Iontophoresis is a way to use electrical impulses to push a steroid drug through the skin to the site of local inflammation. The hoped for goal is to pass the antiinflammatory drug through the skin to a specific area (usually a tendon or bursa). In this way, the local effects might occur faster and without the side effects of an oral drug (taken by mouth).
Research on iontophoresis has been unable to answer your question. Animal studies have been tried but animals seem to absorb the chemicals through the skin seven times more than humans. Clearly there are some differences between humans and animals when it comes to iontophoresis.
Even when humans have been studied directly, the amount of steroid transmitted through the skin varies considerably from one person to another. In one study, half the patients had no steroid whatsoever detected after iontophoresis. In that same study, skin thickness was not shown to be a factor. In other words, thin skin didn't transmit the drug faster or better.
Research shows that iontophoresis has the potential to affect skin, blood, nerve tissue, and tendons. The mechanism by which this occurs is unknown. More studies are needed to help identify who is a good candidate for this treatment.