It is always a good idea to get blood work when trying to determine and resolve a suspected medical issue. I took my mare to the University of Florida for evaluation and while they checked her for hyperthyroidism,apparently there are few documented cases of hyperthyroidism in horses. One of the clinical signs of hyperthyroidism is weight loss despite a good appetite. As I mentioned previously my mare was not excited about eating, as I think is the case here. According to the vet notes that I just pulled out, other symptoms of hyperthyroidism in horses are: hyperexcitability,tremors,sweating, tachycardia,polyphagia, enophthalmos and inexplicable hair loss (Alopecia).