It's important to first study all changes in environmental characteristics. The hamster may be allergic to the substrate (cedar, for example, is largely considered toxic, and fur loss is an early indicator of toxicity). Consider the cleanliness of the cage as well. A hamster in a dirty environment may develop a skin condition, and just as worrisome, may abandon its normal cleaning habits, which leads to a slow deterioration of skin and coat. If you clean regularly, look at the substances you're using to clean the cage. Has there been any change in these, or in your cleaning routine? Look for environmental changes outside the cage as well. Is the hamster downwind of something that could be damaging to its respiratory or metabolic functions?
The next check is for parasites. Inspect balding areas thoroughly. If there are parasites present the skin will appear rough, red, weathered, leathery, splotchy or some combination of these features. If the skin looks clean and smooth, that's a good sign. Get a good magnifying glass and look as closely as possible. Blow lightly on the fur to part it and look at various parts of the hamster's body that way. Parasites are often plainly visible, and when they are not, their destruction usually is.
Once these external factors have been ruled out, common pattern baldness may be the only conclusion. I've never read a satisfactory explanation of why this occurs, but it is implied in some literature that hamsters may suffer from nutritional deficiencies, not because they aren't eating properly, but because their bodies, for one reason or another, fail to utilize the full nutritional content of their diet.
This is where yeast has been helpful. It contains such a broad spectrum of nutrients and minerals, that it may fill the gaps in the hamster's nutrition profile. It would likely be nearly impossible to distinguishthe specific nutrient not being metabolized, but yeast offers a full complement of vitamins, minerals and amino acids, and the lack of one or several of these together might account for the loss. In any case, yeast has often worked wonders.
First, about the yeast, brewer's yeast is fairly commonly sold as a human supplement, especially in health food stores. However, there's a somewhat more refined and palatable type that goes by the name of "Nutritional Yeast" on the label, and is sold in the US by many companies, but the most popular brand seems to be Twinlab. It's available in many health food stores.
They have a bunch of varieties for different purposes, but here's a page with just their original "plain" variety, and you can see the breakdown of nutrients it contains (rather impressive):
http://www.vitacost.com/Store/Products/ProductDescription.cfm?SKUNumber=027434011051
The main difference between ordinary brewer's yeast and the Twinlab line is that the latter is "debittered", and probably tastes a bit more inviting.
You just sprinkle a small pinch on the food every day for two weeks. If the hamster has a favorite food (for mine it's tofu, but apple works well too) sprinkle it on that to be sure the pet gets it. If the food is moist the powder will adhere better. In the case of tofu, mine eat all of what I give them every day, so I can be sure the ham is getting the benefit of the yeast.
Yeast works wonders in cases of non-blight related fur loss. I have seen amazing results in as little as two weeks. You drop a pinch onto the food daily for two weeks. After two weeks reduce the amount to thrice weekly for two weeks, then twice weekly, if you still feel it's necessary at all. The stuff loses effectiveness if too much is fed over too long a period. It's also a lot for the ham to digest - potent and nutritious, but powerful.
