What is Secondary Hyperthyroidism ?

Primary hyperthyroidism in horses is a rare condition in adult horses.  As noted above, the secondary hyperthyroidism is caused by overproduction of thyrotropin, also known as thyroid stimulating hormone (TSH), by the pituitary gland.  This can be caused by several things which can include dietary as well as systemic disease factors.  This condition is treatable but only after ascertaining the root cause of the excessive production of the thyrotropin by the pituitary gland.  For the purposes of this guide, we will focus on the nutritional causes of the secondary hyperthyroidism in horses.

Stimulation of the thyroid gland by an excess of thyrotropin causes secondary hyperthyroidism in horses. Systemic disease, as well as other reasons for the condition, will cause your horse to present signs of being unwell such as a tendency to gain weight, a change in performance, and trouble breathing.

Symptoms of Secondary Hyperthyroidism in Horses

The symptoms that you will likely notice for hyperthyroidism in your horse are similar to those found in hypothyroidism. Here are some of them:

  • Lethargy
  • Obesity
  • Slower heart rate
  • Decreased performance
  • Infertility
  • Enlargement or thickening of muscle around the face and lower limbs
  • Enlarged thyroid gland
  • Problems with regard to breathing
  • Increased sensitivity/tolerance to cold temperatures
  • Clumsiness or a noted lack of coordination


The types of nutritional secondary hyperthyroidism are basically those relating to:

  • Goiter - Sometimes caused by dietary elements which can cause goiter to develop
  • Excess iodine in their diet - Affects how the other hormones are produced and recognized by the body
  • Other masses on or near thyroid gland - Affect the basic function of the thyroid gland as it interacts with pituitary gland


Causes of Secondary Hyperthyroidism in Horses

Most of the causes of nutritional secondary hyperthyroidism are just as the name suggests, they are rooted in the nutrition or dietary elements in the horse’s daily regimen.  Here are some of the causes you and your veterinarian might consider:

Excessive amounts of iodine in the equine’s diet

Iodine’s function is to help the thyroid gland produce hormones needed by the various parts and systems of the body.  When the iodine intake is either too high or too low, the thyroid gland responds by either over or under producing its hormones. The pituitary gland monitors these thyroid hormones and, when it senses too much or too little of the necessary hormones, it will be stimulated to produce more or less of the thyrotropin or thyroid stimulating hormone (TSH).  This will cause the thyroid to produce more or less of their hormones and the cycle keeps repeating, causing either hypo or hyperthyroidism.

Goiter development

Certain food elements contained in some plants which may be ingested by the equine can cause the goiter to develop.  Goiter is simply an enlargement of the lobes of the thyroid gland located in the throat of the equine. The substances found in soybeans, kale, rape, turnips and cabbage are called goitrogenic and, when enough iodine is not present in the equine’s diet, can be helpful.  However, when enough iodine is otherwise present in the diet and when sufficient quantities of these plants are consumed in the diet, the goitrogenic substances actually interfere with the normal production of the thyroid hormones by the thyroid gland.  

Since the pituitary gland is monitoring the circulating thyroid hormone levels, when the levels of thyroid hormone are reduced (which can happen when the production is out of whack with these substances), more TSH is produced by the pituitary gland which enables the cycle to continue.  The goiter (enlargement of thyroid gland) grows and further interference occurs with the hormone production and the thyroid results in the over-production of its hormones.



Diagnosis of Secondary Hyperthyroidism in Horses

Diagnosis can be difficult because equine blood tests for thyroid hormones are not a perfect science.  Because there are systemic reasons for the thyroid hormones to be abnormal (tumors, infections, medications, high-protein diets, imbalance of copper and zinc in the diet and starvation), and because the ways in which these situations can affect the production of the hormones, sometimes levels of thyroid hormones will be measured at levels higher or lower than what they may actually be.  This can cause a horse to be diagnosed with hypo or hyperthyroidism when actually the thyroid is functioning in a normal capacity.  

When this occurs, the medications given can cause the hypo or hyper condition to exist when it really doesn’t.  Blood testing will likely need to be done by your veterinary professional and he may need some imaging tests to determine if there is an enlargement of the thyroid gland or goiter and any potential systemic reasons for it.  Once the root cause of the out of whack thyroid test results has been established, then a treatment plan can be developed to deal with the root cause which will ultimately help the secondary hyperthyroidism of your equine.



Treatment of Secondary Hyperthyroidism in Horses

Treatment will be based upon the root cause of the secondary hyperthyroidism.  If it is a systemic root, your vet will develop the necessary treatment plan to address the systemic cause.  Once that cause has been dealt with, then the secondary hyperactivity of the thyroid should resolve itself.  The treatment may require surgery if the primary cause is a tumor, either benign or malignant, or it may require adding medications to the daily regimen to address that cause, or it may require a close examination of the feed being given to your horse.  Pasturing habits may require some adjustments if the equine is getting too much iodine or goitrogenic substances from his grazing and perhaps even the feed being given may need to be changed.



Recovery of Secondary Hyperthyroidism in Horses

The most important nutritional thing you can do for your equine is to make every attempt to achieve and maintain the best balance of nutrients, vitamins and minerals possible to give your horse all it needs for good health and great performance whether for work or show or race.  Actual true thyroid conditions aren’t generally found in adult horses and that’s the good news.  Most of these situations occur due to nutritional imbalances or systemic diseases and the ways these things can affect an otherwise normal, healthy thyroid gland.

Once the root cause that has lead to the secondary hyperthyroidism diagnosis has been established and treated, your adult horse will be able to return to his normal performance levels.  A word of caution here...thyroid conditions aren’t usually found in adult horses, but attention must be given to your foals as these thyroid conditions can be problematic for proper growth and development and can even be fatal if not found and treated appropriately.



Secondary Hyperthyroidism Questions and Advice from Veterinary Professionals

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© 2022 Wag Labs, Inc. All rights reserved.