The thyroid gland is a paired gland that normally sits on the top (dorsal) of the trachea in the upper 1/4 of the neck region. Here it cannot be seen or easily palpated. Irregularities of position and tumors of the thyroid gland are common in the horse. A common swelling found on the bottom of the neck up close to the head is the thyroid gland where its attachments have failed and it falls down just ventral to the trachea. It is about the size and shape of a golf ball cut in half, firm, and is not a cause of concern.

Most tumors of the thyroid of the horse are benign and do not secrete hormones, though there are exceptions. The benign tumors usually are static or grow slowly but can get quite large over time. Differentiation of the tumors requires monitoring serum T3 / T4 concentrations and biopsy. If there are clinical signs of hyperthyroidism a T3 suppression test should be done. Small static tumors of the thyroid with normal T3 / T4 levels are frequently just watched for change without any harmful long term effects, but if the tumor is malignant, the earlier the diagnosis the better.