This is a bone disorder caused by excess parathyroid hormone (PTH) usually as a result of poor nutrition. This condition is also known as Big Head, Bran Disease or Miller’s Disease as it is common in horses fed high levels of bran. Bran is high in phosphorus, low in calcium and contains chemicals called phytates, which reduce calcium absorption. When bran is fed in high levels, without appropriate mineral supplementation, it can result in calcium deficiency. This in turn stimulates the production of PTH and causes fibrous growths to form in the bones, typically on the face, to compensate for mineral loss. Thankfully, due to balanced commercial feeds, few cases of this condition are seen today.