hey there in this video I am going to discuss the synthesis and secretion of the thyroid hormones and the synthesis and secretion of the thyroid hormones and its various steps can be easily understood by just looking at this diagram and this is a diagram I have taken from Guyton so let's understand as to what is this diagram is showing us the cell what we are seeing is a follicular epithelial cells which is lining the follicles of the thyroid glands this is what is called as the apical membrane of the epithelial cell that's why this portion is going to become the follicular Lumen okay and this is the basol lateral membrane of the epithelial cell and this basalateral membrane is closely associated with the blood vessels or the blood capillaries so the first and foremost step in the synthesis of the thyroid hormones is transportation of iodide from the capillaries into the epithelial cell and this is achieved due to the presence of a transporter in the basal lateral membrane of the epithelial cell which is called as sodium iodide symporter as the name itself is telling it's a symporter that means it is going to transfer one iodide and it is also going to transfer to sodium from outside the cell to the inside of the cell and remember that sodium iodide Sim Porter requires energy that means this transportation is an active transport so from where this energy is derived this energy is derived due to the presence of one more pump you are seeing here in the basal lateral membrane of the follicular epithelial cell and this is nothing but our sodium potassium ATP sperm so what is the sodium potassium 80 based pump does is it is going to pump three sodium from inside the cell to the outside and in exchange it is going to pump to potassium from outside to inside so when it is constantly pumping the sodium ions from inside to outside what is going to happen to the concentration of sodium inside the cell that is going to reduce now this is going to create a gradient for sodium so the sodium's concentration inside the cell will be less and outside will be more now that is going to help in the transportation of sodium from outside to inside and along with the sodium even iodide is also entering so this first and very important step is what is called as iodide trapping iodide trapping it's an active process now the second step is synthesis of thyroglobulin with the help of endoplasmic reticulum and the Golgi apparatus now this thyroglobulin which is synthesized it's a glycoprotein and it is made out of tyrosine amino acids remember that thing and once this thyroglobulin is synthesized it is transported out of the cell into the Lumen here you can see this Arrow Mark by the process of exocytosis Naval thyroglobulin is lying in the Lumen of the follicle the third step is actually the third step is a combination of two steps what is going to happen is as you are seeing here the iodide has entered into the follicular epithelial cell swiftly this iodide will be transported towards the apical membrane so when it is being transported towards the apical membrane it undergoes a process which is called as oxidation now this oxidation is occurring due to presence of a very important enzyme in the apical membrane that is called as peroxidase and as it undergoes oxidation it is also transported out of the cell into the Lumen of the follicle by using one more very important transport protein which is present in the apical membrane of the follicular epithelial cell and that is called as pendrin so the third step is oxidation of the iodide and transportation of the oxidized iodide from the follicular epithelial cell into the follicular Lumen so once the iodide is oxidized and it is transported now it is called as iodine okay so now iodine is also lying in the Lumen and thyroglobulin is also lying in the Lumen now what is going to happen is that this iodine is going to combine with the tyrosine amino acid residues which were present in the thyroglobulin even for this process I require the presence of the enzyme peroxidase so once iodine combines with tyrosine it results in the formation of two compounds one is called as mono iodo tyrosine another one is called as Di iodo tyrosine and this process is called as iodination of the tyrosine or it is also called as organification of the thyroglobulin now once this iodination has occurred and there is formation of mono iodo tyrosine and diadotyrosine now what is going to happen is One Mono iodo tyrosine is going to couple with one diodo tyrosine and this is going to result in the formation of what is called as triido tyrosine which is nothing but our T3 similarly one di iodo tyrosine is going to combine with one more diodo tyrosine and this is going to result in the formation of what is called as tetraiodotyrosine or this will be also called as T4 or thyroxine and for this reaction also I require peroxidase and this reaction is called as coupling or this step is called as coupling so once coupling has occurred now there is formation of T3 there is formation of T4 there is also formation of one more hormone which is called as reverse T3 and there is MIT and dit all these are associated with thyroglobulin molecule so this is what is called as Collide now if the body requires T3 and T4 to be secreted now this Collide which is nothing but thyroglobulin along with all these things these are going to undergo a process of pinocytosis and a colloidal recycle is going to be formed in the apical membrane and this will be now called as a colloid droplet now this Collide droplet is going to combine with lysosomes which are present inside the cell now these lysosomes are going to secrete certain enzymes like proteases now what is the function of proteases here is that these proteases help in cleaving out T3 and T4 from the thyroglobulin so once T3 and T4 are cleaved out of the thyroglobulin this T3 and T4 are transported across the basol lateral membrane and they enter into the circulation of course along with little bit amounts of reverse T3 also so these proteases they not only help in cleaning T3 and T4 out they also cleave out mono iodo tyrosine and Di iodo tyrosine from the thyroglobulin now what is going to happen to mono iodo and diadotyrosine is they are going to undergo a process which is called as de-iodination now this is going to occur with the help of an enzyme which is called as D iodinase so now what is going to happen here is that this MIT and dit will be again broken down into tyrosine and iodide now this iodide which is coming out of MIT and dit because of the deionation it can be again reutilized okay reutilized for the synthesis of T3 and T4 so with the help of one single diagram I can explain all the steps now to enumerate all the steps or to summarize the entire thing what we have learned I am just summarizing the steps here the first step was iodide trapping when an iodide from the circulation is entering into the follicular epithelial cell this is occurring via what is called as sodium iodide's importer and remember it's an active transport next step is synthesis of the thyroglobulin and transportation of the thyroglobulin from the cell to into the Lumen next is these two steps one is called as oxidation of iodide this is occurring because of peroxidase and transportation of oxidized iodide into the Lumen this is occurring because of pen drone which is present in the apical membrane of the epithelial cell once the iodine and the thyroglobulin are in the Lumen there is going to be iodination of the tyrosine or also called as organification of thyroglobulin this results in formation of MIT and dit next is MIT combines with did and did combines with dit this is called as the coupling reaction after that these hormones along with the thyroglobulin can be stored and remember they can be stored for a period of 2 to 3 months in the colloid of the follicular Lumen and then when and when the body requires at that point of time they can be also secreted and this occurs because of the pinocytosis and fusion of this colloidal droplet with the lysosomes and then secretion of certain enzymes which are going to cleave out P3 T4 and rt3 and they are going to be coming out of the basal lateral membrane and secreted into the circulation and even MIT and dit are also cleaved out and they undergo the process of de-iodination and they because of dehydination iodide is released again and it can be reutilized for the synthesis of T3 and T4 and if you write all these three all these sorry not three points nine points in your examination I bet you will not lose a single Mark now if you have understood the concept behind the synthesis of the thyroid hormones please do hit the like button subscribe to my channel and share this video as much as possible thank you