The thyroid gland on this horse is seen between the fingers here and is enlarged and non-painful. The owner (a human medical doctor) had noticed this enlargement had recently developed and was worried as to its cause. She had also recently added a supplement with the 1st ingredient of seaweed (kelp). Removing this supplement restored the thyroid to its normal size within a week. The ingredient label can be seen in one of these images. Enlargement of the thyroid has 3 causes: 1) excess iodine as in this case, 2) too little iodine (rare in horses) and 3) idiopathic (no known reason).