AThe size of a horse’s thyroid gland can certainly be affected by a number of dietary factors. Too much iodine in the ration, as seen sometimes in horses fed kelp (seaweed)-based supplements, might cause thyroid gland enlargement (also known as "goiter"). Paradoxically, the thyroid gland might also enlarge as a result of iodine deficiency. Dietary deficiency of iodine is unlikely, unless you are feeding an unusual ration to your horse. Secondary iodine deficiency might also result from ingestion of excessive calcium, Brassica plants (such as broccoli, cabbage, cauliflower, and mustard), white clover, rapeseed, and linseed, or feed contaminated by sewage. However, if a dietary factor were truly responsible for goiter in this horse, the thyroid gland should be enlarged on both the left and the right sides of the neck. Moreover, other signs of iodine imbalance would be evident, such as an abnormal hair coat.

A much more common cause of goiter in mature to older horses is thyroid adenoma, a benign neoplastic state that is very common and typically does not cause any other problems (beyond the swelling). Other types of thyroid cancer are quite uncommon in horses.

It is puzzling to me that the swollen area has decreased in size so dramatically. In some cases, the right lobe of the thyroid gland can be quite mobile in the throatlatch, and it might move between a superficial and a deeper location (quite normally), especially if it is enlarged by benign adenoma development. If it moved from the superficial (under the skin) location to "rest" in a deeper location, it might appear to have shrunken. Most types of benign thyroid enlargements do not spontaneously shrink