Iodine is actively absorbed from bloodstream and concentrated in thyroid gland. Biologically active iodine is bound to thyrosine and then monoiodothyrosine (MIT) and diiodothyrosine (DIT) are formed. By combining two particles of DIT we have thyroxine and by combining one particle of MIT and one particle of DIT we have triiodothyronine.
MIT + DIT = triiodothyronine DIT + DIT = thyroxine
The biologically active and central to regulation of metabolism are thyroxine (T4) and triiodothyronine (T3). Thyroxine is supposedly a prohomone and a reservoir of the most active and main thyroid hormone - triiodothyronine (T3).
Only a very small fraction is free (unbound) - thyroxine (T4)
0.03% and triiodothyronine (T3) 0.3%. This free fraction is
biologically active hence measuring concentractions of free thyroid hormones is
of great diagnostic value.
These values are referred to as : fT4 and fT3.
There are numerous physiological and pathological stimuli that influence the synthesis of thyroid hormones.
The hormones are essential to proper development and differentiation of all cells of human body. To various extent they regulate protein, fat and carbohydrate metabolism. The most pronouced impact, however, is on utilization of energetic compounds by human cells.
Thyrotoxicosis or hyperthyroidism is the clinical syndrome caused by an excess of circulating free thyroxine and free triiodothyronine, or both. It is a common disorder and affects approximately 2% of women and 0.2% of men.