I forgot one more thing. Her heart may be struggling, so a cardiologist or at least the pulmonary doctor could investigate that possibility. But I think seeing the endocrinologist for her hypothyroidism should come before any checks on her heart, as the hypothyroid can affect the heart. If that's ruled out, then her heart may need to be checked.
Dear Rachelk,
Since scans show no changes in the brain, then she hasn't had a bleed or any other insult to her brain. Now, the docs said she was getting insufficient oxygen to her brain. My guess is that her hypothyroidism may have something to do with this, and her endocrinologist needs to recheck her levels and adjust her thyroid medication as needed. It is the condition of an undertreated hypothyroid that will slow the pumping of the blood through vessels, so in general she won't be getting enough oxygen to her brain and to everywhere else.
Another thing that may be figuring into this is if she is having any respiratory problems that suggest a cold or flu, this can affect her ability to breathe well, which in turn results in less oxygen being pumped through the bloodstream. She had chills, so that's why I thought about a possible virus or bacteria affecting her breathing. In addition, any medications she is taking should be reviewed by her regular doctor, to make sure she's not being too heavily sedated or if a side effect is depressing her nervous system too much or anything like that, as it will have a similar effect as a flu on breathing.
She might could use a little oxygen machine, where she would wear a see-thru plastic cover for her mouth and nose whilst she sleeps at night. Giving her more pillows to sort of raise her shoulders and head more makes it easier for people to breathe in general. If she has developed a kind of asthma, which a pulmonary doctor can check out her breathing, he can prescribe a little puffer that will open the lungs if she feels like she cannot breathe if her lungs tend to swell up from allergy, perhaps.
Those would be my thoughts. GG