A condition of low thyroid hormone. The thyroid gland is located in the front of the neck. Thyroid hormone helps control metabolism. Low thyroid hormone is caused by abnormalities of the thyroid gland, pituitary gland, or hypothalamus. For people who take thyroid hormone medication, hypothyroidism may be due to an insufficient dose. Low thyroid hormone can affect all bodily functions and if severe may cause coma (myxedema) and death.