Does the combination of tretinoin and hydroquinone thin the skin?

Various agents can be used for melasma. Topical steroids are often combined within a "Kligman's" or "modified Kligmans" formula, and they can cause some skin thinning. However, hydroquinone does not cause skin thinning, and tretinoin or vitamin A in fact can thicken the skin a little as well as improve its texture and tone.

Studies have shown that Retin A (tretinoin) cream can stimulate collagen to thicken the skin.  Hydroquinone does not thin the skin.  Cortisone creams can thin the skin if used long-term.