Calcium homeostasis involves absorption of calcium from the intestine, regulation by parathyroid hormone (PTH), vitamin D, and calcitonin, and storage in bone. PTH increases calcium levels by stimulating bone resorption and renal reabsorption and vitamin D absorption. Vitamin D increases intestinal calcium absorption. Calcitonin decreases calcium by inhibiting bone resorption. Hypocalcemia causes neurological symptoms and hypercalcemia causes gastrointestinal and renal issues. Conditions are diagnosed by calcium levels and other tests and treated by calcium supplementation or intravenous calcium for hypocalcemia and hydration and medications for hypercalcemia.

1. CALCIUM HOMEOSTATIS

2. OVERVIEW
Intro.
Absorption.
Function.
Regulation of calcium homeostasis:
 PTH.
 Vitamin D.
 Calcitonin.
Hypocalcaemia.
Hypercalcaemia.
References.

3. INTRO
Fifth most common element in the body.
99% present in skeleton (reservoir).
1% intracellular.
0.1% extracellular.
Forms of Ca in the blood:
 50% is free ionised calcium.
 40% is protein bound (mainly albumin).
 10% is complexed to small diffusible ligands (e.g. bicarbonate,
citrate, phosphate, lactate and sulphate).
Normal level 2.20 -2.6 mmol/L.

4. ABSORPTION
Widely distributed in food substances e.g. Milk, Cheese, Egg-yolk,
Fish , Beans, and Lentils.
The 2 mechanisms of intestinal absorption of calcium are:
 Active: is a saturable, transcellular process which involves calbindin
(calcium-binding protein) – regulated by the active form of vitamin D.
 Passive: is a nonsaturable, paracellular low efficiency process, which is
not affected by calcium status or parathyroid hormone.
 Both processes occur throughout the small intestine.
 Absorption is related to calcium intake.

5. ABSORPTION
Factors stimulating calcium absorption:
An acidic pH.
High protein diet – Lysine and Arginine cause maximal absorption.
Presence of vitamin D.
State of health and intact mucosa.
PTH stimulates the activation of vitamin D, consequently indirectly
increasing absorption of Calcium.

6. ABSORPTION
Factors inhibiting calcium absorption:
Alkaline pH
High fat diet – fatty acids form calcium soaps that cannot be
absorbed.
Excess phosphates, magnesium and iron.
Glucocorticoids reduce intestinal absorption of calcium.
Calcitonin decreases calcium absorption indirectly by inhibiting the
activation of vitamin D.
Advancing age and intestinal inflammatory disorders.

7. FUNCTION
 Major structural element in the bones and teeth.
Essenial for several physiological processes such as neuromuscular
transmission, smooth, skeletal, and cardiac muscle contractions,
nerve function, and cell division and movement.
Co-factor in Blood coagulation.
Plays an important role in the action of other intracellular
messengers e.g. cyclic adenosine monophosphate (cAMP) and
Inositol -triphosphate, which are responsible for mediating the
cellular response to various hormones including epinephrine,
glucagon, ADH, and secretin.
Release of neurotransmiters and hormones.

8. REGULATION OF CALCIUM HOMEASTASIS
The 3 main hormones involved in the homeostatic regulation:
PTH
Vitamin D
Calcitonin
Acting at 3 target organs: intestine, bone and kidneys.

9. PTH
Polypeptide containing 84 amino acids residues.
Secreted by the chief cells in the 4 parathyroid glands.
Part of a negative feedback loop to maintain calcium ions in the
extracellular fluid.
 In hypocalcaemia, parathyroid hormone secretion is stimulated.
 In hypercalacaemia, secretion is inhibited, and the calcium is deposited in the
bones.
PTH increases serum calcium levels through:
 Increasing bone resorption by activating osteoclastic activity.
 Increasing renal calcium reabsorption by the distal renal tubules.
 Increasing renal phosphate excretion by decreasing tubule phosphate reabsorption.
 Increasing the synthesis of 1,25-dihydroxyvitamin D (also called calcitriol) by
increasing the activity of alpha-hydroxylase enzyme in the kidney.

10. VITAMIN D

Group of closely related sterols produced by the action of UV
light.

Vitamin D3 (also called Cholecalciferol) is produced by the
action of sunlight and is hydroxylated to 25-hydroxy-
cholecalciferol (25-HCC) in the liver.

Further hydroxylation of 25-HCC occurs in the proximal
tubules of the kidney to form the active metabolite, 1,25-
hydroxy-cholecalciferol (1,25-DHCC) or calcitriol.

Calcitriol's formation is facilitated by PTH.

11. VITAMIN D
Vitamin D maintains calcium homeostasis by:
-Increasing calcium absorption from the intestine (by interacting with a
calcitriol receptor located in intestinal epithelial cells.
-Stimulating renal tubular reabsorption of calcium.
-Increasing bone calcification and mineralisation (by acting on
osteoclasts in the bone)

12. CALCITONIN
 32 amino acid polypeptide secreted by the parafollicular cells in
the thyroid gland.
 Major stimulus for calcitonin secretion is increased serum calcium.
 The major action of calcitonin is to inhibit osteoclastic bone
resorption, which decreases the serum calcium concentration.
 Clinical use:
 Used in the treatment of Paget's diease.

13. CAUSES AND SYMPTOMS OF
HYPOCALCAEMIA
 Causes:
 Hypoalbuminaemia,
 Hypoparathyroidism
(idiopathic,surgical, or
transient)
 Magnesium deficiency
(alcoholism, chemotherapy
treatment)
 Pseudohypoparathyroidism
 Vitamin D deficiency
 Malnutrition
 Renal insufficiency
 Acute pancreatitis
 Bacterial/viral infections.
 Drugs including
anticonvulsants (e.g.
phenytoin, phenobarbital and
rifampin)
 Symptoms:
 Asymptomatic.
 Neurological (tingling,
tetany, mental changes)
 Muscle cramps (changes in
muscle excitability)
 Cardiac signs (abnormal
ECG)
 Seizure

14. CAUSES OF HYPERCALCAEMIA
PTH mediated:
Primary hyerparathyroidism.
Lithium induced.
Tertiary hyperparathyroidism.
 Calcitriol mediated:
 Granulomatous disease.
 Milk alkali syndrome.
 Exogenous Vitamin D.
 Other causes:
 Vitamin A toxicity.
 Paget's disease.
 Kidney stones.
 Adrenal insufficiency.
Cancer:
Multiple myeloma.
 PTHrp mediated-Breast, lung,
renal cancer.
Bone metastases.

15. SYMPTOMS OF HYPERCALCAEMIA
 Neurological & psychiatric (lethargy,
confusion, irritability, depression).
 GI issues (anorexia, abdo pain, nausea
& vomiting, constipation).
 Renal issues (polyuria, renal stones).
 Muscoskeletal issues (bone/joint pain,
muscle weakness, cardiac
arrhythmias).

16. DIAGNOSIS
Total (amended) Calcium
amended[Ca]=measured total[Ca]+0.02(40-[albumin])
Albumin
PTH ( high hypocalcaemia, low hypercalcaemia)
Vitamin D (high hypocalcaemia, low hypercalcaemia)
LFT (ALP is raised hypercalcaemia and normal in hypocalcaemia)
(Phos raised in hypocalcaemia and low in hypercalcaemia)
U&E (urea and creatinine increased in hypercalcaemia)
Iron (raised in hypocalcaemia)
PH (Acidosis increases calcium and alkalosis decreases calicum)
24-hour urinary calcium (elevated in hyperparathyroidism, renal
failure and decreased in hypoparathyroidism, malabsorption disorders)

17. TREATMENT
H
Hypocalcaemia:
O
ral calcium and vitamin D
supplements and Mg
supplements (mildly
symptomatic and chronic
hypocalcaemia).
I
ntravenous calcium gluconate
(acute symptomatic
hypocalcaemia).

Hypercalcaemia:
Mildly symptomatic patients (3.0 – 3.5
mmol/L) must avoid factors that can
aggravate hypercalcemia, e.g. thiazide
diuretics and lithium carbonate
therapy, and a high calcium diet.
Ca >3.5mmol/Lalways requires urgent
treatment, including volume expansion
with isotonic saline, administration of
salmon calcitonin and zoledronic
acid .
Follow-up therapy to treat underlying
disease e.g. Cancer.

18. References

Baynes J. (1999) Medical Biochemistry. Basildon.Harcourt Brace and
Company Limited.

Baker S. et al.(2002) The essentials of Calcium, Magnesium and Phosphate
Metabolism. Critical Care and resuscitation. 4(4) pp. 301-306.[online]

Nessar A. (2010) Clinical Biochemistry. New York. Oxford University Press.

Mundy G. et al. (1999) Hormonal Control of Calcium Homeostasis, Clinical
Chemistry, 45 (8), pp.1347-1352.

Warrel D. et al.(2003) Oxford Textbook of Medicine. London.Oxford University
Press.[online]

Peacock M. (2010) Calcium Metabolism in Health and Disease, Clinical
Journal of American Society of Nephrology,5(1), pp. 23-30.[online]