Secretariat died in 1989 at the age of 19 due to laminitis, a painful and debilitating inflammation of the tissue bonding the horse's bone to the inner wall of the hoof. This condition was first diagnosed on Labor Day of 1989, and despite treatment, his condition rapidly worsened, leading to his euthanasia on October 4, 1989

. Laminitis is known to be an extremely painful hoof disease with no effective cure once it progresses severely. Secretariat's laminitis was described as the worst case seen by the attending veterinarian, Dr. Thomas Swerczek

. Given the severity and pain caused by the condition, euthanasia was considered the only humane option

. At the time of his death, an autopsy revealed that Secretariat had an extraordinarily large heart, estimated to weigh about 21-22 pounds-roughly two to three times the size of a normal thoroughbred's heart. This large heart was believed to contribute to his exceptional athletic ability during his racing career

. In summary, Secretariat died from complications of laminitis, a severe hoof disease, which led to his euthanasia due to the incurable and painful nature of the condition