Q:My 27-year-old mare has a growth on the right side of her neck that is now the size of a small Nerf football. We first noticed this mass in 2003, but I was told that because of her age there would be no point in removing it. Now Cassie is 27 and appears to be very healthy in every other way except for the growth still on her neck.

Her growth/tumor is finally to the size that it inhibits her ability to flex her head back to her chest. She doesn’t flinch at all when I touch the growth, but it is very hard and I assume it is becoming uncomfortable.

My questions are, 1) What do you think this growth is? and 2) Is there any way to remove, treat, or shrink this mass?   

Kimberly Lewis, via e-mail

A:There are several causes of swelling in this area, and it’s not clear from looking at the picture exactly which one is most likely. Possibilities to consider include some sort of thyroid gland abnormality. These are usually benign growths, but occasionally they can be malignant tumors. The thyroids are paired plum-shaped structures that usually sit under the "strap muscles" around the throatlatch area. In older horses they can become more palpable as the attachments loosen with age, but they should not become larger.

An active thyroid tumor can secrete excess hormones and affect the horse’s metabolism, leading to weight loss, high heart rate, and skin and coat abnormalities. However, many thyroid growths do not secrete excessive thyroid hormones. Additionally, this