I don't know anything about this really, but I did a google search and here's what I found. It is very, very rare, but does happen. If it turns out that her antibodies are attacking her brain, then that would be the reason for her issues.
"Some of the most common symptoms of Hashimoto's Encephalopathy include: disorientation, psychosis, tremors, concentration and memory problems, jerks in the muscles and lack of coordination, headaches, partial paralysis on the right side, and speech problems. Sometimes, patients are mistakenly diagnosed as having had a stroke, or having Alzeimer's. Typically, Hashimoto's antibodies levels will be high, and the patient may also have a diagnosable case of Hashimoto's Thyroiditis, but TSH levels may also be normal.
The condition can also appear in adolescents, but is even more likely to be overlooked. The symptoms in adolescents, unlike adults, frequently include seizures, confusion and hallucinations. A drop in school performance is also a common symptom, along with progressive cognitive decline. Thyroid autoantibody levels should be evaluated in these adolescents with these symptoms, even when thyroid function tests are normal. (Distinct Pediatric Manifestations of Hashimoto's Encephalopathy Described)"