In my research I found serotonin is supposed to be between 101 and 281 nanograms per mL. or CC.
That's well and all for peripheral serotonin receptors in your GI tract, spine, platelets, and peripheral sensory neurons, but then how does that relate to this blood-brain barrier that separates your brain's swimming pool of CSF from the rest of the circulatory system? If your hypothesis is that low serotonin affects the brain then wouldnt one want to sample CSF for 5-HIAA through a spinal tap?
I am not a neurologist and before seeking out one for the above, a less physically painful approach might involve your "deeper issues". I dont know if you're in psychotherapy now, but your research might be a good topic to bring up with a psychotherapist.
Also forgot to add that if you're service connected at all then you could goto the DC VA Medical Center or Bethesda Naval hospital since the VA is boosting mental health resources to vets.