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The major form of thyroid hormone in the blood is thyroxine (T4). This is converted to the active T3 within cells by deiodinases.
Most of the thyroid hormone circulating in the blood is bound to transport proteins :
Only a very small fraction of the circulating hormone is free (unbound) - T4 0.03% and T3 0.3%. This free fraction is biologically active, hence measuring concentrations of free thyroid hormones is of great diagnostic value. These values are referred to as fT4 and fT3. Another critical diagnostic tool is the amount of thyroid-stimulating hormone that is present.
When thyroid hormone is bound, it is not active, the amount of free T3/T4 is what is important. For this reason, measuring total thyroxine in the blood can be misleading.
The thyroid hormones are essential to proper development and differentiation of all cells of human body. To various extent they regulate protein, fat and carbohydrate metabolismMetabolism in the most general sense, is the ingestion and breakdown of complex compounds, coupled with the liberation of energy, and the consequent generation of waste products. It is major process of living organisms, and because this process can happen. The most pronounced impact, however, is on utilization of energetic compounds by human cells.
There are numerous physiological and pathological stimuli that influence the synthesis of thyroid hormones.
Thyrotoxicosis or hyperthyroidismHyperthyroidism (thyrotoxicosis or "fast thyroid gland") is the clinical syndrome caused by an excess of circulating free thyroxine (T4) and free triiodothyronine (T3), or both. Major causes in humans are Graves' disease (the most common etiology with 70- 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.
Thyroxine (3:5,3':5' tetraiodothyronine) is produced by follicular cells of the thyroid gland. It is produced as the precursor thyroglobulin (this is not the same as TBG), and this cleaved by enzymes to produce active T4.
Thyroxine is produced using the ring structures of tyrosine, to which iodine is attached. Thyroxine contains four iodine atoms, triiodothyronine is identical to T4, but it has one less iodine atom per molecule.
Iodide is actively absorbed from the bloodstream and concentrated in the thyroid follicles. (If there is a deficiency of dietary iodine, the thyroid enlarges in an attempt to trap more iodine, resulting in goitreA goitre (or goiter) ( Latin struma is a swelling in the neck due to an enlarged thyroid gland. The most common cause for goitre in the world is iodine deficiency. Other causes are: Hashimoto's thyroiditis Graves-Basedow disease juvenile goitre neoplasm o) Via a reaction with the enzyme thyroperoxidase, iodine is covalently bound to tyrosine residues in the thyroglobulin molecules, forming monoiodotyrosine (MIT) and diiodotyrosine (DIT). By linking two moieties of DIT we have thyroxine and by combining one particle of MITand one particle of DIT we have triiodothyronine.
MIT + DIT = triiodothyronine (usually referred to as T3) DIT + DIT = thyroxine (referred to as T4)
The iodinated thyroglobulin is digested by proteases, releasing the hormones - T4 and T3 are the biologically active agents central to regulation of metabolism. Thyroxine is supposedly a prohormone and a reservoir for the most active and main thyroid hormone T3, T4 being converted as required in the tissues by deiodinases.