Dr. Murray covered the conditions associated with low serotonin quite well. What many patients (and physicians who treat patients with low serotonin) are not aware of is the importance of measuring serotonin levels clinically. Likely less than 1% of clinicians who treat these patients with low serotonin actually measure it.
The question arises, how do you measure it. There are several ways: brain tissue biopsy, spinal tap of CNS fluid, serum or urine. The first two are not recommended in an outpatient setting obviously, and serum measurements tend to be less stable (unless you're collecting in a research setting). Thus, urine is the ideal specimen for most clinicians in an outpatient setting.
Is it accurate? Valid question, but not the right question. Yes, urine is extremely reliable and accurate as a specimen choice. The best question, however, is does the urine accurately reflect what's happening in the brain? Animal research and clinical experience suggest it does.
Where this rubber meets the road is for the average patient with a mood disorder presents to the clinic. Without testing, how would the clinician know how to treat the disorder? Answer: they don't, they guess. The problem is that the clinician ends up treating a diagnosis, not the biochemical imbalance underlying it. Does the patient have a predominantly serotonergic depression? Or is it dopaminergic? The treatment would be quite different. And this is the benefit of measuring multiple neurotransmitter levels in the clinical, outpatient setting.
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