If you feel more confused than ever about thyroid hormones in horses, you're not alone. Less than 10 years ago, obese and laminitic horses and ponies were believed to be hypothyroid and even easy keepers were often given supplemental thyroid hormone. As we came to understand insulin resistance, use of thyroid supplements dropped off but recently the pendulum has swung back to using them again. What's the story'
To begin, you need to understand different types of hypothyroidism. Primary hypothyroidism is disease of the thyroid gland itself. Secondary hypothyroidism is a pituitary problem involving low levels of TSH, thyroid stimulating hormone.
Tertiary hypothyroidism involves the hypothalamus, another area of the brain. In tertiary hypothyroidism, output of the hormone TRH, thyroid releasing hormone, is low. TRH causes release of TSH, which in turn stimulates the thyroid.
Yet another form of hypothyroidism is called euthyroid sick syndrome. This is a condition where thyroid hormones are low in an animal that has a chronic illness.
Horses rarely, if ever, have primary hypothyroidism. Researchers have found that horses with low levels of thyroid hormone respond normally to an injection of TSH, proving the gland is normal and capable of producing the hormone. This means that something is causing hypothyroidism, or the horse has euthyroid sick syndrome.
We don't have the diagnostic tools readily available to figure out what the mechanisms are in horses but in other species it is known that things like high cortiso
l and inflammatory proteins (cytokines) can result in suppressed thyroid function. That would certainly explain why horses with Cushing?s disease (high cortisol) and/or insulin resistance (inflammatory cytokines) are often hypothyroid. Their hypothyroidism is caused by the primary problem and typically corrects when the real problem is treated.
If low thyroid hormone levels are actually caused by something else, why supplement them at all' Many veterinarians continued to use thyroid hormone supplementation because it makes the horse feel better. There are also several human studies that documented better improvements in insulin sensitivity when hypothyroid patients were supplem
ented with replacement hormones in addition to exercise and diet.
There is now preliminary data in horses that suggests the same thing, but they used high doses. Some vets use high doses outside the research setting in an attempt to jump start weight loss. it's also reasonable to supplement horses that are testing low, to get them into a normal range. This is especially true for horses that aren't responding as well to diet changes and exercise.
Nutrition also has a key role to play in thyroid function. Low levels of T4 and T3 can be caused by iodine deficiency. Iodine is not easily measured in hays. Soil levels vary widely and are particularly low along the Great Lakes and in the middle of the country. The National Research Council also increased the estimated minimum iodine requirement by almost 100%, making it impossible to meet the horse's needs from voluntary consumption of iodized salt.
Selenium is required by the enzymes that convert T4 into the more active T3 hormone. Since this is a very common deficiency, dietary levels should always be evaluated if a horse has a normal T4 but low T3.
Hazards in the feed may also contribute to low thyroid hormone levels. For example, nitrates may be present in well water, or in hay/feed that was grown under stressful conditions. Nitrates are nitrogen-containing chemicals produced by bacterial breakdown of organic matter in soil. Plants absorb them and convert them into protein.
The protein analysis in hays and feeds is actually a measure of nitrogen, not protein. It cannot distinguish between nitrates and actual protein. However, the test for actual nitrate is widely available in hay and feed testing labs. You can check nitrates in your well water with an inexpensive kit from the hardware or garden store.
Nitrates interfere with thyroid function by blocking utilization of iodine. Thiocyanates do the same thing. These chemicals are present in the mustard family of weeds and in varying levels in virtually all seeds and seed meals. How often these chemicals influence thyroid function in horses, and the levels required to do that, are unknown. However, if a horse has low thyroid function they should be put on the list of possible contributing causes.
Supplements for hypothyroid horses fall in two general categories, prescription or over-the-counter. The prescription supplements contain synthetic T4 (levothyroxine). Your veterinarian will give you information on dosing. Both supplements are used in small amounts and are palatable.
While there is anecdotal ?traditional use? support for some Western herbs, and equivocal support for Chinese herbs, in treating hypothyroidism, their effects are very poorly understood and they haven't moved into the realm of proven herbal therapies, especially for horses.
The raw materials the thyroid gland needs to manufacture thyroid hormones are the amino acid tyrosine and the minerals iodine and selenium. Tyrosine is an amino acid the horse can synthesize from phenylalanine, and both tyrosine and phenylalanine are present in abundant levels in all but the most restrictive or poor quality diets. Deficiencies of manganese, zinc, copper and iron can also influence thyroid function. Of these, only zinc and copper are likely to be deficient in diets of only hay and unsupplemented grains, or minimal amounts of supplemented grains.
Iodine and selenium are the heavy hitters here, both playing pivotal roles and both commonly deficient. They're commonly found in most multi-supplements and hoof supplements, which would be your first choice.
The minimum for selenium is 2 mg/day, and for iodine 3.8 mg/day for an average-size horse. There's reluctance to supplement for fear of toxicity, but the toxic level for long-term intake is at least 10 times those bare minimum levels.
Kelp and other seaweeds are good sources of iodine. However, the levels vary widely, including up to levels that would be toxic for long-term intake. Therefore, don't supplement kelp with undisclosed iodine levels.
If you live in a selenium-adequate area, or are already providing adequate selenium from a supplemented feed or multi ingredient supplement, go with Source or Uckele's Ocean K. For targeted selenium supplementation, Platinum Performance's Selenium Yeast gets the nod.
Article by Eleanor Kellon, VMD, our Veterinary Editor.