‘Low thyroid’ can be the result of thyroid/goiter surgery.
Per the Mayo Clinic, you are correct
Thyroid surgery. Removing all or a large portion of your thyroid gland can diminish or halt hormone production. In that case, you'll need to take thyroid hormone for life.
Not to dismiss the possibility but also on that list is
Medications. A number of medications can contribute to hypothyroidism. One such medication is lithium, which is used to treat certain psychiatric disorders.
We do know that she does have problems with her brain. Is there any evidence "out there" of her having had thyroid surgery? (besides this scar)