Therefore, serum thyroxine and triiodothyronine concentrations do not change, nor do serum thyrotropin concentrations.
When thyroid hormones are needed, thyroglobulin is reabsorbed from the colloid in the follicular lumen into the cells, where it is split into its component parts, including the two thyroid hormones thyroxine (T4) and triiodothyronine (T3).
Because each molecule of tyrosine binds one or two iodine atoms, two tyrosines are used to synthesize both thyroxine and triiodothyronine.
As a result, all of the thyroxine and about 20 percent of the triiodothyronine produced each day come from the thyroid gland.
The conversion of thyroxine to triiodothyronine significantly decreases in response to many adverse conditions, such as malnutrition, injury, or illness (including infections, cancer, and liver, heart, and kidney diseases).
For example, thyroxine (T4) may be converted to triiodothyronine (T3), a change that substantially increases its hormonal potency, or it may be converted to reverse triiodothyronine (reverse T3), a molecule with the same three iodine atoms that has minimal biological activity.
Therefore, thyroglobulin serves not only as the structure within which thyroxine and triiodothyronine are synthesized but also as the storage form of the two hormones.
Within minutes after birth there is a sudden surge in thyrotropin secretion, followed by a marked increase in serum thyroxine and triiodothyronine concentrations.
Thyroxine, also called 3,5,3′,5′-tetraiodothyronine 3,5,3′,5′-tetraiodothyronine hormones secreT4d by the thyrone gland (the ofher is theiodotworonine).
The clearance of thyroxine and triiodothyronine decreases somewhat and is matched by a decrease in their production.
