I have the same problem. I'm 46, and for more than 5 years I have been struggling with melasma. Started with a little dark spot on my cheeks and now I have it all over my face. I don't smoke or drink, a practice yoga 3 times a weak, eat a good diet and I have never taken birth control pills or had kids. Two years ago I had a copper IUD removed which I used for 7 years. I don't know if copper is related to melasma problem but since the melasma started approximately the time when I had the IUD, I asked my doctor to check the copper level in my system.
I little info about my journey with doctors.
I started with dermatologist which didn't help me at all. I had 4 chemical peelings, used Tri-Luma cream, cover my face with sun block like a crazy woman, and off course avoid sun exposure forever - no results, just spend my money and time. My melasma got a little bit lighter and in a couple weeks came back the same way it was. I realized that my problem is more complicated and my dermatologist told me to find an endocrinologist. Next doctor that I visited was my gynecologist no help, she didn't care. Now I am trying an endocrinologist. So far I have been tested for thyroid and, adrenal problems. These tests came back normal and I didn't get my melasma problem fixed. Now, I asked my doctor to have a more complex adrenal test (the one you have to use contrast intravenously). I also asked to check the levels of copper in my system and a thyroid auto-immunity test. I am not looking for a cosmetic solutions but the root cause of my melasma problem. I will let you know about my results...