What could be the cause of my Mom's (90) frequent hot/ cold sweats during the day?

Thyroid? Someone on thyroid medication will typically metabolize it less well with aging and need a dose reduction. But yes, infection or blood sugars do come to mind first. Unfortunately it could be even more serious than that. Knowing whether or not she has fever could be helpful to your doctor, and knowing what all meds she is on and what labs were actually done might point to the stones left unturned. You may have a very good doc who is just taking it one step at a time and wanting more information after ruling out anything bad and treatable it could be, or maybe they are not taking things seriously enough - hard to tell!

It could be caused by many things. When she has it happen, you might check her blood sugar. I'm diabetic and when my blood sugar goes low, I get shaky and break out in a sweat. She could be hypogylcemic even if she isn't diabetic. You can get a blood meter pretty cheaply and keep it handy to check it. Anything under 70 would be concerning. If it's below that, I would have some juice and snack. Then discuss with her doctor.

Sometimes medicine can cause hot flashes in a person. Second idea it could be a food allergy. A third idea is that she is having a panic attack for some reason, the sweats, feeling shaky and sick feeling are also symptoms.

Did he check for an UTI?