Understanding the T3 and T4 Thyroid Hormones

Facts about Thyroxine and Triiodothyronine

Jul 27, 2009 Jim Lowrance

The two major thyroid hormones are called "thyroxine" (T4) and "triiodothyronine" (T3). Their purpose is to regulate the rate of metabolism in every cell of the body.

The thyroid gland consists largely of iodine and from it, produces the T4 and T3 thyroid hormones. This process is accomplished in the liver and kidneys and within the thyroid hormone cells themselves with the aid of iodide peroxidase enzymes (thyroidperoxidase) and thyroglobulin, which are also referred to as thyroid proteins.

Thyroxine-T4 Converts into T3

Thyroxine, known as the T4, is the one most abundant in the body representing approximately 80% of thyroid hormones but is also a precursor to T3. While medical research shows that T4 does have a purpose at the tissue-level, in regulating bodily metabolism, its other major function as a “prohormone” is to convert into the T3 hormone (triiodothyronine).

There are four iodine molecules in T4 but it loses one of the atoms (single element) of iodine from these molecules (combined elements), which is removed from its outer ring once converted into T3 which will then have three iodine molecules. The T3 it converts into is at least four times more powerful in spite of having one-less iodine molecule and is more active in regulating bodily metabolism. When looked at it from this point of view, T4 is less active, with the purpose of being on stand-by as a reserve hormone, to be converted into the active T3 as needed in the body.

Excess Triiodothyronine-T3 is converted into Reverse T3

T3 represents approximately 20% of thyroid hormones found in the body. When the body has enough T3 available, any excess T4 remaining in reserve, will be rendered inactive by the conversion of it into “Reverse T3” (RT3). This process is ongoing due to the fact that the thyroid gland continues to absorb iodine from things consumed in the diet, from which to continue producing hormones. In the case of T4 being converted into RT3, one iodine atom is removed from the inner ring of the iodine molecules. This process which also occurs mainly in the liver and kidneys is a safe-guard against over-stimulation of bodily metabolism from an excess of T3 hormone or “hyperthyroidism”.

Low T3 Syndrome

In some cases a disease process in the body, a medication being taken or chronic stress, can cause excessive conversion of T4 into RT3 and this will cause T3 levels to drop too low, causing hypothyroidism. While this cause of low T3 is rare, it does occur and is referred to by several names, including “Low T3 Syndrome”, “Euthyroid Sick Syndrome”, “Reverse T3 Syndrome” and “Wilson’s Temperature Syndrome”. In most cases of excessive RT3 conversion, the hypothyroidism is temporary and can be corrected by supplementing short-term with T3 hormone and correcting the problem in the body causing the condition.

Impaired Conversion of T4 into T3

Usually when a person becomes hypothyroid, the T4 itself becomes low but because it is the forerunner to the manufacture of T3, this in turn causes both levels to become low over time. In some cases however, adequate T4 will be produced by the thyroid gland but then fails to be converted into an adequate amount of T3. The term often used for this condition is “impaired conversion” and will cause hypothyroidism due to a lack of the more metabolically active T3 hormone.

As mentioned in the previous subheading, the resulting condition is referred to as “Low T3 Syndrome” but in this case it is due to failure of conversion of T4 into T3. The condition cannot be detected by testing the TSH level alone (pituitary hormone that reflects thyroid levels) but tests of the T4 and T3 must be ordered to detect the problem. The impairment can be due to a number of factors but in some cases may be due to kidney and/or liver disease, the organs in which the conversion process takes place. In addition to organ problems, an autoimmune disease process affecting thyroid function may result in a lack of iodide peroxidase enzymes and/or thyroglobulin. The antibodies that attack these enzymes/proteins are called the anti-thyroidperoxidase (Anti-TPO) and the anti-thyroglobulin (anti-TG).

Also see: Understanding the TSH Hormone-Suite101

The copyright of the article Understanding the T3 and T4 Thyroid Hormones in General Medicine is owned by Jim Lowrance. Permission to republish Understanding the T3 and T4 Thyroid Hormones in print or online must be granted by the author in writing.

Thyroid Front View