the endocrine system uses some complex negative feedback loops to regulate the hormones in our body in this video we're going to look at a particular set of hormones involved in the hypothalamus pituitary thyroid axis or hpt axis for short we're going to talk about two conditions hyperthyroidism which could make you jittery and kind of shaky and hypothyroidism which could cause somebody to have low energy become sort of lethargic maybe put on weight but both of those conditions hyper and hypothyroidism are caused by a disorder of the endocrine system in particular the hpt axis we've got a lot to cover so let's jump to the white board first thing like i said there's three organs involved here two in the brain and one in the neck we have the hypothalamus which is going to be kind of the controller here the pituitary just below that that will receive instructions from the hypothalamus and finally we have the thyroid gland which will release a hormone that's going to regulate our metabolism starting with the hypothalamus the way that i think about this is as if the hypothalamus is sort of normally on the on setting if nothing else changes it's going to be releasing its hormone and the hormone that the hypothalamus releases in this axis is called thyrotropin releasing hormone or trh for short like i said the hypothalamus is sort of on and it's going to release its hormone unless it's told not to by something else so it's going to release trh or thyrotropin releasing hormone and that hormone is going to move from the thalamus down to the pituitary gland and it's going to be excitatory meaning that it's going to tell the pituitary gland to release its own hormone trh has an excitatory or stimulatory effect on the pituitary gland in the diagram i represent that with the circle and the plus sign in the middle trh is basically telling the pituitary gland yeah go ahead do the thing that you're thinking about doing release your hormone the trh will bind to the receptors and stimulate the pituitary gland to release its own hormone now the pituitary gland once it receives this input from trh it's going to release its own hormone which is called tsh or thyroid stimulating hormone now i love the name of this hormone because it tells you exactly what it does thyroid stimulating hormone get this stimulates the thyroid pretty self-explanatory thyroid stimulating hormone is a thyrotropin let's break down that term thyro refers to the thyroid gland and tropin just means it targets so a thyrotropin is a hormone that targets the thyroid gland tsh is a thyrotropin because it targets the thyroid gland so why is trh a thyrotropin releasing hormone if it doesn't target the thyroid gland right trh targets the pituitary well we call it thyrotropin releasing hormone because it causes the release of a thyrotropin or in other words it causes the pituitary to release tsh therefore trh is a thyrotropin releasing hormone in the diagram i represent tsh with green triangles tsh which is released by the pituitary gland will travel through the bloodstream until it reaches the thyroid gland here in the neck again this is a stimulating or excitatory effect which is going to cause the thyroid gland to produce its own hormone it also generally causes the thyroid gland to grow or increase in size over time a lack of thyroid stimulating hormone would cause the thyroid gland to get smaller and diminish over time and too much tsh would cause the thyroid gland to start to grow larger over time now the thyroid gland is going to release two hormones they have long names triodo thyronine and thyroxine but in this video we're going to refer to them as t3 and t4 which is a lot quicker to write and to say t3 and t4 have very similar functions so for the rest of this feedback loop i'm going to treat them as if they're the same thing so t3 and t4 are hormones that regulate your metabolism let's define metabolism real quick metabolism is somewhat complicated but we can simplify this to say that metabolism is all of the chemical reactions in the body that convert food into energy t3 and t4 are going to travel through the bloodstream and stimulate lots of cells throughout the body to increase the metabolism or the breakdown of food into usable energy throughout the body if we have too much t3 and t4 that will lead to hyperthyroidism or an overactive metabolism where we're breaking down food into usable energy too quickly that excess energy could cause our heart rate to beat really fast it could cause our hands to be jittery it could cause our mind to be sort of overactive those all being common symptoms of hyperthyroidism if our t3 and t4 levels are too low it causes the opposite hypothyroidism that would be an underactive metabolism it could cause weight gain because our body is going to stop breaking down as much food and usable energy and it could cause a low energy state or sort of lethargy where we don't really have the energy to do much hypo and hyperthyroidism can both be debilitating diseases even though they have opposite effects so t3 and t4 will travel throughout the body and stimulate our metabolism but how does our body regulate this what prevents our metabolism from being too high earlier i said we'll treat the hypothalamus as if it's sort of always on well if it's always releasing trh making the pituitary gland always release tsh and therefore making the thyroid gland always release t3 and t4 why do we all not have hyperactive metabolisms well t3 and t4 has another function which is to inhibit the previous glands so the t3 and t4 as it travels throughout the bloodstream will also interact with receptors on the hypothalamus and the pituitary gland the more t3 and t4 we have the more it will inhibit or slow down the hypothalamus and the pituitary that'll cause the hypothalamus to release less trh and the pituitary to release less tsh bringing the trh tsh and t3 and t4 levels back down to the set point where they need to be and therefore bringing our metabolism back into our normal range so let's run back through this a couple times the hypothalamus and its normal on state will release trh the trh will bind with receptors in the pituitary to stimulate it to make and release tsh now tsh levels are kind of high and they're going to travel down to the thyroid gland and make the thyroid gland release t3 and t4 as t3 and t4 levels get a little bit too high they're going to inhibit the other glands by traveling to the inhibitory receptors in the hypothalamus that's going to slow the hypothalamus down and cause it to release less trh with less trh there's nothing to tell the pituitary gland to make tsh so our tsh levels will decrease and as our tsh levels decrease that causes t3 and t4 production to decrease as well but what if the t3 and t4 gets too low well if there's less t3 and t4 to inhibit the pituitary gland and hypothalamus well the hypothalamus will revert back to its own state and start making more trh and we're kind of back to where we started if we have more trh that'll cause more tsh and that'll cause more t3 and t4 and so we get this dynamic equilibrium where our t3 and t4 levels maybe go up for a little bit and then they go back down and then back up and back down but they're going to stay about at that set point or the normal range all right to recap let's run back through this one more time the hypothalamus in the brain releases thyrotropin-releasing hormone which will travel down and bind with receptors on the pituitary gland to stimulate it this causes the pituitary gland to release tsh or thyroid stimulating hormone thyroid stimulating hormone will travel through the bloodstream down to the thyroid gland in the neck and bind with receptors there causing the thyroid gland to grow a little bit over time but also telling the thyroid gland to release t3 and t4 to increase our metabolism t3 and t4 will travel throughout the whole body and tell ourselves to increase our metabolism but t3 and t4 also will travel back to the hypothalamus and the pituitary gland and bind with receptors there which will inhibit or slow down the hypothalamus and the pituitary gland this will bring trh and tsh and therefore t3 and t4 levels back down to a level they need to be at this whole process of negative feedback is how we regulate our metabolism make sure it doesn't get too high or too low now i want to take a look at two things that could happen that could cause hyper and hypothyroidism one is a tumor on the pituitary or the thyroid gland and another is an autoimmune disease that could either destroy or block receptors on the thyroid gland or the pituitary to see how that all works click on the video link right here where we'll look at some of those endocrine disorders of the hpt axis do you point to your thyroid gland where's your thyroid gland no that's not where it is yeah there's where my thyroid gland is where's your thyroid gland can you wave high wave high