The thyroid is one of several glands in the endocrine system. It is a butterfly-shaped gland located in the front of the neck. The glands of the endocrine system control many of the body's functions through chemical substances called hormones. These hormones are released into the bloodstream where they circulate and regulate the function of specific organs and organ systems. The hormones produced by the thyroid gland regulate how the body's cells use energy and how "fast" the body's metabolism works. This gland also affects the rate of growth of the hair and bones; the body's weight, temperature and energy level; as well as the function of the heart and digestive system. Thyroid disease is a very common endocrine disorder, especially in women. Hypothyroidism, or underactive thyroid, is a disease in which the thyroid produces less hormones than needed for normal body function. Symptoms include thin, brittle nails and hair, weight gain, fatigue, decreased heart rate, constipation, and feeling cold. Hyperthyroidism, or thyrotoxicosis, is a disease in which the thyroid produces too many hormones. Symptoms include hair loss, weight loss, increased heart rate, nervousness, frequent bowel movements, perspiration, and menstrual irregularities in women. A doctor may diagnose thyroid disease by assessing a patient's symptoms, palpating the neck to check for changes in the thyroid gland, and blood testing to determine the body's levels of circulating thyroid hormones.