The thyroid gland wraps around the front of the windpipe (trachea) and expands into a lobe on either side (see Figure 42.2). The thyroid gland produces the hormones thyroxine and calcitonin. It contains many round structures, called follicles, that produce, store, and release thyroxine. Cells in the spaces between the follicles produce calcitonin.
Thyroxine is synthesized from two molecules of tyrosine, which then have four atoms of iodine chemically bonded to them. Thus, the thyroxine molecule is also called T4:
tal blood vessels, activates the TSH-producing pituitary cells. The hypothalamus uses environmental information, such as temperature or day length, to determine whether to increase or decrease the secretion of TRH. This sequence of steps is also regulated by a negative feedback loop. Circulating thyroxine inhibits the response of pituitary cells to TRH. Therefore, less TSH is released when thyroxine levels are high, and more TSH is released when thyroxine levels are low. To a lesser extent, circulating thyroxine also exerts negative feedback on the production and release of TRH by the hypothalamus.
Was this article helpful?
This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.