Thyroid stimulating hormone (TSH) is a hormone produced by the pituitary gland that regulates the production of thyroid hormones thyroxine (T4) and triiodothyronine (T3) by the thyroid gland. TSH levels increase when thyroid hormone levels are low, and decrease when thyroid hormone levels are high. Abnormal TSH levels can indicate problems with the thyroid gland, such as under or overproduction of thyroid hormones. It can also be a result of pituitary gland disorders.