Thyroxine Function - What Is Thyroxine

What is thyroxine ?

Thyroxine is a prohormone that is produced and secreted by the thyroid gland. It is more commonly known as T4. It is also marketed as a drug called levothyroxine. This is prescribed when the thyroid gland does not work or too slow.


Tyrosine residues of thyroglobulin are iodinated. A tyrosine residue with an iodide is an MIT (MonoIodineThyrosine); a tyrosine residue with two iodides represents a DIT (DiIodineThyrosine). Two DIT's T4 form. An MIT and DIT form T3. T4 and T3 are secreted from the thyroid gland in the blood. The thyroid gland makes relatively more T4 than T3. T4 is by the liver, but also in all tissues, converted into T3. T3 is more potent than T4. In case of insufficient iodine intake, the thyroid is relatively and absolutely produce more T3.

Thyroxine Function - What Is Thyroxine | thyroxine t4 side effects | thyroxine free medication

Thyroxine function

Thyroid hormones are essential for normal growth and development and have many effects on metabolic processes. For example, they play an important role in the development of the nervous system, the synthesis, mobilization and breakdown of fats, and it increases the number of catecholamine receptors on the cell membrane (effect on heart rate).


The majority (99.98%) of the thyroxine in the blood is bound to transport proteins:
  • thyroid binding globulin (TBG)
  • pre-albumin
  • albumin
Only thyroxine that is not bound to transport proteins is able to achieve biological effects. This is free T4 (fT4).


About half of T4 is broken down by means of deiodination and metabolism in tissues. The other half is conjugated in the liver and excreted in the bile. A little thyroid hormone is secreted by the kidneys.


Thyroid hormones can be determined in a clinical chemistry laboratory. Usually, T3 and fT4 is determined, they are biologically active. A too low fT4 is called hypothyroidism and too high fT4 is called hyperthyroidism. In addition, the TSH will be determined to assess the cause of the abnormal value of the fT4.

Thyroxine medication

Thyroxine is like medicine prescribed when the thyroid gland does not work or too slow. It is on the market under the name of levothyroxine, also known as L-thyroxine, T4 synthetic or 3,5,3 ', 5'-tetra jew-L-thyronine. The medicinal product is the monohydrate of the sodium salt of thyroxine. This substance has a greater half-life than the mixture of the two natural thyroid hormones T3 and T4 (= levothyroxine). As mentioned above, T4 is converted to T3 in the body. T4 is thus a prodrug.

Warning after registration
In the Netherlands arose late 2014 concern among users of the brand drug Thyrax®, then this means a different pattern of side effects seemed to get after delivery in blister pack instead of the usual loose tablets. The MEB obliged manufacturer Aspen Pharmacare in February 2015 to post a letter on its website to users.


This mirror image molecule was chemically produced in the 20th century, under the brand name Dethyrone® on the market as a means to lower cholesterol. It disappeared from the market due to cardiac side effects. Dextrothyroxine at a dose of 4 mg showed in a study equipotent to 0.15 mg levothyroxine as well as lipid ever lower and as fighter too low T3 levels.

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