Question: How strong is the evidence that people with clinical depression have lower-than-normal levels of Serotonin (or other neurotransmitters)?

Answer:

Clinical and research implications

A meta-analysis of molecular imaging studies demonstrated significant reductions in serotonin reuptake sites (5-HTT) in the midbrain and amygdala in depressed patients. However, due to the unknown reliability of the studies included in the meta-analysis, no definite clinical implications should be made from the current evidence. In terms of research implications, the authors stated that their results "may encourage imaging studies of the specific role of serotonergic pathology in the striatum in the context of geriatric depression." They also stated that larger, more adequately powered studies are needed. The results from this meta-analysis also demonstrated that age matching in molecular imaging studies is critical, as even small differences in mean age between groups effect the validity of results.