I understand your predicament. I have not been tested but have all the symptoms. I am trying to find someone as you are. It started in my 40's. I am wondering if it is hormonal?
I don't know if this will help you, but I will not stop to look for a answer even if the doctors never heard of TMAU. Keep trying. So yes, go to a endocrinologist, gastroenterologist, etc. Be prepared for the tests to come back negative and they will think you are a head case, but that is just me. I am going to an endocrinologist later in the month.
I am curious if you have lost any weight. I ask because after years of trying to get an answer, I believe I am in a slight Depression and am losing weight. I'm slender, so it doesn't help.
Good luck to you and I will be wishing you the best.