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Hormones from the thyroid gland play a major role in regulating metabolism; how the body burns its fuels. They also influence how sensitive the cells are to the "flight hormones," epinephrine (adrenalin) and norepinephrine. In the fetus and young growing horse, thyroid hormones are necessary for normal development of the nervous system. Both under (hypothyroid) and over (hyperthyroid) production of thyroid hormones can have devastating effects. These disorders are much more common in people than in horses. The mechanisms, and therefore management, are also very different.
For many years horses that gained weight easily and were prone to laminitis were believed to be hypothyroid. However, surgically removing the thyroid does not lead to weight gain or laminitis. Studies also found that when laminitic horses were injected with TRH or TSH, hormones which stimulate the thyroid, their thyroid glands responded normally by increasing production of thyroid hormones. This showed that the thyroid gland itself was normal.
We now know the real cause of easy weight gain and laminitis is insulin resistance. However, many of these horses do have low levels of thyroid hormones. What they have is euthyroid sick syndrome, a form of secondary hypothyroidism. To understand this, you need a little background in how the thyroid works. The thyroid gland rests high in the horse's neck, straddling the trachea (wind pipe). It is directed to produce thyroid hormones by the pituitary hormone TSH - thyroid stimulating hormone. Release of TSH is under the control of a hormone from the brain's hypothalamus called TRH - thyroid releasing hormone. To complete the circuit, thyroid hormone levels control the release of TRH and TSH. When high, TRH and TSH production slows, and vice versa.
When stimulated by TSH, the thyroid gland makes thyroid hormones from the amino acid L-tyrosine and iodine. The major hormone produced is T4, aka tetraiodothyronine or levothyroxine. The thyroid also produces smaller amounts of T3, triiododthyronine. T3 is the more potent/active hormone. It is created by removing one iodine molecule from T4. In the thyroid gland, T4 levels are 20 times higher than T3.
Getting back to euthyroid sick syndrome, this is a condition where TSH is normal, T4 normal or reduced, and T3 low. T3 is produced from T4 in the body tissues by enzymes called deiodinases which strip one iodine off T4. Selenium is an essential mineral for the deiodinase enzymes. T3 production from T4 occurs in all body tissues. Under normal conditions, fasting decreases the uptake of T4 by tissues, thus reducing T3 because there is less T4 available for the deiodinase enzymes to process.
Hormone and Enzyme Primer
T4/levothyroxine - Hormone from the thyroid gland.
T3/triiodothyronine - Hormone produced both in the thyroid gland and all the body's cells by removing one iodine molecule from T4. Most active thyroid hormone.
Deiodinase - Enzyme which removes an iodine from T4 to convert it to T3.
TRH - Thyroid releasing hormone, comes from the hypothalamus in the brain. Levels decreased by high T4/T3; increased by low T4/T3.
TSH - Thyroid stimulating hormone. This hormone is produced by the pituitary, when stimulated by TRH. It directs the thyroid gland to increase production of thyroid hormones.
Low levels of the active form of the antioxidant glutathione also reduce the activity of the deiodinase enzymes so oxidative stress is important in avoiding low T3. Oxidative stress may be the mechanism behind euthyroid sick syndrome in horses with insulin resistance since it is well documented that IR increases oxidative stress. Since selenium is important to enzyme systems that keep glutathione in its active form, selenium levels pack a double whammy on T3 levels.
The hormone cortisol from the adrenal gland, which is elevated in Cushing's disease, also causes secondary hypothyroidism. In people, corticosteroids like cortisol only suppress T4 conversion to T3. In horses, the same thing happens but T4 is also suppressed. In a study that applied the synthetic corticosteroid, dexamethasone, to the skin of horses for 10 days, the T3 dropped quickly and more significantly but T4 was also lower and neither had recovered even at 20 days after the treatment had been stopped.
Many veterinarians only test blood T4 levels when screening for thyroid function. However, T4 is not as active as T3 and drug effects could lead to a misdiagnosis of hypothyroidism. Phenylbutazone, and likely other NSAID drugs (nonsteroidal antiinflammatory drugs) cause low blood T4 levels by displacing T4 from its binding sites on circulating serum/blood proteins. However, T3 levels stay normal.
Horses are more likely than humans to have problems with low thyroid function related to nutrition. Iodine and selenium are the major factors here. Malnutrition and chronic diseases or infections of any type also cause secondary hypothyroidism. This makes sense as lowering metabolism preserves key nutrients.
Most of the body's iodine is concentrated in the thyroid gland and deficiencies clearly influence thyroid function. Excesses are also harmful. Selenium is required by the deiodinase enzymes and by the glutathione antioxidant that protects those enzymes. Both iodine and selenium are deficient at baseline in many areas of the country.
To summarize, thyroid problems in horses are different from other species. Hyperthyroidism is very rare, and limited to senior horses with malignant tumors in their thyroid. Hypothyroidism can occur, but is not related to any problems with the gland itself. It is called secondary hypothyroidism because it is secondary to/caused by other disorders.
Eleanor Kellon, VMD, currently serves as the Staff Veterinary Specialist for Uckele Health & Nutrition. An established authority in the field of equine nutrition for over 30 years, Dr. Kellon is a valuable resource in the field of applications and nutraceuticals in horses. She formerly served as Veterinary Editor for 'Horse Journal' and John Lyons 'Perfect Horse' and is owner of Equine Nutritional Solutions, a thriving private practice. A prolific writer, Dr. Kellon is the author of many best-selling books on a variety of medical and nutritional topics and has contributed to both lay and professional publications.
Founded in 1962, Uckele Health & Nutrition has been a trusted leader in the formulation, development and manufacture of quality nutritional supplements for fifty years. With leading edge experience in nutritional research and science, the Uckele team manufactures quality formulas from concept to shelf, formulating a vast array of high potency, balanced nutritional supplements to support optimal health and performance at the highest level.
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