In the blood to the target cells

In this lesson, we’ll look into the function of the thyroid gland, how the hormone thyroxine works to stimulate your body’s metabolism, and what can happen if the thyroid gland and its hormones don’t function properly.

The Endocrine System

How much time do you spend thinking about your metabolism? How about the development of your nerves? Most people would probably say, ‘Not much.’ We have amazing chemical reactions taking place inside of our bodies every second of every day, and we really don’t pay attention until something goes wrong. One of these chemical processes involves the hormone thyroxine.

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Before we get into the specifics of thyroxine, let’s review the basics of the endocrine system. The endocrine system is one of the master systems of the body, along with the nervous system, that regulates the activities of other systems. It’s structured around the manufacturing and delivery of chemical messengers called hormones.Hormones are made in various spots in the body in specialized tissues called glands, and often act like switches: when they arrive at their target cells, they can turn on or off certain functions, like speeding up your heart rate. Some of the more powerful ones will enter the nucleus of a cell and turn on and off certain genes.Hormones are delivered through the blood and connect to special receptors on the cell that are specific to the hormone.

It’s a very complex system. Imagine if every lamp in your house had a specialized plug, and you could only use that lamp in a certain plug within a certain room. That’s the kind of complexity and specialization that we see in the endocrine system.

The Structure of Thyroxine

Thyroxine is a hormone that is manufactured in your thyroid gland. The thyroid gland is located in the neck just in front of the larynx. Thyroxine is made from two linked tyrosine amino acids. In order to form thyroxine, your thyroid gland needs to have iodine in your blood. One molecule of thyroxine contains four iodine atoms.

This is why the word ‘thyroxine’ is often shortened to T4.

The Function of Thyroxine

Thyroxine is one of two hormones that together form what’s referred to collectively as the thyroid hormone. Thyroxine travels through the blood to the target cells and is then converted to triiodothyronine, shortened to T3; think of T4 as the messenger, and T3 as the worker that carries out the order.

T3 is the active form of thyroid hormone and is primarily responsible for your metabolism, which is the process by which your cells break down food and other substances into smaller molecules they can use.Thyroid hormone works by entering a target cell’s nucleus, binding to receptors, and starting transcription, or the copying of DNA, for making proteins. The target genes are the ones involved in the metabolism of glucose in your body. Thyroid hormone increases your metabolism and body heat production.

In addition, the thyroid hormone also regulates blood pressure, the development of skeletal and nervous tissues, and it affects reproductive function.

Controlling Thyroxine

The thyroid gland is controlled by thyroid stimulating hormone, or TSH for short. TSH is manufactured in the pituitary gland in the brain. The pituitary gland receives information from the body about metabolism, blood pressure, and so forth, and then responds by either increasing TSH secretions, which travel to the thyroid gland and promote thyroxine production, or shutting off TSH secretions, which then decreases thyroxine production.

When a patient is diagnosed with potential thyroid problems, one of the first tests is to check the TSH levels in the blood. If not enough TSH is being made, which is known as hypothyroidism, the patient is prescribed synthetic TSH in pill form. If too much TSH is detected, which is known as hyperthyroidism, further tests are conducted and treatments may include surgical removal of the thyroid gland or taking radioactive iodine to slow down the gland’s activity.Another potential issue with thyroxine occurs if a person doesn’t have sufficient iodine in their diet.

If you don’t have sufficient iodine in your blood, you can develop what’s called a goiter. A goiter can form when your thyroid gland tries desperately to make functional thyroxine, but all it can make is the protein base, which swells up the gland with useless material, and a large growth appears on the neck. This growth can be removed surgically and the person is usually treated with iodine to help the thyroid gland function properly.

Lesson Summary

Thyroxine is a hormone made in the thyroid gland for the regulation of metabolism, body heat production, blood pressure, and the normal development of the skeletal and nervous systems. Thyroxine is an amino acid-based hormone that targets the genes involved in the metabolism of glucose in the body.

Iodine in the blood is a key component of making thyroxine, and if not enough iodine is present, it can lead to the development of abnormal growths called goiters.Thyroxine is regulated by another hormone called thyroid stimulating hormone, or TSH. TSH is made in the pituitary gland in the brain and is released in response to the metabolic needs of the body. Thyroxine levels will go up or down in response to the amount of TSH arriving in the thyroid gland.

Not enough TSH leads to hypothyroidism, and too much TSH leads to hyperthyroidism.


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