Hypothalamic–Pituitary–Thyroid Axis

00:00 Thyroid hormones.

00:02 Tyroxine or T4.

00:05 And triiodothyronine which is T3.

00:10 When we want to think about the hypothalamic pituitary thyroid axis we need to start in hypothalamus.

00:20 In the hypothalamus we have two structures that we need to take into account.

00:24 The paraventicular nucleus here and arcuate nucleus which is seen below.

00:34 These are the places in which we are going to be able to secrete the substances the thyrotrope.

00:42 So paraventicular nucleus secretes somatostatin and arcuate nucleus secretes thyroid releasing hormone.

00:50 These are going to then be placed within the circulation of the anterior pituitary.

00:56 And then the thyrotropes are what are going to respond to either somatostatin or TRH.

01:01 So if we start off with the hypothalamus, we have TRH.

01:07 That is going to be released to stimulate pituitary thyrotropes.

01:13 But remember you have somatostatin which acts as an inhibitor to those very same thyrotropes.

01:19 So if there is enough TRH around and you're able to allow those thyrotropes to release their hormone, which is thyroid stimulating hormone, that will then engage the thyroid gland.

01:33 The thyroid gland will then produce T3 and T4.

01:37 There is negative feedback with T3 and T4.

01:40 The first portion of the negative feedback, is that it inhibits both the anterior pituitary thyrotropes to release thyroid stimulating hormone.

01:52 And it also inhibits the stimulatory nuclei.

01:56 And what this means is that even though thyroid releasing hormone normally is released to stimulate thyrotropes, you can inhibit that nuclei so it doesn't release thyroid releasing hormone.

02:12 Now in terms of the feedback control, you also have one more arm and that arm positively stimulates the hypothalamic nuclei that inhibits the anterior pituitary thyrotropes.

02:25 So what you do is you stimulate the inhibitor or somatostatin to be released.

02:33 This acts as a nice negative loop on multiple levels.

02:38 Both along the hypothalamic and the pituitary level as well as effecting both the inhibitor and the stimulator.

02:48 There are few other things we need to discuss with T3 and T4.

02:52 So the most important one is, is that only the free T3 or T4 participate in this negative feedback.

03:01 What do I mean by that? The majority of T3, T4 is bound up with proteins.

03:08 In fact about 99% of it is bound up.

03:13 So you only have about 1% of your T3, T4 that is actually circulating in the free form.

03:20 So you always have to know where you're at in terms of where that 1% is.

03:27 If you have a reduction in the amount of binding proteins, you may look like you have an elevation in your T3, T4, but you're not producing it.

03:36 It's simply not being bound as well anymore.

03:42 So where does thyroid hormone come from.

03:44 It comes from the thyroid gland.

03:47 The thyroid gland if we looked at the histology of it, is broken out with many of the cells wrapped around what we call a colloid.

03:55 These follicular cells are going to be the ones that are going to be actively producing T3 and T4.

04:03 There are lot of blood vessels in the area.

04:05 And the blood vessels will do a couple of things.

04:09 One is it is going to deliver substrate that is needed for thyroid hormone synthesis such as iodine.

04:18 It's also going to be removing the T3 and T4, so that it can then circulate throughout the body and do it's target end organ functions.