Hypothyroidism is a type of thyroid disorder. It is defined as having too little thyroid hormone. It involves slowing of body functions, and symptoms include weight gain, dry skin, constipation, cold intolerance, puffy skin, hair loss, fatigue, and menstrual irregularity in women. Severe untreated hypothyroidism, called myxedema, can lead to heart failure, seizures, and coma. In children, hypothyroidism can stunt growth and delay sexual development. In infants, it can cause mental retardation. For this reason, hypothyroidism testing is performed in the United States as part of newborn blood screening programs since early detection and treatment can minimize long-term damage.
For further detail, including information on tests, see the Thyroid Diseases article.