see also Thyroid hormones
Thyroxine is a hormone produced by the thyroid gland. The structure of thyroxine is loosely derived from the combination of two residues of the amino acid tyrosine, a reflection of how the hormone is synthesised in the thyroid gland. Thyroxine is also known as tetraiodothyronine and is often denoted as T4, the '4' referring to the number of iodine atoms in the structure.
Thyroxine itself has limited biological effects and can be regarded as precursor to the biologically active triiodothyronine (T3) which is produced mainly by de-iodination of T4 in the peripheral tissue cytoplasm and cell nucleus itself, although some exists in the blood. The biological processes involved are covered in the article on thyroid hormones.
The bulk of circulating thyroxine is bound to several different serum proteins including thyroxine binding globulin, thyroxine binding albumin and serum albumin. The actual amount of free thyroxine is considerably less. This consideration is important when interpreting the thyroid function tests.
This is not supported by science or evidence and indeed is a hypothesis that has been re-examined over many decades and to date found wanting. Ensuring their thyroxine replacement is adequate is a first step in treating them.