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I should have framed my question better, actually. What I meant to ask is if there is a reference to support the claim 'Most serotonin is found outside the body"?, i.e, if there is a publication which reports this?
Mostly, people refer to this review while making the statement but having checked for the references in this review, I couldn't find any.
Cirillo C, Vanden Berghe P, Tack J. Role of serotonin in gastrointestinal physiology and pathology. Minerva Endocrinol. 2011 Dec;36(4):311-24.
My suggestion is that for detailed information about Serotonin, read The Medical Biochemistry Page By King MW Indiana University School of Medicine in below given link. You will get diverse information in this link page about what you need to know that most serotonin is found in gastrointestinal tract where it controls intestinal movements.