Dealing with IR and laminitis

Laminitis and insulin resistance usually go hand in hand, and are two conditions that strike fear into any horse owner, says Kim Dyson.

Hormones from the thyroid gland play a major role in how the body burns its fuels. Both underproduction (hypothyroid) and over-production (hyperthyroid) of thyroid hormones can have devastating effects.

For many years, horses that gained weight easily and were prone to laminitis were believed to have hypothyroidism. However, we now know that the real cause of easy weight gain and laminitis is insulin resistance (IR), although many of these horses do have low levels of thyroid hormones.

To better understand IR, a horse owner needs to know how the thyroid works.

The thyroid gland is situated high in the horse’s neck, straddling the windpipe. The pituitary thyroid-stimulating hormones (TSH) stimulate the thyroid to produce hormones. Release of TSH is under the control of the thyroid-releasing hormone (TRH) from the brain’s hypothalamus.

When stimulated by TSH, the thyroid gland synthesises thyroid hormones from the amino acid L-tyrosine and iodine. The major hormone produced is T4 (tetraiodothyronine or levothyroxine). The thyroid also produces smaller quantities of T3 (triiododthyronine).

Many vets test only 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. NSAID (nonsteroidal anti-inflammatory) drugs cause low blood T4 levels by displacing T4, whereas 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.

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.

Too much sugar
IR can occur in horses for the same reasons as in humans. The IR horse has enjoyed too much sugar and starch in its diet and may have mineral imbalances.

IR causes the pancreas to produce excessive amounts of insulin in an effort to get cells to respond to the presence of the hormone. The clinical signs of IR include fatty deposits in various places on the horse’s body (resulting in a rounded body shape), excessive drinking and urination, and laminitis.

It can take many months for a horse to return to its normal life after an episode of laminitis. To understand why laminitis is so frequently found with IR, we need only look to human diabetics who struggle with poor circulation in their feet, hands and legs.

This occurs because insulin-resistant cells are also defective in the production of nitric oxide. When less nitric oxide is present, the blood vessels cannot dilate properly, especially in the feet – or the hooves.

This creates a breeding ground for fatty deposits in capillaries, which cause the release of inflammatory compounds, further exacerbating the problem.

The value of fenugreek and ginkgo
The most effective method for preventing IR-dependent episodes of laminitis is to treat the IR. A number of herbs have shown excellent efficacy; fenugreek (Trigonella foenum-graecum) is one.

Enriching your horse’s diet with high-quality fenugreek and herbs that promote circulation in the extremities, such as ginkgo (Ginkgo biloba), will help to regulate your horse’s blood sugar and increase circulation to the sensitive bones in its hooves. The animal may finally end up grazing where the grass truly is greener (and sweeter).

Be very wary of feeding concentrates laced with sugar. Horses have evolved to live off roughage.

Dr Mac is an academic, a practising equine veterinarian and a stud owner.