The levels of thyroid hormone that are produced in the thyroid gland are tightly regulated by a feedback loop, whereby an increased amount of thyroxine and triodothyronine inhibits further production of these hormones. The control of thyroid hormone synthesis is controlled by two main hormones, thyrotropin-releasing hormone from the hypothalamus and thyroid-stimulating hormone from the pituitary gland, both structures being located in the brain.
Once thyroid hormones are produced in the thyroid gland and released into the bloodstream, a further level of control is elicited by plasma binding proteins, which bind to the hormone and limit its availability to body tissues. Only free hormone is able to diffuse across cell membranes and affect gene expression in the cells.
For a brief overview of thyroid hormone regulation through these feedback loops and the effects of thryotropin-releasing hormone and thyroid-stimulating hormone, please see the summary sheet below.