I think thyroid problems can cause and also found this...
"Continuous exposure to the sun damages your skin. Dark patches appear on face due to sun exposure. Face is mostly exposed to the sun. This is the main reason why dark patches appear mostly on face. Stress is also one of the reasons for dark patches. Hormonal changes during pregnancy or due to any other reason in women cause dark patches. Lack of nourishment, lack of proper skin care is also some of the reasons. Sometimes due to illness or allergy the dark patches appear. Taking contraceptive pills can also be a cause for dark patches. Selection of soaps, deodorants, face products and cosmetics affects the skin".