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Evaluation of Mineral Balance
Introduction: Minerals are naturally occurring nonorganic (noncarbon-containing) solids, many of which are classed as electrolytes. Most minerals must be acquired in the diet (essential minerals), and some are very toxic if present in more than trace amounts. There are at least fifteen essential minerals:
This lecture will concentrate on three essential minerals that are often disrupted: calcium, phosphorus and magnesium. These mineral levels are controlled by the hormones parathormone, calcitonin and Vitamin D3. Parathormone is produced by the parathyroid glands and functions to increase blood calcium and magnesium and decrease blood phosphorus. Osteoclasts are activated, resulting in calcium resorption from the bones. The kidney responds by conserving calcium and magnesium while increasing the excretion of phosphorus. In addition, the kidney increases activation of vitamin D. Calcitonin is produced by the thyroid gland and is sometimes called thyrocalcitonin. Its function is to decrease blood calcium. The osteoclasts are inhibited and the osteoblasts are stimulated, increasing calcium deposition in the bones. The kidneys conserve phosphorus and excrete calcium in response to increased blood levels of calcitonin. Vitamin D3 is the third component of calcium and phosphorus regulation. It has a permissive effect on osteoclasts, making them more responsive to parathormone and increasing blood calcium. It also facilitates the absorption of calcium and phosphorus from the intestinal tract.
Evaluation of Mineral Levels: Calcium: Calcium has many functions, including nerve impulse transmission, coagulation and bone structure. Animals with hypercalcemia may exhibit weakness, vomiting, depression, anorexia and polyuria / polydipsia. If an animal has hypocalcemia, tremors, tetany, seizures, and restlessness may be seen. Serum or heparinized plasma should be used to evaluate calcium levels. Lipemia and hemolysis invalidate calcium tests. The reference range for calcium in dogs is 8.5 - 11.0 mg/dl and in cats is 8.0 - 10.5 mg/dl. Because calcium in the bloodstream is bound to albumin for transport, hypoalbuminemic animals must have the calcium level corrected. Hypercalcemia is associated with:
Hypocalcemia can occur with:
Phosphorus: Phosphorus has many similarities with calcium. Serum or heparinized plasma should be used to evaluate phosphorus levels. Hemolysis can invalidate phosphorus tests. The reference range for calcium in dogs is 2.5 - 5.5 mg/dl and in cats is 3.0 - 6.0 mg/dl. Because calcium in the bloodstream is bound to albumin for transport, hypoalbuminemic animals must have the calcium level corrected. Hyperphosphotemia is associated with renal disease (phosphorus retention occurs) and hyperthyroidism in cats. Hypophosphotemia can be seen with animals with malignancies, diabetic ketoacidosis, milk fever and primary hypoerparathyroidism. Magnesium: Magnesium should be evaluated in any animal with hypercalcemia, hypokalemia or myopathies. Serum or heparinized plasma should be used to evaluate magnesium levels. Hemolysis invalidates magnesium tests. Because of high levels of magnesium in erythrocytes, serum or plasma should be harvested immediately. The reference range for magnesium in dogs is 1.0 - 2.5 mg/dl and in cats is 1.5 - 2.5 mg/dl. Hypomagnesemia in small animals is most commonly associated with renal wasting due to chronic renal disease. In cattle, grass tetany is the usual cause.
Pathologies of Faulty Mineral Metabolism: Rickets is a failure of mineralization of bones due to a deficiency of calcium, phosphorus and/or vitamin D and is most frequently seen in young animals with poor nutrition. The long bones soften and bend under the stress of supporting the animal's body. Alkaline phosphatase, an induced liver enzyme, is increased in animals in rickets. Most mineral abnormalities are not associated with bones, however. Parturient paresis or milk fever results in severe hypocalcemia, hypophosphotemia and muscular paresis in dairy cattle. It occurs near parturition in high producing cattle. In small breed dogs, a similar condition is called eclampsia or puerperal tetany. Affected bitches show restlessness, panting and nervousness. Within eight to twelve hours it progresses to ataxia, tetany and seizures. Hyperparathyroidism results in excessive production of parathyroid hormone. Rarely, however, is the problem with the parathyroid gland. Secondary hyperparathyroidism can be cause by chronic renal failure or improper diet. Renal secondary hyperthyroidism is most commonly seen in dogs in chronic renal failure. The kidneys are unable to excrete sufficient phosphorus, so the parathyroid gland is stimulated to release parathormone, resulting in demineralization of bone. Blood phosphorus levels are increased and calcium is decreased. The bones become soft and pliable, causing "rubber jaw" disease. The animal develops lameness and a stiff gait, and fractures may occur as a result of minor trauma. Nutritional secondary hyperparathyroidism develops when animals eat a diet with an improper balance of calcium and phosphorus. Diets with inadequate calcium, such as all-meat diets fed to dogs or calcium-poor diets fed to reptiles, results in hypocalcemia, causing secretion of parathormone. Signs include lameness, bone pain and long bone fractures. Grass tetany most commonly occurs in cattle and sheep feeding on early spring grass. Phosphorus and potassium in the grass competes with magnesium for uptake. Affected animals may be restless and have tremors, which may progress to paresis, tetany, convulsions and death.
Study Guide: 1. Complete this chart
2. Describe the sample needed to test for 3. Complete this chart
4. Differentiate between renal and nutritional secondary hyperparathyroidism. |