Dark Knuckles

For the past several years I keep going through several months of the year (normally in late winter early spring) when my knuckes get very dark. I'm a white male, I'm 6'1" tall and way 270lbs. I've read that being over weight can cause this and I know I'm over weight but i'm not a huge person. I have controlled type 2 diabetes. I've lost 20 lbs over the last few months but just within the last 2 weeks my knuckles have gotten really dark again, the dark skin feels thick and dry. Its only on my hands. Please offer me some good advise. Thanks