Stands for 5-hydroxyindole acetic acid. The major metabolite of 5-HT, or serotonin, in the brain and cerebrospinal fluid. Serotonin does not cross the blood brain barrier, so blood cannot be assayed to determine the psychologically relevant serotonin level in the body. However, 5-HIAA does reflect the serotonin level in the brain, and it does show up in representative quantities in blood and urine. When you hear about someone having their serotonin levels tested, and they didn't undergo a spinal tap. they probably had a 5-HIAA assay.

(note - There are quantities of serotonin in the blood, where it raises blood pressure by causing blood vessels to contract. This is actually where it gets its name (english root analogs: serum, tone). I find that amusing.)

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