If you are not already doing so, consider keeping a food log. In my case, the rash I had for decades was gluten intolerance (now celiac [14 years]). I also have MGUS (no rash), hypothyroid, and colageneous colitis (7 years). Because of the food log I saw that legumes and NSAIDs triggered the colitis. I'm also sensitive to sap of some trees (woodturner), perfumes, some processed foods, and other odds and ends. Sometimes sensitivities can worsen rashes and other autoimmune syndromes. I'm 69 and just take synthroid and various vitamins. I'm on mostly a FODMAP diet.