Description

The thyroid gland is a large endocrine gland that produces hormones that help determine the metabolic rate  by which energy is provided for vital processes in the horse's body.

Generally, thyroid gland problems are classified as Primary: inadequate thyroid hormone production; Secondary: inadequate production of thyroid stimulating hormone; or Tertiary: the inability to use thyroid hormones within the body.

True primary hypothyroidism, or below-normal levels of thyroid hormones, in horses is quite rare because iodine, an essential element for the manufacture of the two main thyroid hormones, is provided in adequate amounts in most commercial feeds.

However, other factors can contribute to an imbalance in the horse's system. Many body illnesses affect the production of hormones and a number of medications can affect the levels of these hormones in the blood.

Foals appear to be vulnerable to thyroid disease and may be affected while in the womb if the diet of the mare is deficient in iodine.

Hyperthyroidism occurs when the gland becomes overactive. Again, this condition is extremely rare in horses and has been reported in only a few horses that have been over 20 years of age.

The condition is usually due to a tumor in the thyroid gland and produces the same signs seen in humans, including enlargement of the thyroid gland, increased heart rate, increased thirst, inability to sweat, excitability, hair loss, and weight loss, despite having a healthy appetite.

Symptoms

• Dullness and lethargy
• Obesity
• Slow heart rate
• Poor performance
• Muscle problems
• Thickening of tissues around face and lower limbs
• Hypothermia (tendency to feel cold)
• Infertility
• Weakness at birth
• Lack of coordination
• Stunted growth
• Developmental bone and muscle problems
• Respiratory problems
• Enlarged thyroid gland

Causes

Thyroid gland conditions are most often caused by too much or too little dietary iodine. Thyroid functions can also be affected by tumors, infection, certain chemicals in forage that block hormone production, medications, excessively high protein diets, too much zinc or copper in the diet, and starvation.

Prevention

Making sure the horse's diet has a balanced amount of iodine, especially for mares that are producing foals is the best method of preventing thyroid gland conditions. Most commercial feeds and supplements contain sufficient iodine, so there is rarely a need to add more iodine to the diet.

According to some sources, seaweed-based products have been linked to iodine overdose. Consult with a veterinarian before using products that might contain additional iodine, including washes, creams, and lotions.

Treatment

Treatment depends on whether the thyroid gland is not functioning correctly or whether a secondary problem exists elsewhere in the body. Cases of confirmed primary hypothyroidism should be treated with a dietary supplement recommended by a veterinarian and monitored on a regular basis. Many horses with the condition appear to recover spontaneously, given time.

Secondary causes of thyroid gland conditions should be treated based on the medical concerns related to that specific condition. A veterinarian should be involved in treatment of tumors and infections. Corrective measures need to be taken if the problem is caused by dietary excesses or insufficiencies.