Unexplained weight loss for mom, who is 87 years old. Why is this happening?

I am glad you are in the care of a physician. I hope you will find answers from the testing. When there is this kind of "wasting disease" it does indicate that something is going on; often hard to find out just what, and every patient is a mystery. You may be looking soon at a palliative care situation. And yes, you can be exceptionally thin, and still live. Joan Didion, the wonderful author is in her late 80s now, I do believe, and likely weighs no more than 80 pounds sopping wet. I hope you will update us on any findings and wish you good luck.

Like Geaton my first thought was Thyroid. An overactive one you will lose weight. There are meds to slow it down. But they need adjusting because my Mom started gaining back too much weight. I would think the doctor would do a full Lab workup and this will show the Thyroid function.

There are many things that can cause weight loss, a very general symptom. Is she on any thyroid meds or has her thyroid ever been checked? Hyperthoid can cause weight loss and if she's already on medication for it, losing that much weight will require her thyroid meds prescription to be adjusted or else the overmedication will cause her to continue to lose weight.

Is she telling you she's eating 3x a day? Or is someone else telling you, or are you there to see it yourself? If she has cognitive or memory impairment then she may mistakenly believe she is eating (my MIL had this problem). I think rehab and facilities usually keep a record of what their patients eat. You may want to request this info and talk to the DOM based on what you find out.

These suggestions are only because they are easy and non-invasive to discount. There are many other reasons she could be losing weight but may require more aggressive testing, which you will need to decide to green light or not. Does your mom have a Living Will (Advance Care Directive) on file with her doctor? I'd check to find out. May you receive wisdom for these decisions and peace in your heart that you're doing the best for your mom.

I think it is all just guesswork at this point, you need to wait to see what is discovered. Prepare yourself for what will be done if it is something like cancer - at 87 choosing hospice or palliative care may be the kindest option.