Can an untreated stomach ulcer cause the thinning of all hair (scalp and body) and hair loss?

It's important to see your physician about this issue as a full evaluation is needed. Is it possible for a stomach ulcer to cause hair loss. Yes but it is certainly not very common. If an ulcer causes massive bleeding such that hemoglobin drops many points or the ulcer is associated with significant inflammation that micronutrient absorption is affected then yes, this can cause hair loss. Stomach ulcers are common (millions of Americans have peptic ulcer disease). For most, the ulcer and hair loss are unrelated. See your doctor to review if there could be a connection in your case. 

A stomach ulcer or any illness that causes stress can cause hair loss if you are genetically prone to hair loss