The risks associated with thyrotoxicosis depend on the severity and duration of exposure to excess amounts of thyroid hormone. Long-term exposure to excess amounts of thyroid hormone may be associated with an increased risk of developing a rapid irregular heartbeat called atrial fibrillation. This may develop as a result of overstimulation of the cardiovascular system that interferes with the normal contraction of the upper chambers of the heart, also known as the atria. In the normal state, the atria contract in an organized pattern, discharging electrical signals that trigger the regular contraction of the lower chambers of the heart, also known as the ventricles. Overstimulation of the cardiovascular system may cause short circuits to develop in the tissue that conducts electricity through the atria. As a result, they may begin to contract in a rapid disorganized pattern. This may lead to acceleration of the heart rate as rapidly discharged electrical signals trigger rapid irregular contractions of the ventricles. Atrial fibrillation may disrupt the normal flow of blood through the heart and lungs. It may also increase the risk of suffering a stroke, as disorganized contraction of the atria may lead to the formation of blood clots that can travel to lodge in the arteries that supply the brain, cutting off the flow of blood and oxygen.
Long-term exposure to excessive levels of thyroid hormone may also be associated with an increased risk of developing loss of bone tissue. In the normal state, the bones that make up the skeleton are constantly being broken down and built up again through a cyclic process called remodeling. Disorders that tip the balance of remodeling to the point where the bones are broken down at a faster rate than they are built up again may lead to a progressive loss of bone tissue. Loss of a significant amount of bone tissue may weaken the bones in key areas to the degree where there may be an increased risk of sustaining a fracture with little or no trauma. Key areas that may be particularly susceptible to fracture in the setting of osteoporosis include the spine, the hips, and the forearms.