What Causes Dark Patches On The Ankles And Legs?

I am a fair skinned 62 year old female in relatively good health. In the last few months have noticed several dark patches on my ankles and lower legs. Not scaly, itchy, raised or sore- each about the size of a quarter - 8 altogether. Just very unsightly. Blood and urine tests done 6 mo. ago in routine physical did not show any concerns about diabetes or thyroid function - nothing other than slightly elevated blood pressure. Thyroid problems however are widespread among the females in my family and I have annual check ups to make sure all is well there. Topical cosmetic creams have had no effect. What are possible causes and treatments? Than you.