KDM says we aren't making serotonin ( or maybe much of it?). I forget the pathway that is involved.....
I went into studying serotonin early on, because I had such drastic responses to serotinergic precursors like 5-HTP.
1) The serotonin in the general bloodstream has zero to do with the serotonin behind the blood-brain barrier.
2) Your serotonin may be low, but if your receptors have increased sensitivity, then it may not be something you need to increase.
3) Patients may have serotonin autoantibodies, which makes increasing serotonin problematic (one study found 2/3 of patients in a decent-sized cohort had autoantibodies to serotonin).
4) Check your SNPs. If you break down serotonin slowly, low may be more normal for you.