Primary vs. Secondary Hypothyroidism (Nursing)

00:01 The feedback loop plays a really important role in primary versus secondary hypothyroidism. The HPT axis.

00:09 Right. That's the hypothalamus the pituitary thyroid axis and the hormone cascade that comes afterwards obviously is a key player in your endocrine system because that hypothalamus releases TRH, which targets the anterior pituitary and causes the anterior pituitary to release TSH, which targets the thyroid gland and causes the thyroid gland to release the thyroid hormones T3 and T4.

00:34 So now let's look at what happens with the feedback loop in both primary and secondary hypothyroidism.

00:42 Now, to help you understand this with the feedback loop, we're going to start with primary hypothyroidism.

00:47 So there's decreased T3 and T4 production by the thyroid gland in primary hypothyroidism.

00:54 Remember that's because the thyroid gland has likely been attacked by some type of autoimmune disorder. So it can't respond to that thyroid stimulating hormone. Now the hypothalamus says, hey, look, we don't have adequate levels of T3 and T4, so it's going to rev up the engine and increase the production of TRH.

01:14 Okay. Well the TRH production goes up.

01:17 That means the anterior pituitary gland is even more stimulated and it increases TSH. But again, the thyroid gland can't respond.

01:26 And that's why the TSH elevation is compensatory.

01:30 But it doesn't work. It's ineffective because the thyroid has been damaged and cannot respond. So that's what happens to the feedback loop when the thyroid can't respond. The hypothalamus and the anterior pituitary are doing their jobs and they're trying to make things happen.

01:45 But the thyroid just can't respond to the TSH.

01:49 Now let's look at this same feedback loop and how it works in secondary hypothyroidism. Now remember in secondary hypothyroidism this isn't an autoimmune disorder that's taken out the thyroid.

02:00 This is going to be a problem a little bit higher up in that axis chain.

02:04 This is going to mean there's decreased TSH production by the pituitary, or there's less TRH stimulation from the hypothalamus.

02:12 So this is a problem somewhere in the hypothalamus or in the pituitary gland. Now that means that the thyroid gland which may be okay it's not being stimulated. So that's the problem in primary means it's getting stimulated the heck out of it and it can't respond.

02:28 But in secondary hypothyroidism the problem is the thyroid gland isn't being stimulated.

02:35 There's a problem with the pituitary gland or with the hypothalamus.

02:39 So there's no compensatory TSH elevation because that's our problem.

02:44 So this ends up with the patient having reduced amounts of T3 and T4 even though their thyroid tissue is healthy.

02:53 So keep in mind that feedback loop is what keeps us in balance and in homeostasis. And remember the differences between how it works in primary hypothyroidism, where the hypothalamus is putting out everything the anterior pituitary is, it just can't respond to TSH.

03:09 And in secondary where the thyroid is fine.

03:12 But there's either some problem with the hypothalamus or the anterior pituitary gland.