Generally, the T3 and T4 thyroid hormones increase metabolic activity in almost all the cells throughout the body. Thyroid hormones stimulate the growth of many organs, especially the bones and the brain.
The concept of “negative feedback” declares that if you have two objects, the first object will always stimulates the second object (positive influence) but the second object always inhibits the first (negative feedback). An Inhibited first object decreases the stimulation of the second object which leads to its inhibition. An Inhibited second object then decreases the inhibition of the first, which leads to stimulation of the first again. Stimulation of the first object then stimulates the second...and the cycle repeats itself again.
In the case of the thyroid gland the two components of the “negative feedback” control mechanism are the Thyroid stimulating hormone (TSH) and thyroxin (T4). TSH stimulates the thyroid gland to increase production of T4. Increased T4 then inhibits the production of the TSH by the pituitary gland. Decreased secretion of TSH then decreases secretion of T4. Decreased secretion of T4 then stimulates production of TSH by the pituitary gland. This control mechanism control levels of the hormone thyroxin so that they stay within normal range in the body.