Thyroid hormone and thyroid gland

Editor's Notes

1. Trace elements are elements that occur in very small amounts in living organisms and are necessary for their health, growth and development. Whereas their shortage in the body may result in stunted growth and disease, their excess is also harmful. If deficient they need to be taken with diet.
2. It looks like a bow tie & is even located in the appropriate place for a bow tie lying over the trachea just below the larynx! It develops from an evagination of the floor of the pharynx, and a thyroglossal duct marking the path of the thyroid from the tongue to the neck sometimes persists in the adult.
3. These 2 secretory cells are derived embryologically from 2 different sources: Follicular cells from endoderm of primitive pharynx & C-cells from neuroectoderm.
4. When the gland is inactive, the colloid is abundant, the follicles are large, and the cells lining them are flat. When the gland is active, the follicles are small, and the cells are cuboid or columnar.
5. Iodide is thus transported against its concentration gradient driven by the favorable electrochemical gradient for sodium! Energy is then expended by the Na/ K ATPase ( the sodium pump). Outward diffusion of k maintains the membrane potential. Like other transporters, Na-I symporter has a finite capacity & can be saturated. Consequently, other ions as perchlorate & thiocyanate compete for binding sites on the symporter, can block the uptake of iodide. This property can be exploited for diagnostic purposes!
6. This reaction is catalyzed by Thyroperoxidase.
7. Thyroglobulin stored within the follicular lumen is separated from ECF & the capillary endothelium by a virtually impenetrable layer of follicular cells. It is a complicated process for 2 reasons: Before their release, T4 & T3 are still bound within the thyroglobulin molecule. These hormones must be transported completely across the follicular cells to reach the capillaries that course through the interstitial spaces b/w the follicles.
8. This highly specific enzyme will remove iodine only from the worthless MIT & DIT and not from T3 & T4. Deiodinase provides almost twice as much iodide for hormone synthesis as Iodide pump & is therefore of great significance in hormone biosynthesis. Patients who are genetically deficient in thyroidal tyrosine deiodinase suffer symptoms of iodine deficiency & excrete MIT & DIT in their urine. Normally, virtually no MIT & DIT escapes from the gland.
9. It is remarkable that such less percentage is carried in free form as it is only the free pool of the hormone that has access to the target organ receptors & thus can exert their effect! Also the bound protein cannot escape the blood stream through the capillary endothelium as it is too large! The other binding proteins are: Thyroxine binding globulin (TBG) (binds 70% of the hormone) Transthyretin (TTR) (binds 15% of hormone) Albumin (binds 15%) The bound hormone provides substantial reservoir of extrathyroidal hormone! Therefore, when there is a deficiency of thyroid hormone due to a defect of the gland, the deficiency is not even noticed for weeks to months as the unbound form is slowly released. Also, the total amount of bound T3 & T4 will be affected if the plasma protein concentration is decreased as with kidney & liver diseases.
10. Like all lipophilic hormones, TH crosses the cell membrane and binds with an intracellular receptor, which in this case is a nuclear receptor. The thyroid nuclear receptor has a 10 times greater affinity for T3 than for T4. Because a hormone’s potency also depends on how strongly a hormone binds to its receptor therefore T3 is far more potent.