How To Understand the Function of Thyroid-Related Hormones

There are four major hormones having to do with thyroid function and the resulting metabolism these regulate in the body. In the four steps below, we will look at the thyroid related hormones called the T-4, T-3, Reverse T-3 and TSH.

1. Thyroxine (T-4). The most abundant thyroid hormone in the body is the one called “thyroxine” (T-4). This is the hormone produced in larger amounts by the thyroid gland, so that there is an adequate supply of it to be converted into a more active form of thyroid hormone that is more responsible for regulating the metabolism of the body. T-4 itself, in its pre-conversion form, is less metabolically active. Thyroxine is the thyroid hormone containing four molecules of iodine, which is why it is commonly abbreviated as “T-4”. You could say that the T-4 hormone is a reserve hormone that is ready to be converted into the more metabolically active hormone, which will be described in step two.
2. Triiodothyronine (T-3). The second major thyroid hormone, and the one that is more active in the body in regulating metabolism, is “triiodothyronine” (T-3). This is the more powerful of the thyroid hormones and is converted from the T-4 hormone described in step one. This one contains only three molecules of iodine, but is many times more powerful than the T-4 hormone. T-3 is up to 10 times more active in regulating the metabolism of the body than is the T-4. The conversion process from T-4 to T-3 is accomplished through enzymes and proteins that enter the T-4 hormone cells and cause them to convert into T-3 hormone cells. This process takes place mainly in the liver, and is one of the many important purposes of the liver in helping to regulate the body’s metabolism.
3. Reverse R-3 (R T-3). When the body has converted enough T-3 from the T-4 hormone, it will convert any of the excess T-4 hormone into another type of inactive hormone called “Reverse T-3” (R T-3). This is another function that takes place in the liver and helps to eliminate any extra T-4 produced by the thyroid gland. The conversion of the extra hormone into Reverse T-3 protects the body against having too much of the powerful T-3 hormone. If this elimination of extra T-4 did not take place, too much T-3 would be converted and accumulate in the body, causing an abnormally sped up metabolism, which results in a type of “hyperthyroidism”.

   Certain diseases in the body and periods of extreme prolonged stress can cause the body to convert too much of the T-4 hormone into Reverse T-3, rather than into T-3, which is sometimes referred to as “euthroid sick syndrome”. This will cause a slowed-down metabolism from lack of active thyroid hormone, which results in a type of “hypothyroidism”. Problems with imbalances of Reverse T-3 are less common causes of hyperthyroidism and hypothyroidism than diseases directly affecting the thyroid gland.

4. Thyroid Stimulating Hormone (TSH). The hormone that regulates the rate at which the thyroid gland produces the T-4 hormone is called “Thyroid Stimulating Hormone” (TSH). The production of T-4 will then result in the conversion of it into the needed T-3 and Reverse T-3. This hormone comes from a major gland in the brain called the “pituitary gland”, which monitors how much T-4 hormone is in the blood. If the pituitary gland detects that the thyroid gland is producing too much T-4 hormone, it will send less TSH to stimulate the thyroid gland to do so. If the pituitary gland senses that the thyroid gland is not producing enough T-4 hormone, it will send more TSH to stimulate it into doing so.

   A healthy thyroid gland will have normal amounts of TSH stimulating it; when blood levels of TSH are tested in people suspected of having either an over-functioning or under-functioning thyroid gland, they can see from this test if the pituitary gland is sending too much TSH (hypothyroidism) or too little TSH (hyperthyroidism) to stimulate production of T-4. TSH tests will detect thyroid hormone imbalances earlier than any other blood test, including all of those listed in the steps above.