The hypothalamus produces TRH releasing hormone, which tells the pituitary gland to stimulate the thyroid gland. When the level of thyroid hormones drops to low, the pituitary gland produces TSH. This tells the thyroid gland to produce more hormones. Under the influence of TSH, the thyroid will secrete T3 and T4, which will raise their blood levels The pituitary gland senses this and, decreases the TSH production. T3 and T4 are then released into the blood stream, where they control metabolism.