Calcium absorption, which takes place in your horse's small intestines, can be enhanced by vitamin D activity, and is inhibited by the presence of phosphorous. This is one reason why you don't want to feed your horse too much bran or other sources with excessive phosphorous. In general, grains are much higher in phosphorous than is roughage. Once the calcium has been absorbed into the bloodstream, it is further regulated in the kidney.

Horses ordinarily lose large amounts of calcium in their kidneys, but their diets are so high in calcium that it doesn't usually matter. Calcium can be filtered and re-absorbed in the kidneys – this is dependent on the presence of Vitamin D and another hormone called parathyroid hormone (PTH).

The biggest store of calcium is in the bones, where it is in constant flux. Cells called osteoclasts, under the influence of PTH and vitamin D, break down bone to supply calcium to the body, and cells called osteoblasts build bone back up, storing calcium in the process. This constant breakdown and repair of bone is completely normal and necessary for proper function.

Even though most blood tests measure total calcium, all calcium is available for use by muscles and nerves. Some is stored in the blood in a useless form. There is a big functional difference between free and bound calcium, otherwise known as ionized and non-ionized calcium. Only ionized calcium can fulfill the physiologic processes that are needed for muscles, nerves, and cellular messenger systems to work. This is why the body's acidity level, or pH, is important. When the pH is low, meaning that the body is acidic, there is more ionized calcium. When the body's pH is high, or alkalotic, there is less ionized calcium. So, the endurance horse who is alkalotic at the end of a long ride because of water and electrolyte losses is also more likely to suffer from hypocalcemia.

Causes

Bicarbonate overdose. Sometimes bicarbonate is given to horses in order to make them alkalotic, thus protecting them against alkalosis. However, the alkalosis itself it can precipitate hypocalcemia.

Blister beetle toxicity. These beetles are found in alfalfa hay, usually from southern or western United States, and cause colic, oral ulcers, and renal disease in addition to hypocalcemia.

Excessive Lasix administration – Lasix causes the body to lose water, chloride, hydrogen and calcium.

Very rapid intravenous tetracycline administration. Tetracycline is a broad spectrum antibiotic that is frequently given to horses when they have tick-borne diseases. It can temporarily bind calcium, but rarely causes problems if it is given slowly.

Oxalate toxicity. This is found in some weeds, as well as rhubarb. They bind calcium, causing hypocalcemia in the horse.

Hypoparathyroidism. In horses this is usually a secondary process, due to excessive phosphorous in the diet (such as with a high bran diet).