The serotonin hypothesis of depression is still influential. We aimed to synthesise and evaluate evidence on whether depression is
associated with lowered serotonin concentration or activity in a systematic umbrella review of the principal relevant areas of
research. PubMed, EMBASE and PsycINFO were searched using terms appropriate to each area of research, from their inception
until December 2020. Systematic reviews, meta-analyses and large data-set analyses in the following areas were identified:
serotonin and serotonin metabolite, 5-HIAA, concentrations in body fluids; serotonin 5-HT1A receptor binding; serotonin transporter
(SERT) levels measured by imaging or at post-mortem; tryptophan depletion studies; SERT gene associations and SERT geneenvironment
interactions. Studies of depression associated with physical conditions and specific subtypes of depression (e.g.
bipolar depression) were excluded. Two independent reviewers extracted the data and assessed the quality of included studies
using the AMSTAR-2, an adapted AMSTAR-2, or the STREGA for a large genetic study. The certainty of study results was assessed
using a modified version of the GRADE. We did not synthesise results of individual meta-analyses because they included
overlapping studies. The review was registered with PROSPERO (CRD42020207203). 17 studies were included: 12 systematic reviews
and meta-analyses, 1 collaborative meta-analysis, 1 meta-analysis of large cohort studies, 1 systematic review and narrative
synthesis, 1 genetic association study and 1 umbrella review. Quality of reviews was variable with some genetic studies of high
quality. Two meta-analyses of overlapping studies examining the serotonin metabolite, 5-HIAA, showed no association with
depression (largest n = 1002). One meta-analysis of cohort studies of plasma serotonin showed no relationship with depression,
and evidence that lowered serotonin concentration was associated with antidepressant use (n = 1869). Two meta-analyses of
overlapping studies examining the 5-HT1A receptor (largest n = 561), and three meta-analyses of overlapping studies examining
SERT binding (largest n = 1845) showed weak and inconsistent evidence of reduced binding in some areas, which would be
consistent with increased synaptic availability of serotonin in people with depression, if this was the original, causal abnormaly.
However, effects of prior antidepressant use were not reliably excluded. One meta-analysis of tryptophan depletion studies found
no effect in most healthy volunteers (n = 566), but weak evidence of an effect in those with a family history of depression (n = 75).
Another systematic review (n = 342) and a sample of ten subsequent studies (n = 407) found no effect in volunteers. No systematic
review of tryptophan depletion studies has been performed since 2007. The two largest and highest quality studies of the SERT
gene, one genetic association study (n = 115,257) and one collaborative meta-analysis (n = 43,165), revealed no evidence of an
association with depression, or of an interaction between genotype, stress and depression. The main areas of serotonin research
provide no consistent evidence of there being an association between serotonin and depression, and no support for the hypothesis
that depression is caused by lowered serotonin activity or concentrations. Some evidence was consistent with the possibility that
long-term antidepressant use reduces serotonin concentration.