No.This is NOT a serotonine syndrome.Serotonine in the blood serum is used for diagnositcis of triptofan metabolism, and 2 times increased serotonine fortunately does not indicate nothing serious but maybe some slight methabolic disorder related to B-group vitamine problems.There is also a chance for so called dumping syndrome but this means you have to undergo bariatric surgery which you did not report.Normaly 10 % of triptofan (amino acid) we recevie with food like cheese, milk, nuts, choclolate, fish etc,. is used for making serotonine. But in some conditions in the thyrpid or guts up to 60 % of triptofan to be metabolised and therefore those people demonstrate 5-10 higher serotonine increase. As this is not your case and you have only mild increase, you should not be bothered with idea of having those conditions. Slight increase in serotonine indicates also chronic hyperactivity of parasympatic nerve system which is rather expected for BFS people (as we all here have dysautonomias).serotonine syndrome never happens per se, it is a specific condition related to drugs abuse, use of SSRI and IMAO together (which is alreasdy a known medical case and ususally, if you report to all doctors properly all drugs you are taking by medical prescription, this should not happen, cause serotonine syndrome could be potentially lethal), or if oyu are on antipsychotci drugs of IMAO class and eat cheese, salmon or some other food rich in triptofan after which extra serotonine goes to your brain. Some of our fellows (could not rememer who exactly) had reported serotonine syndrome due to improper prescription medicametns change, and it was really a disaster.so, summarizing what I would like to say: this is not a serotonine syndrome as usually it could not last for months without getting really bad and possibly lethal, also doctros should be aware of incmpatible ani-psychotic medication when making your prescriptions; 2 times increase does not indicate usually any serious problems in thyroid or in GIT (one needs 5-10 times increase to be alerted about serious problems); most possible reason is B group metabolic disease or hyperactivation of parasympatic autonomic nerve system due to chronic fears.I would suggest to ask your doctors about this anyway - if they ordered it, they must have some ideas to check for.