Hi,
I suffer from dark pigmentation where my clothing rubs due to insulin resistance. Do you have any sugar issues like hypoglycemia or diabetes? It is a long shot but, insulin resistance could be a possibility. Symptoms of insulin resistance are; high cholesterol, dark pigmentation in folds of skin or like mine where my clothing rubs, skin tags, trouble with being over weight, and high blood pressure.
Hi
Welcome to the MedHelp forum!
Dark spots on buttocks which are not itchy could be due to hyperpigmentation, friction with clothes, sweat dermatitis, vitamin deficiency, dry skin, adrenal gland disorders, thyroid problems, seborrhaic warts, moles etc. It is difficult to comment beyond this at this stage without examining them.
I would strongly recommend you to consult a skin specialist. Please let me know if there is any thing else and do keep me posted. Take care!