Do blood clots cause diarrhea?


It’s not an uncommon clinical scenario. A patient has a diarrheal illness, feels weak, lies on the couch for several days, then presents with complaints of swelling and redness in their leg. An ultrasound confirms the diagnosis of a blood clot (thrombosis) in the veins of the leg.


Sounds straightforward. Immobility and dehydration are commonly implicated in increasing the risk of thrombosis. Or is it more complex?


As always, first some background. Platelets are circulating cells in the bloodstream that are heavily involved in the formation of blood clots. They also contain large amounts of serotonin. There are unable to synthesize it on their own and are constantly on the lookout for serotonin in the bloodstream to take up and store. Platelets then release it at sites of tissue injury and bleeding.(1) Serotonin causes blood vessels to narrow and platelets to aggregate forming a blood clot. (Aside, if you’re taking an SSRI and preventing platelets from uptaking serotonin and also wonder why you bruise easily, look no further)


Serotonin also has a major effect on the cells that line the intestine. Increased serotonin levels cause diarrhea by increasing intestinal motility and fluid secretion.


So now the question arises, did the platelet release of serotonin during the blood clot cause the diarrhea instead of the other way around? Could blood serotonin levels be useful in the diagnosis of thrombotic events?  Perhaps… remember, you learned it here first.



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