The pituitary, an endocrine gland in the brain, controls the thyroid gland and, in doing so, ensures that the hormones produced end up in the bloodstream. By way of the bloodstream, the thyroid hormones reach all of the cells in all parts of the body.
This secretion of thyroid hormone into your blood is a process that is very precisely regulated so that you always have the perfect amount of hormone in your body. Not too much and not too little. The secretion of thyroid hormone is regulated by the pituitary and by the hypothalamus, two small glands in your brain which also synthesize hormones. The hypothalamus synthesizes TRH (thyrotropin-releasing hormone). TRH signals the pituitary to make TSH (thyroid-stimulating hormone). TSH, in turn, makes sure that the thyroid gland secretes thyroid hormone into the bloodstream and that more hormone is then synthesized to replace it. The amount of thyroid hormone in your blood and tissues is continuously being 'measured' by the hypothalamus. Once the thyroid hormone level is sufficient, the production of TRH declines. As a result of this, the pituitary produces less TSH which means that the thyroid gland ultimately synthesizes and secretes less thyroid hormone. After all, there is already enough of it to go around. The control of the thyroid, including this feedback loop, is called the hypothalamus-pituitary-thyroid axis.