The thyroid gland makes thyroxine, a hormone that controls the body's metabolic rate, which is the rate of the chemical reactions in cells. The amount of thyroxine in the blood is kept within narrow limits by negative feedback, where a change is detected and a response reverses it.
When the level of thyroxine is low, the hypothalamus in the brain is stimulated to make TRH. TRH causes the pituitary gland to release TSH. TSH acts on the thyroid gland, which then produces more thyroxine, so the level rises.
When thyroxine levels are normal, thyroxine inhibits the release of TRH and the production of TSH. Less TSH means the thyroid makes less thyroxine, which stops the level from becoming too high. The system swings back and forth around the normal level.
