Equis ISSN 2398-2977


Synonym(s): Transport tetany

Contributor(s): Graham Munroe, Vetstream Ltd


  • Definition: serum ionized magnesium <1.1 mg/dl (<0.45 mmol/l).
  • Magnesium is an essential intracellular cation involved in many intracellular enzymatic reactions including regulating the calcium channel function, neurotransmission, vasomotor tone, muscular contraction and cardiac excitability. 
  • Because of its importance, homeostatic mechanisms normally maintain intracellular and extracellular magnesium concentrations within narrow limits, and overt hypomagnesemia is rare in horses. 
  • Magnesium is primarily an intracellular cation, with only 1% being found in the serum. 
  • Extracellular magnesium exists in three forms - protein-bound, complexed with anions (such as phosphate) and ionized. 
  • Ionized magnesium is the physiologically active form. 
  • Total serum levels therefore may not reflect either the active moiety or the total body concentration. Intracellular magnesium is primarily in the bone (67%), muscle (2%) and other soft tissues (11%).
  • Magnesium is antagonistic to the actions of calcium.



  • Decreased intake due to anorexia. 
  • Gastrointestinal loss from profuse diarrhea, malabsorption, maldigestion. 
  • Urinary loss due to renal tubular acidosis, primary renal disease, diabetes mellitus   Pancreas: chronic pancreatitis - diabetes mellitus  /diabetic ketoacidosis, hyperthyroidism   Endocrine: hyperthyroidism  , hypokalemia, hypocalcemia   Hypocalcemia  , hypophosphatemia Link: Hyperphosphatemia (NEW).
  • Parathyroid hormone increases magnesium reabsorption via the renal tubules. 
  • Iatrogenic due to parathyroidectomy, drug administration (insulin, loop diuretics, cisplatin, aminoglycosides) or inadequate supply in intravenous fluids.

Predisposing factors

  • Ingested magnesium is absorbed into the body from the proximal (25%) and distal (35%) small intestine, and the colon (5%). Prolonged malnutrition, anorexia or significant intestinal disease may lead to hypomagnesemia.
  • High levels of dietary potassium or nitrogen reduce magnesium absorption. 
  • Renal homeostasis is vital for regulation of magnesium levels therefore any condition causing excessive renal loss can also result in hypomagnesemia.
  • Calcium-containing compounds lower serum magnesium concentrations.
  • Exhaustion, hyperaldosteronism and hyperthyroidism   Endocrine: hyperthyroidism  can all lead to hypomagnesemia.


  • Magnesium is secreted into mares milk and may be associated with hypocalcemia   Hypocalcemia   in early lactation.


  • Hypomagnesemia is a common finding in the critically ill patient but whether it is contributory to mortality, or a result of, the severe disease is unknown.
  • Magnesium homeostasis is mainly determined by renal elimination and reabsorption. 80% of total serum magnesium is filtered by the glomerulus. 10-15% is reabsorbed in the proximal tubule, 60-70% in the cortical thick ascending loop of Henle, 10-15% in the distal convoluted tubule. 
  • Magnesium deficit is most commonly associated with conditions leading to increased glomerular filtration rate or other reasons for decreased renal function. 
  • Digestive secretions contain considerable magnesium (0.5-1.0 mg/kg/day). Magnesium levels will decrease if resorption by the distal small intestine does not keep pace with this loss.
  • Magnesium is secreted into the milk of lactating mares, especially in early lactation.
  • Substantial amounts of magnesium can be lost in sweat.
  • A significant role for intracellular magnesium is as a co-factor for the calcium-ATP pump that moves intracellular calcium into the sacroplasmic reticulum. It performs a similar role with the sodium-potassium-ATP pump that moves these electrolytes across cell membranes. Reduction of the activity of these ion pumps is likely to be the major cause of the clinical signs of hypomagnesemia.


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Further Reading


Refereed papers

Other sources of information

  • Marino P (2007)Renal and Electrolyte Disorders: Magnesium. In:The ICU Book. Lippincott Williams & Wilkins. pp 625638.
  • Stewart A J (2004)Magnesium Disorders. In:Equine Internal Medicine. Eds: Reed S M, Bayly W M & Sellon D C. 2nd edn. Saunders, Missouri, USA. pp 1365-1379.