in this video I'm going to talk about thyroid hormone feedback so let's look at the mechanism by which thyroid hormone is released the hypothalamus releases TR on to the pituitary thyrotrope cells if you don't know what these words mean have a look at the thyroid hormone control video the thyri cells then release TSH into the bloodstream and then it diffuses out of the bloodstream into the thyroid gland the thyroid gland is then induced to release thyroxine which is T4 it also releases T3 but that's not important here now T4 enters the bloodstream and feeds back onto the pituitary and onto the hypothalamus and it reduces the release of TSH and TR respectively now this has important implications for the diagnosis of thyroid problems as it's possible to diagnose the location of a problem depending on the blood levels of thyroxine and TSH so I'll draw up a table with each of the possible situations so if thyroxine is high this is what we call hyperthyroidism and if thyroxine is low this is what we call hypothyroidism if thyroxine is high and TSH is high because we know that thyroxine should be suppressing the release of TSH then there must be a problem in the pituitary which means that this feedback mechanism doesn't work therefore the problem must be in the pituitary if thyroxine is low but TSH is high then there must be a problem in the thyroid which means that it's not responding to the high levels of TSH if TSH is low and thyroxine is high then it must be also a thyroid problem because it's not responding properly to the TSH and it's still releasing high levels of thyroid hormone and if thyroxine is low and TSH is low because TSH should be high when thyroxine is low then the problem must be in the pituitary as the pituitary is not responding to the feedback action of thyroxine and that's an overview of thyroid hormone feedback and how it can be used in clinical diagnosis