Hamsters can get a number of diseases that can cause them to be itchy and lose their hair. Some of these diseases include mites such as demodex. Other causes could include urine scald (from wet bedding/lack of grooming), dermatitis (infection/inflammation of the skin caused by bacteria), or even fleas. Another issue can be cancer of the adrenal glands or over activity of the glands (Cushing's diesease). If your hamster's bedding is changed regularly, and you haven't gotten a different type of bedding recently (some hamsters can have reactions to certain beddings such as pine and cedar), I would recommend a trip to your vet. They can do a skin scrape to check for mites, and depending on how old your hamster is they can look for other problems such as the adrenal cancer I mentioned. These mites are normally present on hamsters, but an increase in stress or a change in other health can cause they to overgrow and start to cause a problem. They are fairly easy to treat however, so I would definitely recommend getting your little guy checked out! Here is some more info on hamster care and common ailments including pictures of cushings disease, mites, and a bacterial infection of the skin. http://www.petcarevb.com/hamsters.html

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I hope your little guy is back to his old self soon!