Thyroid problems, specifically hypothyroidism, can cause a lot of what your Nan's going through right now. Lethargy, swelling in hands and feet, and even anemia can be symptoms of hypothyroidism.
Do you have a copy of her test results handy or can get a copy of them? It'd be interesting to see her levels. Her GP or the lab may be using the "old" ranges and going by TSH level alone. The new range for TSH, approved by the ****, is now .3-3.0. And even if her TSH level is in the "normal" range, she can have symptoms of hypothyroidism. She needs to get tested for her FT4 and FT3, as well as thyroid antibodies. At her age, she probably has nodules as well (this is quite common among older folks).
Do searches online. Educate yourself. You'll have to be able to ask the right questions if you're acting as your Nan's advocate. Push for the tests to be done. At her age and with her health, she shouldn't have to suffer needlessly because a doctor sees her as a test result and not a person.
Please keep us posted!