vascular giving it a reddish color blood reaches the thyroid via superior and inferior thyroid arteries superior middle and inferior thyroid veins carry thyroid hormones from the gland beneath the fibrous capsule the thyroid gland is composed of spherical structures called thyroid follicles the follicle wall is a simple cuboidal epithelium of follicular cells that surround a central human the follicle human is filled with a viscous colloid that contains a glycoprotein called viral globulin capillaries and parafollicular cells are located in the loose connective tissue between thyroid follicles parafollicular cells also called c cells secrete the hormone calcitonin which is involved in regulating calcium levels in body fluids calcitonin is the antagonist to parathyroid hormone thyroid hormone or th is a general term that encompasses two specific hormones try iodophyrene known as t3 and tetraiodothyronine known as t4 or thyroxine thyroid stimulating hormone or tsh from the anterior pituitary maintains th synthesis and secretion in the first phase of th production follicular cells secrete thyroglobulin which contains numerous tyrosine amino acids into the follicle lumen iodide molecules i negative from the blood cross the follicular cell and enter the lumen where they are converted to iodine i the tyrosine molecules within the thyroglobulin combined with iodine to form iodinated thyroglobulin which can be stored in the follicle lumen for two to three months in the second phase follicular cells absorb iodinated thyroglobulin by endocytosis and lysosomal enzymes hydrolyze the thyroglobulin to produce t3 and t4 which are secreted into the bloodstream the primary effect of th is to maintain normal cellular metabolic activity in children th is especially important for the growth and development of many tissues through stimulation of the pituitary gland to release growth hormone excess production of th hyperthyroidism causes increased metabolic rate which can lead to weight loss hyperactivity heat intolerance and goiter or enlargement of the thyroid insufficient th production hypothyroidism results in low metabolic rate and resultant weight gain sluggishness and sensitivity to cold temperatures um just a little thing about hyper and hypothyroidism again this is this too much production or under production of t2 t4 right thyroid hormones metabolism acutely this is not going to kill you but chronically it can't kill you so you got gotta maintain those normal levels of t3 t4 also called thyroid hormones right so you want to maintain this again acutely could be serious but chronically can be deadly uh if you don't if you don't control what's causing the over production of the production of t3 t4 another thing it could be a problem with the thyroid but it could also be a problem with the anterior pituitary because it might be secreting too much or too little tsh or thyroid stimulating hormone so it could be a pituitary gland issue or it could be a thyroid gland issue that's causing overproduction or under production of t3 t4 also called thyroid hormones just so you could be aware of that so again we'll first talk about t3 t4 and how that's made just like the video showed you and then we'll focus on calcitonin uh remember t3 is thyroxine t excuse me t4 thyroxine and then we have t3 uh member thyroxine is also given as a medication so people are hypothyroidism uh you know there is a medication called thyroxine which is a synthetic t4 now the thyroid gland just like the video talked about c-shaped raptor on the trachea inferior to the larynx see the model nicely here there's the larynx there's a trachea and you can see it's wrapped around there it's highly vascularized like all other glands and organs and here's a cadaver you can see a little there and there it's connected by something called the isthmus but i'm not going to stress that just appreciate that c-shape gland wrapped around the trachea um inferior to the larynx so here's your larynx there's a trachea you can see part of it is wrapped around it right it goes around like and wraps around it here it's missing on the other side but it doesn't touch right it wouldn't touch it leaves it the c-shaped grain and there's your little and then those little three dots one two three is representing your parathyroid glands right one two three four sometimes there's three sometimes two you can see parathyroid glands are inferior or posterior to the thyroid one two three this is your thyroid gland and those little dots are your parathyroid glands again the other side is missing it's symmetrical and it doesn't go around completely posteriorly so let's talk about the thyroid follicles these round circular structures right so we see look at the histology this is coming from the thyroid you should be able to recognize that another thing you should be able to recognize is that these round structures are called thyroid follicles and these thyroid follicles are composed or lined by epithelial cells which are cuboidal called follicular cells these are called follicular cells and then these follicular cells are making something called colloid or which is also called thyroid globulin so colloid another name is thyroglobulin which is a protein being secreted by these follicular cells in response to tsh thyroid stimulating hormone to make these colloid and this chyloart collar which is called thyroglobulin this pink stuff is stored within this lumen of the follicle so these follicular cells are being are the ones responsible for making the colloid but also the production of t3 t4 from that colloid right and they're influenced by tsh which is thyroid stimulating hormone where is that coming from well it's coming from the anterior pituitary that's what they're showing you here right there's your tsh and this tsh current anterior is going to stimulate the thyroid but you got to be specific right stimulating this thyroid follicular cells that's what they're stimulating and they're the ones busy making the t3t4 with the introduction i've done of course but again the process you got to make thyroid globulin first uh and then what happens as these follicular cells make the thyroid globulin uh with the introduction iodine which is coming from the blood just like the video showed you it comes from our diet we need iodine because if you're iodine deficient you can get hypothyroidism so you need iodine to make t3 t4 iodide is component of those hormones and these follicular cells will extract that iodide from the blood and then make t3 t4 again they initially make the colloid stored that in the lumen and then where they're making t2bt4 they take some of that colloid and they change it to t3 t4 with their introduction to iodine which we saw in the video so that's what we see here right it's called uh t4 is called thyroxine and t3 is called triado thyronine because it has a component of iodine or the ioda ions are found there it's important where did they get the iodine from they got it from the blood where did we get it from when we get it from our diet and again thyroglobulin another name is uh colloid it is a globular protein and it's secreted by the follicular cells and they need that to make to be t3t4 so again here we see nice thyroid follicles we see colloid also called thyroid globulin and then we see these follicular cells which are simple cuboidal epithelial cells lining the thyroid follicle this is probably the best one to show you that like you really appreciate those follicular cells being simple cuboidal now the parafollicular cells also called clear cells which you know they're pointing to something here but they're found in between the thyroid follicles and parafollicular cells if you look here these are the ones making calcitonin the hormone that helps regulate calcium levels of blood so parafollicular c cells are found between the thyroid follicles now i don't know what they're pointing at but it would be found somewhere in there right i guess that's what they're trying to tell you between the thyroid follicles it's called parafolliculus lc cells they're making the calcitonin which we'll be talking about now this one is not so great this histology why because yeah you can see the thyroid follicles you can see the the the the colloid but these simple cuboidal epithelial cells kind of looks squamous to me and some this is a very kind of low magnification this was a better one uh right what is number one well let's parapholical what's number four look at himself what is number three well the trinitalia is parafollicular c cell what is number two well that's cholera or thyroid globulin and we know that these thyroid hormones which are talking about t3 t4 are responsible for metabolic functions uh such as uh you know elevated rates of oxygen consumption energy consumption increased heart rate from a force of contraction increased sensitivity to sympathetic stimulation uh you know these are effects on peripheral tissues uh you know heat intolerance stuff like that so definitely uh be aware of at least that they're recall thyroid hormones t3 t4 make sure you know the cells that make them and the process of how they get about made with the introduction iodine and of course with the formation of colloid before that and then also be aware that these hormones uh t3 t4 are under the influence of the anterior pituitary tra or tsh right now tsh is thyroid stimulating hormone antibiotic made in antipituitary who regulates tsh well that's a hypothalamus is a thyrotropin-releasing hormone that's a regulatory hormone that regulates a tsa secretion tsh gets created into the blood targets the thyroid what in the thyroid targets the follicular cells right these number four targets the follicular cells and then you get a production of t3t4 which goes into the blood right always remember if you get hypo hypothyroidism it could be a thyroid issue or it could be a pituitary gland issue or even a hypothalamic regulatory issue but the point is again you got to find the source of what's causing under under or over production of t2 t4 it's primary secondary cause primary supplement and there's a good thyroid hormone video you can watch on your own since i already showed you one i'll watch this one on your own and now we'll talk about calcitonin so remember calcitonin let's look at the chart we'll come back to that so calcitonin comes from the c cells those parafollicular cells remember they're called parafollicular c cells two names they have and they target the bones kidneys and the small intestine so that's missing you can see small intestine two targets so c cells parafollicular cells and hormonal effect decreases calcium concentrations in the body in the flu in the body fluids thus the blood and what stimulates its secretion is high levels of calcium in the blood now these c cells or parafollicular cells are basically coming from the thyroid gland between the thyroid follicles that's where they're found and if a patient is hypercalcemic meaning they have too much levels of calcium blood calcium is going to get secreted and it's going to help lower the levels of calcium now you can see that we normally have normal levels of ions in our blood calcium is extremely important it's one of those important ions that we've got to have normal levels of because if you have too much of it or too little of it it could hurt you it could kill you so we want to maintain normal levels so here in this case it's greater than the normal level you don't need to memorize a normal level but again if it's hypercalcemic if you're above the normal levels calcitonin is going to get secreted from the c cells the parafollicular cells and it's going to target the kidneys the small intestine and the bone cells what is the goal of calcitonin well if it's coming from if it's being secreted it's because you're hypocalcemic so the goal will be to decrease calcium levels how does it do that well the mechanism so here's the three targets and the mechanism is to lower levels of calcium well in the kidneys it excretes calcium into the urine the small intestine decreases absorption calcium from the urine those two are the major methods now your skeleton remember it's a reservoir for calcium you need calcium to make strong bones hydroxyapatite the mineral crystal but remember your skeleton is a reservoir for it so when you need some calcium you can extract it from it or sometimes you can put it back if there's too much of it the major ways of or lowering levels of course is going to be through the kidney and through the small intestine but appreciate that it also stimulates osteoblasts and osteoclasts right so it would actually inhibit osteoblastic activity and stimulate osteoclastic excuse me inhibit osteoblastic activity sorry the osteoclasts are going to be inhibited and osteoblasts continue to basically lock calcium ions into the bone matrix which makes sense right i apologize so remember osteoblasts what do they do they take calcium and they turn it into bone right versus osteoclasts dissolve bone so it makes sense is that if you're hypercalcemic you want to lower levels of calcium you want to inhibit osteoclasts because you don't want to dissolve or release more calcium into the blood you'll want to stimulate osteoblasts to take the calcium and put it back into the bone so but now casatonin has a hormone that counteracts that effect it's called an antagonistic hormone so if a hormone if there's two hormones and one raises it though it lowers the levels of that particular thing that it's doing it's called antagonistic hormones so the antagonistic hormone for calcitonin is called the parathyroid hormone pth parathyroid hormone and parathyroid hormone comes from the parathyroid glands let me show you that so again these are the four parathyroid glands sometimes there's six in terms of total the little dots that i showed you before in the model uh these are your power they secrete parathyroid hormone which is pth parathyroid gland and the name of the cells that make it is called the chief cells you gotta know the name of the cells chief cells and they make pth parathyroid hormone parathyroid hormone also targets three things the same three things of calcitonin targets so it targets the kidneys the bone cells and the small intestine again the table left out small intestine and what is the purpose of calcium of pth to increase calcium levels when they're when you're low when you have low levels of calcium in the blood so if you're hypocalcemic you want to raise the levels of calcium and pth helps you do that with the help of a hormone that's called calcitriol which comes from the kidneys so it gets help from another hormone so we'll i'll talk about that so again the parathyroid glands the name of the cell is called chief cells and they secrete pth parathyroid hormone in response to being hypocalcemic and they help raise the levels of calcium in the blood how do they do that well this they target the same three targets as calcitonin remember calcium tolerance role was coming from the thyroid was to lower the levels here pth coming from the parathyroid glands is to raise levels so if your hypocalcemic pth gets secreted and you raise levels how does it do it same three targets opposite mechanism kidneys it retains calcium it doesn't treat it into urine small intestine increases absorption calcium from small intestine as you eat foods you absorb more calcium and stimulates osteoclastic activity kind of inhibiting some osteoblastic activity but if you stimulate osteoclastic activity you dissolve bone releasing calcium to the bloodstream again your skeleton is a reservoir for calcium when the body needs it the major way it lowers it drastically is through the kidneys in the small intestine of course excuse me raises it because by doing that you were conserving calcium at raising levels uh let me show you a quick video on pth that i have the small parathyroid glands are located on the posterior surface of the thyroid gland there are usually four parathyroid glands although the number varies between two and six they are supplied by the inferior thyroid arteries parathyroid glands are composed of two cell types chief cells or principal cells which synthesize parathyroid hormone and oxal cells whose function is unknown parathyroid hormone or pgh is secreted in response to low blood calcium levels pth raises calcium levels in several ways it promotes resorption of calcium from bone tissue so again here you go activity it increases resorption of calcium in the kidneys should say reabsorption of calcium so you don't excrete the intake or you're reabsorbing so it is not excreted in urine and it stimulates synthesis in the kidney of the hormone calcitriol the active form of vitamin d which increases calcium absorption from the small intestine excess pth secretion hyperparathyroidism may lead to bone fractures due to reduced calcium kidney stones psychological changes such as depression muscular weakness or even cardiac arrest if blood calcium levels are too high inadvertent removal of the parathyroid glands during surgery of the thyroid gland is the most common cause of hypoparathyroidism the lack of pth causes low blood calcium levels which can lead to neuromuscular symptoms such as tingling in the fingers and limbs muscle cramps cardiac arrhythmia and tetany which may result in death by asphyxiation so again i'm not going to ask you about hypo hyper and hypoparathyroidism or its causes but or its symptoms you can appreciate that if you have too much levels of calcium in the blood or too little you know that could kill you you know from hyperparathyroidism you could have basically you know to you know too little right so you're gonna get cardiac arrest muscular weakness depression kidney stones bone fractures um first if you're hypoparathyroidism you could have uh tingling in the fingers limbs muscle cramps cardiac arrhythmias technique you know that cardiac arrhythmia could kill you cardiac arrest could kill you so unlike in hypo and hypothyroidism which had to do with metabolism hormones t3 t4 you know acutely we said before that that's not going to kill you only long term this could kill you acutely right this could be deadly if you mess with those calcium levels so you want to make sure you correct this quickly because the last thing you need is cardiac arrest for a cardiac arrhythmia to develop due to the high levels of low levels of calcium that's being in your blood so we know where they need calcium for proper bone formation to make strong bones but that's not the only reason right we need calcium for your brain to work to release neurotransmitters we need calcium from your heart to work contraction relaxation of the cardiac muscle or even skeletal muscle we know that we need a calcium for many things it's one of the most important ions in our body as well now let's go back to the lecture so let's recap on pth and parathyroid again here's pth secreted by the parathyroid gland and here's calcitonin from the thyroid remember calcitonin if you're hypercalcemic you will secrete calcitonin targets these three lowers levels calcium pth if you're hypocalcemic it raises levels of calcium thus you if you're hypocalcemic same thing here they're trying to show you the same thing right here's the normal levels and again you want to maintain the normal levels of calcium in the blood we already know why and these two hormones help you do that now pth gets help from another hormone because if you look here it says pth with calcitriol and you can see right there and what are they talking about well what is calcitriol so before i explain what cancer trial is it's going to come out we'll do the kidney again but vitamin d3 is called cold calciferol vitamin d3 when exposed when sunlight hits those keratinocytes in the skin you make you change cholesterol cold calciferol and then it goes into our blood we need to be in the sun for at least you know a while which would try not to be because of the radiation the harm of it but you know you could get some some vitamin d uh synthesis from being out in the sun most of us get it from our diet right from absorption to foods we eat whether it's supplements foods whatever now go to the next slide once vitamin d goes into the blood now we need vitamin d3 for everything we need it for our brain for proper immune system development a lot of things this is just one reason we need vitamin d3 so once vitamin d3 goes into the blood it goes to the liver gets converted to calcium dial that goes to the kidney and the kidney makes a hormone called calcitriol casa trio or sometimes say because trio constant trial is a hormone made by the kidneys called the active form of vitamin d3 and you can see that pth influence its production as long as you have vitamin d3 in the blood and what does calcitriol do well it helps raise the level of calcium in the blood it increases absorption calcium from the small intestine that's why if you look here that's why they mention it there right with calcitriol because pth also promotes its synthesis right into production but without vitamin d3 in the diet you're not going to get this hormone called calcitriol also called the active form of vitamin d3 that's where you look here says pth are stimulated by low levels of calcium pth effects enhanced by calcitriol and opposed by calcitonin which is the antagonist over there and again if you have thyroid gland diseases again this is just going back to hyper and hyperthyroidism this is some pathology here many it could be a thyroid issue because there are many causes for that could be a pituitary gland issue in regards like tsh for example hyperthyroidism you know here's an example of a hyperthyroidism this is just one graves diseases autoimmune disease of the thyroid and some skeletal muscles a face in the eye and uh again you know they get a cynical exothalamus bulging of the eyes and you get a goiter which you can see here you can see these these eyeballs are mulching out it's called exothalamus because of the muscles that are being inflamed because of the autoimmune disease that your immune system is attacking part of it now again just if you don't know what autoimmune disease is autoimmune diseases uh your immune system attacking a part of you you can have some genetic predisposition but also environmental triggers that could trigger it uh from graze disease to multiple sclerosis to you know there's so many crohn's disease this is just one example of uh and there's many reasons to get hyperthyroidism right you can get a goiter exothalase is more with goiter when you have uh graves disease hypothyroidism there's many reasons for it as well uh you know you get more of an atrophy there are some cases you get a quarter but more like an atrophy thyroid gland all right and that's the end of this lecture uh and then we'll move on to part two of the endocrine system with the second learning objective