To help evaluate thyroid gland function; to help diagnose hypothyroidism or hyperthyroidism; to screen for hypothyroidism in newborns. Hyperthyroidism is a condition in which the thyroid gland is overactive, and makes too much thyroid hormone. Hypothyroidism is a condition in which the thyroid gland does not make enough thyroid hormone. The thyroid gland is located in the neck. It produces several hormones that control the way every cell in the body uses energy (metabolism). The thyroid is part of the endocrine system. The most common cause of hypothyroidism is inflammation of the thyroid gland, which damages the gland's cells. Neonatal hypothyroidism is decreased thyroid hormone production in a newborn. In very rare cases, no thyroid hormone is produced.