I used to think I got heat rash under my bra, but turns out I had rubber allergies and then developed GD. Steroids never helped me in this area. Even if steroids help temporarily you are only treating the condition and not stopping the cause. Yet, steroids had been a life saver for my ACD before I learned about testing. Many of us have multiple types of eczema, if you have GD don't assume your rash is only from GD, it may be from Allergic Contact Dermatitis, ACD. Get the 5 day extended Patch Test, google True Test.
Cottonique has some 100% cotton undies and bras along with latex free elastic undies and bra and other very soft 100% cotton items. Well made but no glamour and if you are large breasted the elastic free bras offer limited support. Some I know has success with their bra liner, it has a small band of latex free elastic in a casing at the bottom. Women wear these under their bras, yet even that doesn't help me as the elastics off-gas. I not only have GD but ACD and tested positive to an element in rubber with is used to synthesize blended materials like elastic. What I do is remove the elastic at the bottom of the bra liner and expand the casing with a soft 100% cotton knit and run one of their draw strings though the bottom and can wear this as a bra because I am small breasted and support is not an issue. Also their front tie 100% cotton bra has some support because of the shaping, but as it is seamed and a bit lumpy it works if worn with front button or snap shirts which are loosely fitted. Camp type shirts which are 100% cotton or polyester with 2 pockets are the best for camouflage. Columbia makes a few 100% polyester 2 pocket camp shirts which are more tailored and feminine. Really great for my casual lifestyle now retired. Just always read the fabric information as Columbia also makes blended fabric clothing. I wear lots of these camp shirts. I used to think I got heat rash, wrong. I have zero rash in bra area now from ACD and my GD has gone into remission because of the cilantro smoothie detox we have been talking about since the end of March 2019.