Thyroid-stimulating hormone (TSH) is secreted by the anterior lobe of the pituitary gland and induces the production and release thyroid hormones thyroxin (T4) and triiodothryronine (T3). These thyroid hormones exert a negative feedback on the pituitary. The release of TSH is regulated by TSH-releasing hormone (TRH) produced in the hypothalamus. When there are high circulating levels of thyroid hormone in the blood, less TRH is released by the hypothalamus, so less TSH is secreted by the pituitary. The normal concentration of TSH in the blood is extremely low, but it is essential for maintenance of normal thyroid function.