The salivary glands and the stomach also clear iodide and small but detectable amounts appear in sweat and in expired air. This conclusion was challenged by Xiao et al. The endocytosed vesicles fuse with the lysosomes of the follicular cell.
However, one route could simply convey Tg molecules that are not selected for entering the other pathways. Further addition increases the degree of iodination at these sites, iodinates some new tyrosyls, and results in thyroid hormone formation at residues 5, , , and , with a trace found at , in that quantitative order. The colloid droplets then deliver their content to lysosomes. The decrease in thyroid iodide transport resulting from excess iodide administration escape from the Wolff-Chaikoff effect, see further is related to a decrease in NIS expression 40;
Other iodine deficiency disorders include impaired growth and development, decreased fertility, and prenatal and infant death. Ouabain, digitoxin, and other cardiac glycosides block transport in vitro 27; Kimura et al. Iodide concentration by the choroid plexus and salivary glands does not have any obvious physiologic benefit, but needs to be remembered for possible insights into pathways as yet undiscovered.
As now known for the entry of thyroid hormones in peripheral target cells, the exit of thyroid hormones from thyrocytes probably involves membrane transporter s. Starting from highly purified preparations of thyroid lysosomes, Rousset et al. The organ is situated on the anterior side of the neck, lying against and around the larynx and trachea, reaching posteriorly the oesophagus and carotid sheath. Of this, about half is in the complex carbohydrate units, the remainder is present as phosphoserine and phosphotyrosine
It has been shown that the basal activity of both isoenzymes is totally depending on calcium and functional EF-hands calcium binding motifs. Classic Negative Feedback Loop. It was reported that the acetylcholinesterase-homology region of Tg could function as a dimerization domain ; Neosynthesised Tg polypeptide chains entering the lumen of the rough endoplasmic reticulum RER are subjected to core glycosylation, dimerise and are transferred to the Golgi where they undergo terminal glycosylation Fig.
Figure 1. Importantly, this reservoir of peroxidase-processed thyroglobulin is usually enough for months of use explaining why defects in thyroid hormone synthesis often take months to become clinically apparent. Breast milk contains large amounts of iodide, mainly during the first 24 hours after ingestion It is reasonable to think that Tg molecules following this route are the more mature molecules with a high hormone content but, this has not been firmly demonstrated. Calcitonin The thyroid gland also secretes a hormone called calcitonin that is produced by the parafollicular cells also called C cells that stud the tissue between distinct follicles. A potential iodide transporter has been proposed: pendrin 70;
As now known for the entry of thyroid hormones in peripheral target cells, the exit of thyroid hormones from thyrocytes probably involves membrane transporter s. Thyroid follicles serve as both factory and warehouse for production of thyroid hormones. Whatever the precise nature of the iodinating species, it is clear that iodide is oxidized by H2O2 and TPO, and transferred to the tyrosyl groups of Tg. As a result of this hyperstimulation, thyroglobulin accumulates in the thyroid gland follicles, increasing their deposits of colloid. Importantly, peroxidase-processed thyroglobulin within the follicle can act as a reservoir for thyroid hormones in the absence of stimulation for hormone release. A third receptor; megalin, has more recently been discovered in the thyroid and has been the subject of extensive studies yielding convincing data ;
Circulation and transport[ edit ] Plasma transport[ edit ] Most of the thyroid hormone circulating in the blood is bound to "transport protein "s. All tyrosine residues of Tg are not equally accessible to iodination. Its content is directly proportional to dietary iodine.
In summary, contrary to NIS for which the anion selectivity 25 corresponds to what was expected, the ion selectivity of thyroid pendrin remains to be elucidated. The nature and the origin of the reducing compounds acting on Tg are not known. Inside the lumen of RER, newly-synthesized proteins undergo core glycosylation and by interacting with chaperones acquire their conformation. The evolution has first been to consider that it could proceed via a mechanism different from phagocytosis, also named macropinocytosis, evidenced in rats under acute TSH stimulation reviewed in A and B, phase contrast and fluorescence images taken at the time of microinjection. Occasionally, large amounts are found in certain thyroid diseases, including Hashimoto's thyroiditis , congenital metabolic defects , thyrotoxicosis and thyroid carcinoma
The proximal region of the C-terminal half portion of Tg contains five repeats of another type of cysteine-rich motives. There are many objections to this view There, lysosomal enzymes break apart the thyroglobulin colloid, releasing free T3 and T4, which diffuse across the follicle cell membrane and enter the bloodstream. TSH stimulates transport only in the thyroid. The primary structure deduced from cDNA is also known for bovine, rat, and mouse