If you are confused about thyroid function in horses you have a good excuse. As new information from studies becomes available, recommendations have changed dramatically.
Before it was discovered that insulin resistance is the most likely cause of obesity related to hormonal problems, overweight and laminitic horses were believed to be hypothyroid and thyroid supplements widely recommended. However, research has shown that low thyroid functioning related to disease in the thyroid gland itself is rare, possibly nonexistent, in horses.
Veterinarians were now being told that thyroid hormone supplementation was not justified. What was once very common became almost nonexistent. A few veterinarians continued to prescribe it because they saw it made many horses feel much better.
In time, the pendulum began to swing back. We found that while the gland itself might be normal, thyroid function can be suppressed by any acute illness and by other hormonal issues like insulin resistance or Cushing’s Disease. It was also found that a period of supplementing with high levels of thyroid hormone can jump start getting insulin resistance under control by causing weight loss (but only when the diet is appropriately controlled as well).
I personally would not approach one disorder (insulin resistance) by creating another (hyperthyroidism) but it’s reasonable to support thyroid function in IR horses until their IR can be controlled. However, it’s also perfectly reasonable and effective to simply feed a good and mineral balanced diet while addressing the IR and let the thyroid recover on its own, which it will do.
Hyperthyroidism is very rare in horses and linked to malignant thyroid tumors in older horses. Older horses more commonly develop benign swellings/goiters of the thyroid which may be linked to low intake of iodine and/or selenium which are needed for thyroid hormone production and activation. There are typically no symptoms.
The situation is different for both high and low iodine intake in pregnant mares. Their foals are often born with very large thyroid glands and hypothyroidism that makes them weak, unable to nurse, mentally depressed and may even be fatal.
The thyroid is important in all animals but the horse is very different from humans or small animals. We still have a lot to learn and recommendations may well change again as more information comes out, but except for pregnant and newborn animals the thyroid is probably not playing a primary role in equine diseases.
Eleanor M Kellon,VMD