More details on the T4 Thyroid Hormone

T4 is also known as thyroxine when it is produced by the thyroid gland.

Its proper chemical name is 3,5,3',5' tetraiodothyronine but as you can see it is easier to call it T4 (due to its four iodine atoms).

In its synthetic medication form T4 is often referred to as: Levothyroxine, L-Thyroxine or Synthetic T4. However, there are many brand names for T4 medication and sometimes it is just referred to by the brand.

Examples of T4 medication brands are:

  • Thyrax
  • Euthyrox
  • Levaxin
  • L-thyroxine
  • Eltroxin
  • Thyrax Duotab
  • Thyrox
  • Eutirox
  • Tirosint
  • Levoxyl
  • Synthroid
  • Numerous generic versions often called Levothyroxine

However people choose to refer to T4, one thing needs to be made very clear. T4 is what is known as a storage hormone. To all intents and purposes T4 is biologically inert and on its own will not enable us to experience good health:

  • T4 does not regulate our metabolism
  • T4 does not keep us warm
  • T4 does not stop us gaining weight
  • T4 does not give us energy
  • T4 does not allow us to think clearly
  • T4 does not provide any benefit until it is converted to something more useful

These statements apply regardless of whether T4 is a naturally produced thyroid hormone or a synthetic thyroid hormone from a bottle of tablets.

T4 is a pro-hormone and in a healthy person it provides a reservoir or storage for current and future needs. T4 is only useful to us after it has been converted to the biologically active thyroid hormone T3.

In a healthy person a significant amount of the circulating T4 is converted into T3 by the our cells. The thyroid gland itself is responsible for about 25% of our total conversion capability. Hence the loss of the thyroid through thyroidectomy or Hashimoto's is highly significant.

The T3 converted from T4, as well as the relatively small amount of T3 produced by the thyroid gland itself, increases the level of circulating T3 in the bloodstream. Although the thyroid gland does produce some T3, it is thought that around 80% of the circulating T3 is derived from conversion from T4, .

However, the above is not the whole story as there are many problems that can stop the normal, healthy functioning of thyroid hormones, including worsening conversion from T4 to T3, and lead people to descend into ill health and in many, sometimes tragic cases, these people never climb back out again.

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