The thyroid gland is composed of follicles, round sacs that are clustered together like tiny bubbles. The follicle consists of a gelatinous material called colloid in the center and is lined with follicular cells on the outside. These follicular cells produce thyroid hormone by absorbing iodine from the diet and combining it with thyroglobulin, a thyroid protein produced in the thyroid.
Inside each of these thyroid follicles, iodine hooks up with the thyroglobulin and mixes with an amino acid called tyrosine on the thyroglobulin to produce two forms of thyroid hormone, triiodothyronine (T3) and thyroxine (T4). The thyroid hormone is then stored in the colloid. When levels of thyroid hormone dip, the pituitary gland releases more TSH, which arrives at the thyroid follicles and attaches to TSH receptors on the cells. In the presence of TSH, T4 and T3 are cleaved off the thyroglobulin. The T4 and T3 go from the colloid back to the follicular cells and are then released into the bloodstream.
The numbers on these two forms of thyroid hormone reflect the number of iodine molecules in each molecule of thyroid hormone. In healthy people, 80 percent of the thyroid hormone you produce is T4, the less powerful form of thyroid hormone. The remaining 20 percent is T3, the more potent and active form of thyroid hormone.
The bulk of the T3 your body needs is produced by the conversion of T4 into T3. Your body cells do this conversion by removing one of the iodine molecules on T4, with help from enzymes known as deiodinases. There are three different types of these enzymes, each in different tissues. The deiodinase enzymes also convert T4 to reverse T3 (rT3), which is an inactive form of T3. The conversion of T4 into T3 takes place in organs such as the liver, kidneys, and muscles. The scientific name for the conversion process is called monodeiodination.
Follicular cells of the thyroid gland change shape depending on their activity. The cells are rectangular when the thyroid gland is active and square when it is inactive.
Once inside the bloodstream, most of the T3 and T4 hormones attach themselves to blood proteins produced by the liver, which are called thyroxine-binding globulin (TBG); thyroxine-binding pre-albumin, also called transthyretin; and albumin. Only minute amounts of thyroid hormone — 0.04 percent of T4 and 0.4 percent of T3 — circulate in the blood as the free and active portions. The active form of thyroid hormone then travels throughout the body via the blood and binds to thyroid hormone receptors on different tissues. There are two types of thyroid receptors, TR alpha and TR beta, which have different activities.
Every single cell in your body relies on thyroid hormone to do its job. That's because thyroid hormone determines how quickly your body uses oxygen and calories from food to produce the energy cells need to do their jobs. Thyroid hormone begins doing its work at conception and continues until death.
T3 has a shorter half-life than T4, which means T4 remains in the body a lot longer than T3. Animals such as pigs have a higher amount of T3 than humans do and are used as a source for the production of natural hormone used in treatment.
Thyroid hormone has numerous and critical functions in the body, including the regulation of the speed at which individual cells function, or BMR. Here are some other ways that thyroid hormone works in your body.
It ensures the proper growth and development of children.
It aids in proper muscle functioning.
It ensures that the heart pumps effectively and efficiently.
It ensures that the gastrointestinal (GI) system is able to digest and excrete food properly.
It strengthens hair, skin, and nails.
It helps with the development of the brain.
It aids in the growth of strong bones.
It ensures proper development of the body's organs.
As you can see, thyroid hormone is a pivotal player in the proper functioning and well-being of your body. Even the slightest increase or decrease in hormone levels can affect your health.
But T4 and T3 aren't the only hormones produced by the thyroid gland. Tucked between the follicular cells are other cells, known as parafollicular cells, also called chief cells or C cells. The parafollicular cells produce calcitonin, a hormone important to bone health.
Calcitonin inhibits the removal of bone by osteoclasts, a type of cell involved in the constant breakdown of bone. These cells also produce small amounts of somatostatin, which inhibits the release of several hormones, including TSH, growth hormone, and insulin.
You may know calcitonin as a drug, not just a hormone. In postmenopausal women who have osteoporosis, calcitonin may be prescribed as a medication, under brand names such as Miacalcin and Calcimar. It works just like the natural hormone by slowing the breakdown of bone. Calcitonin is also found in salmon.