How Does the Thyroid Work - What is the Difference Between T4 and T3?

Where is the thyroid?

The thyroid is a small bow-tie or butterfly shaped gland located in your neck, close to the windpipe, behind and below your adams apple area

The thyroid produces several hormones involved in the function of the body in particular the metabolism, 2 of these are absolutely essential

These essential hormones are:

Triiodothyronine (T3) and Thyroxine (T4)

What do they do?

The thyroid hormones T3 and T4 help oxygen get into your cells and are critical to your bodies ability to produce energy. The role it has in delivering oxygen and energy makes your thyroid the master gland of metabolism, this then makes sense of many of the symptoms incurred by thyroid sufferers.

The Thyroid contains the only cells in the body capable of absorbing iodine.

It takes Iodine obtained from food, iodized salts or supplements and then combines that iodine with the amino acid tyrosine. The thyroid then converts the iodine/tyrosine combination into the hormones T3 & T4

What does the 3 & 4 mean?

The 3 and the 4 relate to the number of iodine molecules in each thyroid hormone molecule.

The split in percentages between the two is quite large, of the hormones produced by your thyroid 80% will be T4 and 20% will be T3

What is the difference?

The distinct difference between the two is T3 is the biologically active hormone meaning it is the one that has an effect at cellular level, so while the thyroid produces some T3 (20%) the rest of the T3 needed by the body is actually formed when the body converts T4 to T3, hence the larger percentage of T4 in the body.

The mostly inactive hormone T4 undergoes a process often called T4 - T3 conversion to produce the remaining amounts of T3 it requires, this conversion can take place in the thyroid, the liver, the brain and in other organs and tissues.

What is the purpose of T3 & T4

Once it has been released by the thyroid the T3 & T4 travel through the bloodstream, with the purpose of helping cells convert oxygen and calories into energy to serve as the basic fuel of your metabolism

Now as T3 (the biologically active hormone) circulates through your bloodstream it attaches to and enters your cells via receptor sites on the cell membrane.

Once it's inside the cells T3 increases the cell metabolic rate including body temperature and stimulates the cells to produce a number of different hormones, enzymes, neurotransmitters and muscle tissue.

T3 also helps your cells use oxygen and release carbon dioxide; this in turn helps to provide a smooth metabolic function.

How does the thyroid know how much T4 & T3 to produce?

This is quite a clever process; the release of hormones from the thyroid is part of a complete feedback process between the hypothalamus, the pituitary gland, your blood and the thyroid gland.

The hypothalamus, a part of the brain, emits thyrotropin-releasing hormone (TRH) The release of this TRH tells your pituitary gland to in turn produce thyroid-stimulating hormone (TSH).

The TSH that circulates in your bloodstream is the messenger that tells your thyroid to make the thyroid hormones T3 & T4 sending them into your bloodstream.

When there is enough thyroid hormone in your body, the pituitary gland makes less TSH (the messenger), which is a signal to the thyroid gland that it can slow down the production of the 2 thyroid hormones.

It's a smoothly functioning system, which runs on autopilot without any interference from us at all. That is when it works properly.

It is only when something interferes with the system and causes the feedback process to malfunction, thyroid problems can develop.