Thyroid hormone synthesis physiology | Endocrine system physiology mbbs 1st year lecture

Physiology lecture on synthesis of thyroid hormones
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Synthesis of thyroid hormones occurs in multiple steps:
Thyroid hormones are iodinated tyrosine which is an amino acid.
Synthesis of thyroid hormones occurs in multiple steps
1. Synthesis of thyroglobulin: Thyroglobulin is synthesised by follicular epithelial cells just like any other protein synthesis by cells and is secreted into the colloid by exocytosis. So this protein is stored in the colloid.
2. Then next step involves trapping and transport of iodide: Iodine is present in circulation as iodide ion. This iodide is taken up into the cytoplasm by NaI- symporter which is a secondary active transporter present on the basolateral membrane of thyrocytes. This Iodide then enters into the colloid via the apical membrane through the Cl-/I- exchanger which is also known as ‘pendrin’
3. So now we got both iodide and thyroglobulin in the colloid. However, the iodide cannot react with tyrosine residues. For that it should be oxidised to iodine. So this process of oxidation of iodide to iodine and iodination of the tyrosine residues of the thyroglobulin is known as organification. This occurs in presence of the enzyme thyroid peroxidase present on the apical membrane of thyrocytes. So organification is the third step. Many tyrosine residues of thyroglobulin are iodinate either at single site forming monoiodotyrosines i.e MIT or two sites forming diiodotyrosines i.e DIT.
4. MIT and DIT are not thyroid hormones. It is the coupling or combination of these iodinated tyrosines which produces thyroid hormones. One MIT and one DIT couple to form triiodothyronine i.e. T3 and 2 DITS couple to form Tetraiodothyronine i.e T4 or thyroxine in presence of enzyme thyroid peroxidase. This is known as coupling reaction. So basically, thyroid hormones form within the thyroglobulin molecule.
When the gland is stimulated by Thyroid stimulating hormone, all these steps of synthesis of thyroid hormones i.e synthesis of thyroglobulin, uptake of iodide, organification and coupling reaction are stimulated. Also, the follicular cells start taking up the stored iodinated thyroglobulin from the colloid by endocytosis. Intracellularly lysosomes fuse with the phagocytosed thyroglobulin and cleave T3 and T4 from thyroglobulin by proteases which are then released from the basolateral side and enter into the circulation. The separated amino acids are reused to produce thyroglobulin. The MIT and DIT which are not coupled are acted upon by the enzyme ‘deiodinase’ which as its name suggests detaches the iodine from the tyrosine both of which are recycled.
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[@akshathferrara9029]

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[@abhijeetchavan7670]

Very nice lecture mam....u solve my problem and confusion regarding synthesis of thyroid hormone.

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[@atulchoudhary6840]

Mam your teaching style and to the point teaching is really awesome...it really helps me to resolve my confusion

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Your videos helps pharmacy student too.
I learnt alot from your arrhythmia series..

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[@jagannathpolai1622]

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[@PhysiologyOpen]

Thank you so much. Will definitely upload as many. There are few on pharmacology available

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[@pragyaskitchen2911]

ma"am you are a fabulous teacher
pls upload videos on pituitary and adrenal hormones

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[@kamaltejwani2814]

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Coming soon just after 2 videos on thyroid. Sorry for the delay

[@kamaltejwani2814]

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Glad it was helpful!

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