Thyroxine, commonly known as T4, is a hormone produced within the human body. It plays a role in regulating the body’s metabolism, which is the process of converting food into energy for cellular function.
The Thyroid Gland
The primary site for T4 production and initial storage is the thyroid gland. This butterfly-shaped endocrine gland is situated in the front of the neck, below the Adam’s apple. It wraps around the trachea, or windpipe, with its two lobes connected by a narrow band of tissue called the isthmus.
Specialized follicular cells within the thyroid gland synthesize T4. These cells utilize iodine, an element obtained from the diet, to create the hormone. The thyroid gland then releases T4, and a smaller amount of triiodothyronine (T3), into the bloodstream.
T4’s Journey Through the Body
T4 is released into the bloodstream, traveling throughout the body to nearly every cell and tissue. In the blood, T4 exists in two forms: bound T4 and free T4. Most T4, over 99%, is bound to proteins, which prevents it from immediately entering tissues. This bound form acts as a circulating reservoir.
Conversely, free T4 is the unbound form, capable of entering body tissues and exerting effects. T4 primarily functions as a prohormone, meaning it is largely inactive on its own. To become biologically active, T4 must undergo conversion to triiodothyronine (T3) through a process called deiodination, which involves the removal of an iodine atom. This conversion predominantly occurs in various peripheral tissues and organs, including the liver, kidneys, muscles, and central nervous system.
Regulation of T4 Production
The concentration of T4 in the body is precisely controlled by a complex feedback system known as the hypothalamic-pituitary-thyroid (HPT) axis. This regulatory pathway begins in the brain with the hypothalamus, which releases thyrotropin-releasing hormone (TRH). TRH then stimulates the pituitary gland, a small gland located at the base of the brain, to produce thyroid-stimulating hormone (TSH).
TSH subsequently travels to the thyroid gland, prompting it to synthesize and release T4 into the bloodstream. When T4 levels in the blood rise sufficiently, they signal back to both the hypothalamus and the pituitary gland, inhibiting further release of TRH and TSH. This negative feedback loop ensures that T4 levels remain within a healthy range, adapting to the body’s changing needs and maintaining metabolic balance.