Thyroid-stimulating hormone (TSH) is produced by and stored in the pituitary gland, which is located beneath the brain. The release of TSH into the bloodstream stimulates the thyroid gland to release its hormones, called thyroxine (T4) and triiodothyronine (T3).
When the pituitary gland detects that thyroid hormone levels are too low, it secretes more TSH. If the pituitary gland detects too much thyroid hormone, it releases less TSH.
WebMD Medical Reference from Healthwise