My knuckles are getting darkened? (Photos)

I was diagnosed with acanthosis nigricans (not sure) was told to test insulin level n thyroid levels but found to be normal... I have lot of dark patches in neck region.. Now my knuckles r getting darkened for past 6 months only... What would be the reason? Is this also due to acanthosis nigricans or it s sign of any other underlying problem? Note: I see this old on my left hand