It is synthesized and secreted by our hypophyse gland (also called pituitary gland), a major gland in the brain. It's a hypophyse hormone, NOT a thyroid hormone. It regulates the thyroid gland, telling it to produce more thyroid hormones if our blood levels are too little, or telling it to produce less thyroid hormones if our blood levels are too much. When the thyroid gland fails to do its job adequately (called hypothyroid), the TSH lab will show a high number. In the contrary, hyperthyroid has a very low TSH level, the thyroid needs no stimulation.