I too am on RX thyroid medication. I take mine during fasts. Thyroid is very gentle and if you need medication to regulate it, it will not have any side effects. If she is hypothyroid (doesn't make enough thyroid) then she will feel really icky without her medication, which could give her a bad feeling about fasting. Thyroid can be taken on an empty stomach, so there won't be any upset tummy. Although I am not a doctor or anything, I have taken my thyroid during a fast and been much happier becuase of it.
Tillie