now when we're considering the endocrine system we think need to think about negative feedback because homeostasis is absolutely vital we need just the right amounts of the endocrine hormones present in the blood because they're very physiologically active so how is this achieved well we can start off at the top here with the hypo alus at the base of the brain then the next level of control down is the pituitary gland the pituitary gland and we're thinking about the anterior lobe today the anterior lobe of the Petri gland which is the glandular part that's sometimes called the Adeno hypo fisis because it's glandular and if we start off with thinking about the example of the thyroid gland so there we've actually got three anatomical structures we've got the hypothalamus which is an anatomical structure we've got the anterior lobe of the pituitary gland which is a definite anatomical structure and we've got the thyroid gland which is a anatomical structure in the neck as you know now if we think about thyroid hormone essential to regulate the amount of metabolism going on in the body essential during growth and development for normal growth of the body essential for normal development of the nervous system but we don't want too much because we don't want it to overstimulate metabolic processes so when the levels of thyroid hormone are low the hypothalamus will start things off and it will produce a releasing hormone called thyrotropin releasing hormone or releasing Factor thyrotropin releasing factor and this will stimulate the pituitary gland and when the pituitary gland is so stimulated the pituitary gland is going to release TSH thyroid stimulating hormone and the thyroid stimulating hormone will stimulate the thyroid gland and as a result the thyroid gland will produce more thyroid hormone and that comes in the form of T3 triiodo iine and T4 the difference is the amount of iodine that's got three atoms of iodine that's got four atoms of iodine so I think you can see now we've gone from the hypothalamus releasing the thyrotropin releasing hormon releasing Factor down to the pit gland which is stimulated to release the thyroid stimulating hormone that goes in the blood down to the thyroid gland to increase the amount of thyroid hormone so now we've got what we wanted we've got more thyroid hormone in the blood but of course we don't want too much thyroid hormone we don't want this process to keep going on so what happens is the anterior lobe of the pituitary gland and the hypothalamus are both sensitive to the levels of thyroid hormone in the blood so as the levels of thyroid hormone in the blood crease that will be detected by the anterior Lo of the pituitary gland and perhaps more importantly it will also be detected by the hypothalamus because we now have increasing levels of thyroid hormone and the increasing levels of thyroid hormone will inhibit the release of the thyrotropin releasing hormone from the hypothalamus so let's say that again that's key the levels of thyroid hormone will inhibit the release of thyrotropin releasing factor or releasing hormone from the hypothalamus so now we will have less thyrotropin releasing hormone we'll have less of that less TR so now going to be less TR thyro tropen releasing hormone if there's less thyrotropin releasing hormone there's going to be less stimulation of the anterior lobe of the pituitary gland so there's going to be less thyroid stimulating hormone if there's less thyroid stimulating hormone there's going to be less stimulation of the thyroid gland if there's less stimulation of a thyroid gland there's going to be less thyroid hormone produced by the thyroid gland therefore the levels in the blood will start to drop as the levels in the blood start to drop that's going to remove the inhibitory effect on the hypothalamus which will therefore produce more thyro tropen releasing hormone and that will go down that stimulating pathway so this is all called a negative feedback system because the end product in this case the thyroid hormone is inhibiting the release of its own releasing factors so this is all a negative feedback NE negative feedback system it's inhibiting the release of its own releasing factors and the situation is the same with other some other endocrine glands for example if this was the um pituitary gland it could release um act adrenocorticotrophic hormone so if we thinking that this was the um the system controlling cortisol so there we would have the hypothalamus and the hypothalamus would be releasing corticotropin releasing hormone croen releasing hormone or releasing factor that would then go down to the pituitary gland and the pituitary gland would produce more act more act that is adreno corticotrophic adrenocorticotrophic hormone adrenocorticotropic hormone because that is stimulating the adrenal cortex to produce more cortisol so it's the same sort of thing and of course the cortisol would then inhibit the release of its own releasing factors in a negative way in just the same way that the thyroid hormone would inhibit the release of its own releasing factors so for several of the endocrine products here we've looked at cortisol as an endocrine product in the blood which of course needs to be very finely regulated and if there's too much of that we have a special name for that that's um Cushing's Disease Cushing's Disease would be uh too much hyper cortisol ISM and if there's not enough that's got a special name as well that's Addison's disease and it's it's the same with the thyroid hormone that the regulation is so critical that we have diseases such as thyroid toxicosis or hypothyroidism that result from increased or decreased amounts but that's always the physiological situation the end product is inhibiting release of its own releasing factors via this negative feedback process