I have been on this site posting my distress with many of the same symptoms you describe. I get what I call “spells” of nervousness, nausea and an intense feeling of of illness and doom. It comes out of nowhere, escalates, and passes in 30 seconds or so. This happens 3-4 times a week. I too had a complete work-up at Mayo and all tests were normal. I’m on Synthroid and it is controlling thyroid issues well. I’m currently being treated for anxiety for lack of another label for my symptoms. I’m taking escitalopram and clonazepam, as needed. Seems to be helping but symptoms are not entirely gone. I’m a fit 70-year-old woman otherwise. I do have a history — and family history — of panic attacks (in my 30s). I empathize with you, as these happenings are unsettling to say the least! Let us know if you find something that works!