How does the dexamethasone suppression test work?

Q. I am confused as to why high-dose dexamethasone inhibits a pituitary source, but the lower dose does not. Is it just because the cells are hyperplastic and not functioning up to par? Also I am assuming that dexamethasone produces a metabolite different from those produced from endogenous cortisol in urine, right? Otherwise the test wouldn’t be interpretable.

A. Great questions. Regarding the high vs. low dose dexamethasone and pituitary adenomas: the cells in the adenoma are neoplastic. Benign, but still neoplastic. Neoplastic cells generally are usually pretty insensitive to outside stimuli (but they can, in some cases, be affected). I think of those pituitary adenoma cells as being typical neoplastic cells in that they are insensitive to low-dose dexamethasone.

However, if you give a lot of dexamethasone, well, they do respond a little (and decrease their ACTH production). A similar principle operates in Nelson syndrome: the ACTH-producing pituitary adenoma is somewhat kept in check by the negative feedback from all the cortisol floating around. But if you remove the adrenal glands, you remove the negative feedback, and the pituitary adenoma grows explosively.

Regarding measuring dexamethasone in the urine: dexamethasone is an extremely potent steroid – way more potent than cortisol. So to get the same effect as cortisol, you only need to supply a relatively tiny amount of dexamethasone. The amount used in the suppression test is so tiny that it doesn’t affect the measurement of cortisol in the urine.

How to study for boards, part II

Today we’ll continue on in our little series of posts on how to study for boards. In part I, we talked about how to set up a study plan, and how to pick among all the resources available for students. (more…)

Adrenal cortical hypoplasia vs. hyperplasia

Q. What does it mean when Robbins says the adrenal cortices are hyper- and hypoplastic?

A. Hypoplastic adrenal cortices mean that the adrenal cortices have atrophied; hyperplastic adrenal cortices mean that they have expanded. (more…)

How to memorize the interleukins

I know a bunch of you are studying for boards, or are thinking about studying for boards. Any little memory aid that could help with a question or two is probably most welcome at this point.

Take, for example, all those cytokines with numbers (IL-1, IL-2…). How are you supposed to remember what all of those do?! Well, today’s post, written by a brilliant immunologist, gives you a great little method for remembering the interleukins associated with Th1 and Th2 cells.

You can keep the single digit interleukins straight because they come in sequence.

1. Macrophages can produce IL-1 which activates Th cells (CD4 T cells).

2. If the macrophages also produce IL-12, this will drive the Th cells to become Th1 cells (2 to Th1, or “to” Th1).

3. If Th1 cells are induced, they will produce IL-2 (second cytokine in order) and both IL-3 and IFN-gamma (gamma is the 3rd letter of Greek alphabet and represents the 3, too). The IFN-gamma will activate cytotoxic T cells (CD8 T cells).

4. If the macrophages produce only IL-1, this will drive the Th cells to become Th2 cells. If Th2 cells are induced, they will produce IL-4, IL-5, and IL-6 (note that they follow in order) that drive B cells to divide and differentiate into plasma cells that produce antibody.

5. Th2 cells also produce IL-10 (“zero Th1”) which blocks macrophage production of IL-12 and thus blocks activation of Th1 cells.