Minerals: Calcium, Phosphorus & Magnesium Systems

Flashcards covering the laboratory evaluation of calcium, phosphorus, and magnesium systems, including mineral physiology, hormonal regulation, and abnormalities of these minerals.

What are the three main fractions of serum calcium and their approximate percentages?

50% is ionized, 40% is bound to albumin, and 10% is bound to non-protein anions.

Which form of serum calcium is physiologically important and requires special handling for measurement?

Ionized calcium (iCa).

What are the main effects of Parathyroid hormone (PTH) on calcium and phosphorus regulation?

It causes Ca release from bone, Ca retention by kidneys, renal P excretion, and renal calcitriol formation.

What is the primary role of activated Vitamin D (calcitriol) in mineral regulation?

It increases intestinal Ca and PO4 absorption.

What is the main effect of Calcitonin on calcium levels?

It decreases Ca release from bone.

What is PTH-rP and when is it typically secreted?

Parathyroid hormone-related protein (PTH-rP) is secreted by some tumors, affects fetal Ca balance, and its function in adults is largely unknown.

How do regulatory hormones generally respond to hypocalcemia?

There is no increase in calcitonin, increased PTH, and increased calcitriol (active vit D3).

How do regulatory hormones generally respond to hypercalcemia?

There is increased calcitonin, decreased PTH, and decreased calcitriol (active vit D3).

What three factors should be considered when interpreting calcium levels?

Albumin, Phosphorus, and Acid-Base balance.

List common causes of hypocalcemia.

• Hypoalbuminemia (with normal ionized Ca)

• Pancreatitis

• Kidney failure (hypovitaminosis D in dogs, cats, cattle)

• Hypoparathyroidism

• Decreased intake or absorption (ruminants)

• Milk fever, and hypomagnesemia

What are common clinical signs of hypocalcemia?

Generalized tetany, seizures (dogs); hyperesthesia and tetany early, paresis to flaccid paralysis later (cows), more sensitive to pain; nervousness, anorexia, stilted gait, hyperventilation, and numbness.

List common causes of hypercalcemia.

• Kidney disease in horses

• Vitamin D toxicosis

• Osteolytic bone lesions

• Primary hyperparathyroidism

• Hypoadrenocorticism

• Multiple myeloma

• Neoplasia (PTH-rp production)

What are common clinical signs of hypercalcemia, and what Calcium x Phosphorus product predicts tissue mineralization?

PU/PD, lethargy, weakness, constipation, and mineralization of soft tissue. A Ca x P product > 5.6 (or >70 in US units) predicts tissue mineralization.

What is an important consideration when measuring serum phosphorus?

Avoid hemolysis, as red blood cells contain phosphorus.

What are common causes of hypophosphatemia?

Parturient paresis in cows, Vitamin D deficiency / malabsorption, diabetic ketoacidosis & treatment with insulin, and early hyperparathyroidism and humoral hypercalcemia of malignancy (HHM).

Is serum magnesium a good indicator of total body magnesium?

No, serum Mg is a poor indicator of total body Mg, as it has similar fractions to Ca, and localized deficiencies (e.g., low CSF Mg) can occur despite normal serum levels.

What are the primary mechanisms regulating magnesium balance?

Intestinal absorption, renal excretion(dec GFR → inc Mg), and milk excretion (during lactation). Excretion → feces, kidneys, lactation

List common causes of hypomagnesemia.

Hypoalbuminemia (hypoproteinemia), malabsorption (GI loss), increased GFR (renal loss), and cattle on Mg-deficient or K-rich pasture(NAVLE).

What clinical signs are associated with hypomagnesemia?

Neuromuscular signs such as hyperexcitability, tremors, fasciculations, ataxia, tetany, and cardiac arrhythmias (loud heart sounds).

What typically causes hypomagnesemic tetany in cattle?

Lush spring grass pastures that are low in Mg, often fertilized with Ca, nitrates, ammonia, sulphates, and K, which further decrease Mg in the grass.

Low dietary Mg → ↓PTH → ↓Ca

• neurologic signs 

• rapidly when moved to fresh pasture

• pregnancy/lactation also predispose

List common causes of hypermagnesemia.

Decreased GFR → inc Mg (dehydration, anuric kidney failure, urinary obstruction)

iatrogenic causes (Mg-containing laxatives or antacids).

Explain how hypomagnesemia can lead to hypocalcemia, even if calcium levels are initially low.

In hypomagnesemia, Ca and Mg compete for parathyroid gland receptors. With less Mg, receptors are more available to even decreased Ca levels, 'tricking' the gland into thinking Ca is adequate, reducing PTH production and preventing correction of hypocalcemia.

Why might a horse in end-stage kidney disease have normal calcium while a dog or cat might have hypocalcemia?

Horses do not have 1-alpha-hydroxylase, so kidney tissue loss doesn't affect activated vitamin D, and they ingest high dietary calcium. Cats and dogs have 1-alpha-hydroxylase, so diminished kidney tissue reduces activated vitamin D, causing hypocalcemia.

What is a crucial step before running tests for PTH, PTH-rp, and ionized Ca?

Always call the laboratory to confirm special shipping requirements (e.g., sample type, on ice) to ensure accurate testing and avoid delays.