Enlargements of the thyroid gland are fairly common in horses. There are several different causes, some associated with thyroid-function abnormality and some not.
The thyroid gland sits high in the neck, wrapped around the horse’s trachea (”wind pipe”) and close to the larynx (”voice box”). The exact position varies slightly with individual anatomy. Because of the proximity to the jaw bone and the large parotid salivary gland that sits in that location, there’s sometimes confusion over whether an enlargement involves the salivary gland or the thyroid. This is fairly easy to determine with careful examination and palpation.
Locate the trachea below the area of enlargement. It feels like a round pipe, hard in consistency and has regularly spaced rings. Once you have found the trachea, follow it up to the enlargement to see if the swelling is sitting on top of the trachea, which would indicate it is the thyroid. If you’re not sure, trace the path of the swelling. If it involves parotid salivary gland, you will be able to follow it up toward the ear along the edge of the jaw bone, and probably between the jaw bones as well. If necessary, your vet can also do an ultrasound of the area.
Goiter is a benign enlargement of the thyroid gland. It may or may not be associated with alterations in thyroid-hormone levels. It can be caused by insufficient iodine in the diet, or the presence of high levels of nitrate, which interfere with thyroid function. Foals are more sensitive to low-iodine intake than adults and may be born with a goiter or develop one soon after birth. Foals born with this problem also have a variety of skeletal abnormalities, such as jaw deformity, bossed forehead and tendon/ligament problems.
Excessive iodine intake can also cause goiters in adult horses and neonatal foals. Overfeeding kelp or mixing supplements with generous iodine content is the usual cause. Pregnant mares may abort, and live foals born to these mares may have abnormal bone metabolism. In other species, selenium deficiency can cause goiters as well, even when iodine intake is normal. Goiters will resolve on their own once the mineral excess or deficiency causing them is corrected.
Thyroid enlargement by tumors is particularly common in older horses. In most cases, these tumors are completely benign and don’t involve the cells that actually produce thyroid hormone but rather the C-cells, whose job it is to produce the hormone calcitonin, important in calcium metabolism. However, studies have shown there is little evidence of increased calcitonin secretion by these cells. This type of benign tumor produces an obvious thyroid enlargement but rarely, if ever, causes any hormonal disruption. No treatment is needed for these benign tumors, except in rare cases where they compress the trachea and interfere with breathing.
Much less common is the development of benign thyroid tumors (adenomas) that produce thyroid hormone, resulting in a hyperthyroid state. This has only been seen in older horses and has been described most often in Thoroughbreds. Also possible, although equally rare, is the development of cancerous tumors of the thyroid, called adenocarcinomas.
Treatment of functioning tumors causing hyperthyroidism is usually by surgical removal. Careful screening for functioning tumors of the adrenal gland should be done first, since these tumors increase the risk of anesthesia. Because these are older horses, a full cardiac workup will also be done. In addition to the usual risks of surgery and anesthesia, there’s a chance the recurrent laryngeal nerve, which innervates the larynx RLN (recurrent laryngeal nerve), might be injured either directly or as a consequence of postoperative swelling.
Horses with RLN damage are unable to fully open their vocal cords to allow air into the lungs, resulting in the condition commonly known as ”roaring” because of the characteristic loud noise the horse makes when exercising. If only part of the thyroid is involved with the tumor, the remaining gland will start to function normally after surgery. If the entire thyroid has to be removed, postoperative thyroid supplements will be needed.
Another possible option, either alone or in combination with surgery, is the injection of radioactive iodine, I-131. Because of the high affinity of thyroid tissue for iodine, the radioactivity becomes concentrated in the thyroid. This would be an experimental procedure in horses, since these cases are so rare there’s no experience with its use.
Finally, a recent report from the Veterinary School at the University of Bari in Italy described autoimmune thyroid disease in horses for the first time. This is the most common form of thyroid disease in people and dogs, but it had never been documented in horses before. No cases have been reported in this country, but it needs to be added to the list of conditions to rule out in horses with enlarged thyroid glands. The European study was done on horses that were euthanized, so no treatment attempts made. In other species, radioactive iodine or other thyrotoxic drugs are used to treat this condition.
If your horse has an enlarged thyroid gland, there’s no need to panic. Iodine-related goiters are easily corrected by adjusting iodine intake and selenium status should also be checked. For the rare cases of hyperthyroidism caused by functioning tumors, surgery is the usual treatment.