Thyroxine: What It Is and Associated Diseases

Thyroid hormones play an important role in cellular metabolism and energy production. One of these hormones is thyroxine, which has specific qualities that allow it to perform its function.
Thyroxine: What It Is and Associated Diseases

Last update: 02 September, 2021

Thyroxine is one of the most important hormones in the human body. Hormones are a series of substances that are synthesized in the glands, and whose main function is to maintain the homeostasis or balance of our organism. Unfortunately, a variety of diseases are associated with these substances.

Most of the conditions result from an error in the synthesis or release processes. This will consequently alter the concentration in the bloodstream, and they won’t carry out their function properly.

What is thyroxine?

Also known as tetraiodothyronine or T4, thyroxine is a hormone made by iodine, which is synthesized and released from a gland located in the back of the neck. It has a particular chemical structure, composed by condensing the amino acid tyrosine and adding 2 atoms of iodine.

The amount of T4 produced in this gland is much greater than other synthesized hormones, but it’s not very active biologically. Multiple studies have established that thyroxine behaves like a peripheral prohormone, i.e. it must be transformed into its active form by enzymatic action.

The hypothalamus and the pituitary gland regulate the secretion of tetraiodothyronine. Both structures synthesize substances capable of stimulating or inhibiting the release of T4, depending on the requirements of the organism.

Which gland is responsible for producing thyroxine?

The thyroid gland is responsible for producing and releasing thyroxine into the bloodstream. It’s located in the back of the neck, just below the larynx, and in front of the trachea. The anatomy of the thyroid resembles a shield: it consists of two lobes located on either side of the trachea, and joined by a central region called the isthmus.

Under normal conditions, this gland weighs about 20 grams and is usually not palpable. Its main function is the production of different hormones, among which T4 or thyroxine and T3 or triiodothyronine are the most important.

Specialized structures in the thyroid gland called follicles are responsible for synthesizing thyroid hormones. At the center of these follicles is a colloid substance that contains everything necessary for hormone production. As such, the colloid contains enzymes, iodine, tyrosine, and a special protein: thyroglobulin.

What is thyroxine? A doctor demonstrating a thyroid model and hormone sample.
The thyroid gland has two lobes and a central area that connects them called the isthmus.

What’s the function of this hormone?

The main function of thyroid hormones is related to the metabolism and growth of the human body. However, thyroxine is less biologically active and has a longer half-life than triiodothyronine. These characteristics make T4 the ideal substance for maintaining a constant reserve.

In general terms, triiodothyronine is the active form of the thyroid hormones, while thyroxine functions as the plasma reserve. When the body has low levels of T3, various enzymes in the peripheral tissues extract an iodine atom from thyroxine, transforming it into its active form.

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