Glands at a Glance: A Horse’s Endocrine System

Your horse's life-sustaining endocrine system works in wondrous, if little-noticed, ways.

The Adrenal Gland
Alias:"the shock organ."
Appearance: like lumpy knit caps sitting on the ends of the kidneys.

Structures: a two-part organ that's virtually two endocrine glands in one.

  • The adrenal cortex, the outer layer derived from urinary/genital-tract cells, is divided into three zones which produce numerous steroid hormones.
  • The adrenal medulla, the gland's interior portion derived from neurological cells, produces catecholamines, organic compounds that act both as hormones in the bloodstream and as neurotransmitters in the nervous system.

Hormones and actions: affect a broad range of essential life functions.

  • circulation--The mineralocorticoids, including aldosterone, regulate body salt (sodium levels) and help maintain blood volume and blood pressure.
  • metabolism--The glucocorticoids, including cortisol, cortisone and coricosterone, regulate energy, protein, fat and mineral use as well as help to reduce inflammation.
  • reproduction--The gonadocorticoids have a role in sperm production in stallions but have the greatest influence on secondary sex characteristics, including gender-specific behavior.
  • stress response--The catecholamines (epinephrine, with some help from norepinephrine and dopamine) orchestrate the immediate "fight or flight" response to perceived threats. The glucocorticoids alter metabolism and body functions when stress persists.

Diseases:

  • As the "shock organ," the adrenals can be damaged and permanently scarred by bouts of severe systemic disease, such as endotoxemia and anaphylactic shock.
  • Adrenocortical insufficiency occurs when corticosteroid medications are administered for extended periods at high doses. With plenty of supplemented steroid hormones in circulation, the feedback mechanism keeps adrenal production turned off, and the gland eventually shrivels from disuse.
  • Hyperadrenocorticism, overproduction of corticosteroid hormones, typically occurs because of pituitary hypertrophy.
  • Anhidrosis, the inability to sweat when needed, may have an adrenal-hormone component, as affected horses have more epinephrine in circulation than normal horses.

The Thyroid Gland
Alias: none
Location: on each side of the windpipe, just behind the larynx.
Appearance:rather lake a split golf ball, with the two hemispheric lobes joined by an "isthmus" extending across the underside of the windpipe; isthmus is larger in foals and asses than in adult horses.

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Structures: made up entirely of follicles, hollows lined by cells that produce and secrete hormones.
Hormones and actions: affect all organ systems, adjusting body metabolism, growth, sexual maturation and vital functions to meet environmental conditions.

  • Thyroxine (T4) level determines the body's metabolic rate. The more there is, the more calories will be burned, the faster and more forcefully the heart will beat, the greater will be the appetite, the more active will be the digestive tract and the more alert, even hyperactive, will be the horse.
  • Triiodothyronine (T3) and related hormones have actions similar to thyroxine but are faster acting and one tenth as prevalent.
  • Calcitonin reduces the amount of circulating calcium by moving it out of the blood and into storage in the bones. This hormone plays a modulating role behind the primary calcium regulator, parathyroid hormone.

Diseases: Thyroid malfunctions in horses are considered to be rare, possibly because they're not well documented, and blood hormone levels are difficult to test.

  • Goiter is an enlargement of the gland due to either excess or deficiency of dietary iodine, the essential ingredient of thyroxine. Foals may be born with goiter when their dams get too little or too much iodine in their diets. Often the cause is an iodine excess from kelp-containing feed supplements.
  • Hypothyroidism, thyroid insufficiency, causes broad ranging signs, including failure to grow, decreased appetite, dull haircoat, increased sensitivity to cold, delayed shedding, lethargy and failure to produce milk in broodmares.
  • Thyroid tumors occur often in older horses, but they are usually benign adenomas and have no effect on the gland's function. Cancerous tumors do sometimes develop, and they can be surgically removed, leaving the remaining part of the gland to carry on.

The Parathyroid Gland
Alias: none
Location: accounts differ; horse is believed to have four, at least two of which are in connective tissue on or near the thyroid gland; remaining two may be in the chest, close to the first pair of ribs or elsewhere on the thyroid.
Appearance: obviously hard to identify, but irregularly shaped and quite small, the size of a shirt button.
Structure: a single-function gland of paler, less dense tissue than the thyroid.
Hormone and actions: act independently of other endocrine glands, sensing plasma calcium levels and making minute-to-minute adjustments. Calcium plays a role in clotting, neuromuscular controls, cell-membrane permeability, and muscle contraction, as well as providing the building block of bones and teeth. Parathyroid hormone (PTH) increases plasma calcium by stimulating bones to release the mineral the kidneys to retain calcium in circulation rather than eliminate it in urine, and the gastrointestinal tract to take up more from food. Vitamin D is the essential partner of PTH in calcium retention and absorption. Calcitonin is the antagonist.

Diseases: Horses' parathyroid glands rarely malfunction and cause disease, but dietary imbalances can wreak havoc with calcium chemistry.

  • Tumors affect hormone production, increasing the amount of calcium brought into circulation from bone, gut and kidneys.
  • Hyperparathyroidism occurs when PTH release is no longer responsive to blood calcium levels.
  • Vitamin D deficiency or excess disrupts calcium utilization. With only a little exposure to sunlight, horses synthesize adequate amounts of the vitamin, which is also present in sun-cured roughages; oversupplementation of vitamin D causes the gut to absorb more calcium than the body needs, leading to mineralization of soft tissues.
  • Nutritional secondary hyperparathyroidism results from a dietary imbalance in which phosphorus substantially exceeds calcium, causing shifting lameness and "big head" disease.
  • Hypocalcemia, also called lactation tetany, transit tetany eclampsia and "thumps," depending on the triggering event, occurs when plasma calcium drops precipitously from stress, feedback error or lactation. A low-level disturbance causes excitability in the horse; midlevel deficiency produces incoordination and muscle tremors and spasms; severe deficiency quickly brings the horse down, leading to stupor and death. Calcium given intravenously saves the day.

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