10. Calcium Metabolism and Disorders of Calcium Balance Flashcards
Why is calcium absorption in the GIT considered incomplete?
Requires activated vitamin D
Calcium containing salts aren’t absorbed
What happens to the calcium that is ingested?
1/2 is absorbed
Some lost in stool
In a steady state, remaining calcium is excreted in urine
What form is calcium stored as in bone?
Hydroxyapatite
How is calcium reabsorbed in the kidneys?
Only ionised calcium is filtered
Some reabsorbed passively
Active under endocrine control
What are the forms of calcium in circulation?
40% bound to albumin
15% bound to citrate, nitrate and phosphate
45% ionised/free
How does hypoalbuminaemia affect calcium balance?
Less bound to albumin
How does alkalosis affect calcium balance?
More bound calcium
Which hormone reacts first to a dip in calcium?
PTH: peptide
What effect does PTH have on the GIT?
Increases absorption of calcium and phosphate
What is the overall effect of PTH?
Increase calcium but reduce phosphate
‘Phosphate Trashing Hormone’
What effect does PTH have on bone?
Increases number and activity of osteoclasts
Increased bone resorption
What effect does PTH have on the kidneys?
Increase calcium absorption in the distal nephron
Increase 1a hydroxylase enzyme activity > increased calcitriol
Increased excretion of phosphate into urine
What effect does calcitriol have on the GIT?
Increased TRPV6 channels to increase calcium absorption
What effect does calcitriol have on the bone?
Bind to osteoblasts and osteocytes
Increase pyrophosphate levels which inhibits calcium uptake into bone
What effect does calcitriol have on the kidneys?
Increase TRPV5 channels and calbindin D28K
-increases resorption in distal tubule
No loss of phosphate
What are the clinical effects of hypercalcaemia?
Stones Bones Groans Psychiatric overtones Short QT interval Dehydration (diuretic effects) Chronic kidney disease due to deposition Nephrogenic diabetes insipidus (reduced response to ADH) Type 1 renal tubular acidosis
What are the causes of hypercalcaemia?
PTH excess
Vitamin D excess
Bone turnover
Thiazide diuretics
What is the most common cause of hypercalcaemia in asymptomatic outpatients?
Hyperparathyroidism
What is the most common cause of hypercalcaemia in in-patients?
Malignancy
Immobility
Give causes of PTH excess
Hyperparathyroidism
Familial hypocalciuric hypercalcaemia
PTHrP secreting cancer
Lithium
What diseases can cause vitamin D excess?
Granulomatous disorders can secrete ectopic calcitriol
eg. sarcoid, TB
What are the reasons for hormone independent bone turnover?
Bone mets
Paget’s disease
Hyperthyroidism
Immobilisation
How is a diagnosis of hypercalcaemia made?
Correct for hypoalbuminaemia
Physical exam and CXR
PTH level
-if suppressed check PTHrP, calcidiol, calcitriol
How is serum calcium corrected for hypoalbuminaemia?
(0.02 x (40-albumin)) + serum calcium
How does a physical exam and chest x ray help in the diagnosis of hypercalcaemia?
Cancer should be obvious at this stage
Granulomatous disease on CXR
What does high PTH suggest?
Primary hyperparathyroidism
What does high PTHrP suggest?
Malignancy
What does high calcidiol suggest?
Vitamin D excess
What does high calcitriol but not calcidiol suggest?
Ectopic calcitriol production
What is the treatment for hypercalcaemia?
0.9% saline Furosemide Bisphosphonates Calcitonin Steroids Parathyroidectomy
Why is furosemide given?
Prevents resorption of calcium
What bisphosphonate is given and why?
Pamidronate IV
Reduces bone resorption
What side effect is associated with bisphosphonates?
Osteonecrosis of the jaw
Why is calcitonin given?
Lowers osteoclast activity, but weak action
Why are steroids given?
In granulomatous disease
What are the symptoms of hypocalcaemia?
Tetany, spasms, cramps Perioral parasthesia Chvostek's sign (facial nerve) Hyperreflexia Trousseau's sign (pinching hand)
What are the causes of hypocalcaemia?
Hypoparathyroidism Vitamin D deficiency Increased bone uptake Pancreatitis (saponification) Transfusions Alkalosis Chronic kidney disease
What are the causes of hypoparathyroidism?
Autoimmune
Surgery
Low Mg
What are the causes of vitamin D deficiency?
Lack of sunlight
Low CYP450
How does chronic kidney disease cause tertiary hyperparathyroidism?
Low calcitriol due to reduced 1a hydroxy enzyme and increased phosphate as GFR too low to excrete
Reduced GIT, renal and bone resorption of calcium
Results in low ionised calcium and high phosphate
-increased PTH > Increases calcium
Causes chronic kidney disease mineral bone disorder
Results in vascular calcifications and adenoma in parathyroid
What else should be measured in hypocalcaemia?
PTH Creatinine Phosphate Magnesium Calcidiol and calcitriol
What are the test results for primary hypoparathyroidism?
Low PTH
high phosphate
What are the test results in chronic kidney disease?
High creatinine
Low calcitriol
What is the treatment for hypocalcaemia?
Oral calcium
Vitamin D
Magnesium IV slow infusion
What is the treatment for severe hypocalcaemia symptoms?
10ml of 10% calcium gluconate over 10 mins
What is the preferred form of vitamin D to give in hypocalcaemia?
Calcidiol in general
In chronic kidney disease and hypoparathyroidism calcitriol preferred as they can’t convert to calcidiol
