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Benign Enlargement of Thyroid Gland

Synonyms: Goiter

Enlargement of the thyroid gland in the throat region is fairly common in older horses. In most cases, the cause is benign (non-cancerous).

Enlargement of the thyroid gland is often called “goiter”. This term is confusing, because it generally means that thyroid enlargement is caused by elevated amounts of iodine in the diet. In human medicine, this is a common reason for thyroid enlargement. Horses with iodine excess (usually from overfeeding of iodine) may also have an enlarged thyroid. In rare cases, foals are born with a congenital enlargement of the thyroid gland.

Benign thyroid enlargement may be differentiated from true goiter via ultrasound appearance and blood iodine testing. In most cases, the diagnosis is made presumptively (without any further diagnostics). Ultrasound and/or biopsy of the mass is used to rule out thyroid cancer. In some cases, testing blood iodine levels may be helpful if true goiter is suspected.

Goiter in the horse is rare enough that I do not discuss it further here. Benign thyroid enlargement is usually not considered a problem and you and your vet simply will monitor the growth, without any treatment.

There is no recommended treatment for benign thyroid enlargement. However, in some cases the masses can become quite large. Although unsightly, usually these masses do not seem to bother the horse.


  • How can you be sure this is benign and not cancerous?
  • Is this related to iodine excess or deficiency?

    There is no known prevention. In cases where iodine is thought to be involved, ensuring appropriate levels of iodine in the diet will help remedy the thyroid enlargement.
    Author: Doug Thal DVM Dipl. ABVP


    Smith, BP ed. Large Animal Internal Medicine 4th ed. St. Louis: Mosby Elsevier, 2009.


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