Hypothyroidism, often called underactive thyroid or low thyroid and sometimes hypothyreosis, is a common disorder of the endocrine system in which the thyroid gland does not produce enough thyroid hormone. It can cause a number of symptoms, such as poor ability to tolerate cold, a feeling of tiredness, and weight gain. In children, hypothyroidism leads to delays in growth and intellectual development, which is called cretinism in severe cases.