Why do I have dry, itchy brown spots appearing on thighs and arm?
At first it is dry, and then it became slightly itchy, and then became dark brown.
Thanks for the question. It is hard to be certain without seeing the actual brown spots, but from the description, it does suggest that this is post inflammatory hyperpigmentation (PIH).
PIH is pigmentation following inflammation of the skin from various causes such as eczema, psoriasis and trauma. In your case, with the history of dry and itchy skin, preceding eczema seems likely. The brown spots of PIH are not permanent but may take week or months to gradually settle. In the meantime, moisturize the skin regularly and avoid scratching it. The important thing is to prevent the eczema so that you will not leave new marks.
If not better, you should consider seeing your doctor/dermatologist to confirm the diagnosis and treatment.
From your description, it appears that the brown spots are PIH (post-inflammatory hyperpigmentation), as Dr Theng rightfully mentioned.
Certain individuals may have higher risk of PIH after skin inflammation and may take a longer time to recover.
After your eczema settles, you might want to consider the following treatments for PIH:
1. Creams (tretinoin creams, vitamin C creams)
2. Chemical peels
3. Laser treatments (Q-switched Nd:YAG lasers, Picosecond lasers)
Also, do remember to apply sunblock to prevent worsening of the pigmentation.
Hope that this helps!