I understand that feedback inhibition isn't working and is why ACTH is undetectable/low (and why there is no change after dexamethasone), but wouldn't cortisol levels increase after dexamethasone? Or does dexamethasone not count as cortisol when quantifying serum levels? pls help
misseiliThis patient has Cushing Syndrome and as you stated their feedback inhibition is hampered. Low-dose dexamethasone causes a decrease in cortisol in normal individuals but not in those with Cushing Syndrome, only high dose dexamethasone causes a decrease in cortisol with Cushing Syndrome. Add'l info: In ectopic ACTH (ie Small cell lung cancer) high dose dexamethasone leads to no change in cortisol. Pathoma 2020 Pg 171+2
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I understand that feedback inhibition isn't working and is why ACTH is undetectable/low (and why there is no change after dexamethasone), but wouldn't cortisol levels increase after dexamethasone? Or does dexamethasone not count as cortisol when quantifying serum levels? pls help