There are many types of biosynthesis in the human body; the production of various substances from the action of enzymes and proteins is typically essential for normal biological function. Thyroid hormone biosynthesis is generally significant for regulating the body’s metabolism. Substances produced by the gland, including thyroxine and triiodothyronine, are typically needed for normal development of the lungs, nervous system, and skeletal muscle in infants. Iodine is usually required for thyroid biosynthesis, and is normally absorbed into structures called follicular cells, where it is used for hormone production.
The thyroid gland is typically situated on the front side of the trachea, or wind pipe, below the larynx; and can respond to changes in blood flow and breathing. It is normally composed of microscopic spherical structures called follicles, which have a single layer of cells around a central area called a lumen. A precursor to some thyroid hormones, thyroglobulin, is normally stored here. Thyroid hormone biosynthesis is generally dependent on the transport of iodine through the membrane of the follicular cells. The process is often stimulated by a substance called Thyroid-Stimulating Hormone (TSH), secreted by the pituitary gland.
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Peroxidase enzyme, iodide, and tyrosine are also normally required for thyroid hormone biosynthesis. Thyroxine is usually produced when enzymes alter the structure of thyroglobulin, and when iodine atoms and tyrosine molecules combine. Which hormone is made normally depends on how many atoms attach to the molecule; thyroxine normally has four atoms of iodine, while triiodothyronine has three.
Follicular cells where thyroid hormone biosynthesis takes place can also secrete these substances into the blood. Once in the organs, the hormones are usually activated or inactivated by an enzyme called deiodinase, which can remove an iodine atom from specific parts of a molecule depending on the action taking place. Thyroxine hormones are normally activated by removing an atom from the molecule’s outer ring, which converts them to triiodothyronine. Iodine is usually removed from the inner ring to convert either hormone to an inactive form.
Thyroid hormone biosynthesis is often disrupted when there is a deficiency of iodine. The pituitary gland can then produce more TSH to compensate, sometimes causing the thyroid gland to enlarge and form a goiter. Functionally, the gland can then store more iodide to secrete normal levels of hormones into the body. The hormones secreted by the thyroid are generally essential for normal functioning of every cell in the body; abnormal levels can lead to depression as well as growth and metabolic defects.