How Your Thyroid Works

The thyroid gland is often referred to as a butterfly y-shaped gland, but it is also shaped like the capital letter H, specifi cally, the H of the Honda vehicle logo. Each side of the H or butterfl y is called a lobe, while the center (the body of the butterfl y) is called the isthmus. The thyroid gland is located in the lower part of your neck, in front of your windpipe, and is basically wrapped around the windpipe. Using the butterfl y analogy, the butterfl y “hugs” the windpipe.

The thyroid gland makes two critical thyroid hormones. The fi rst is thyroxine, known as T4, which has four iodine atoms for each hormone molecule. The second is triiodothyronine, or T3, which has three iodine atoms for each hormone molecule. Both hormones are referred to in the singular as thyroid hormone.

Thyroid hormone is essential for our existence, affecting every single cell in the body. Thyroid hormone serves as the speed control for our cells, controlling their “speed of life.” Of the two hormones, it is thought that only T3 has a direct effect on the cells of the body; it is debatable whether T4 has any direct effect. T4 must be converted to T3 by the cells by removing one specifi c iodine atom before it can work. If a different iodine atom is removed from T4, it creates an inactive molecule that does not work as an effective thyroid hormone.

Nearly all cells have special enzymes inside of them called deiodinases that remove the iodine atom from T4 to make it into T3. It is thought that this is one way that each cell customizes how much T3 it will get, even though the blood supply to all the cells generally provides the same level of T4 at the same time. The T3 produced in the body’s cells works in the nucleus of these cells to turn some genes on and some genes off. This is how the thyroid hormone works in the body.

For example, in the cells of the pituitary gland (where thyroidstimulating hormone (TSH) is produced), T3 turns off the gene that

makes TSH. This is why TSH levels are high when thyroid hormone levels are low and TSH is decreased when thyroid hormone levels are high.

The thyroid gland usually releases around 80 percent of its thyroid hormone as T4 and 20 percent as T3. Only 0.03 percent of the total T4 traveling around in the bloodstream is in the free form, in which it can be taken up into each body cell and do the job of effective thyroid hormone. Once the free T4 passes into a body cell of any kind, it is changed into T3 by an enzyme called 5-prime deiodinase.

Once changed, this T3 is absorbed into the nucleus of the cell, where the chromosomes are found. Inside the nucleus are special T3-receptor proteins that are made to stick to distinct spots in the genes of the chromosomes and also to stick to T3. When enough T3 is taken into the nucleus, it sticks to these T3-receptors and controls the genes that they are attached to. Some genes are turned on while other genes are turned off. Thus, each cell is able to convert the correct proportion of T4 to T3 for its own needs.

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Last updated on February 5, 2014