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propylthiouracil in p. 362
agranulocytosis p. 251
aplastic anemia p. 251
thionamides p. 362
for thyroid storm p. 348
There are 2 different thyroid hormones; triiodothyronine or T3, and thyroxine or T4.
Now, if we zoom into the thyroid gland, we’ll find thousands of follicles, which are small, hollow spheres whose walls are lined with follicular cells, or thyrocytes.
Zooming further into these follicular cells, we’ll see their apical side that surrounds a central lumen filled with a viscous fluid called the colloid.
The colloid contains the precursor hormone thyroglobulin.
The basolateral side of follicular cells is in contact with blood vessels that supply these cells.
Synthesis of thyroid hormones begins when follicular cells take in inorganic iodide ions from the blood, along with two sodium ions, via a sodium- iodide symporter.
This step is known as ‘iodide trap’.
The iodide ion is pumped via the pendrin protein, into the viscous fluid inside the follicle called the colloid, which contains thyroglobulin; the precursor of thyroid hormone.
In the colloid, inorganic iodide undergoes oxidation via the enzyme thyroid peroxidase or TPO, to become organic iodide, which then binds to the tyrosine in thyroglobulin.
This step is known as iodination.
Some tyrosine residues bind to only one iodine and form monoiodotyrosine or MIT, whereas others bind to two iodine atoms to form diiodotyrosine or DIT.
These molecules are then coupled together by the same enzyme thyroid peroxidase.
This process is known as coupling.
Coupling one MIT with one DIT creates T3, while coupling 2 DIT molecules creates T4.
T4 is generally created in greater amounts than T3, with T3 being the more active form with a half life of 1 to 2 days, while T4 is less active but has a longer half life of 6 to 8 days.
Once released from the thyroid gland, most of the T3 and T4 travels via the blood by binding with the thyroxine - binding globulin, or TBG, to reach the target cell.
Alternatively, small amounts of T3 and T4 stay unbound, and therefore they are referred to as “free” thyroid hormones.
Hyperthyroidism refers to a condition in which there is excess thyroid hormones. Medications used to treat hyperthyroidism aim at reducing the level of thyroid hormones or the management of associated symptoms. Hyperthyroidism medications include beta blockers, radioactive iodine, propylthiouracil, methimazole, Lugol's iodine, propranolol, and corticosteroids.
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