@dixie122074 Welcome to Connect. You are asking a great question. Asking if there is a relationship between prescription medication and symptoms that may be side effects is always good to check on and so often overlooked.

I did some looking, and what I found suggests that possibly low levels of Vitamin D may be associated with seizures. It is something under study with researchers right now. Possibly it may be the low levels of vitamin D that caused the seizure, and not the act of taking Vitamin D? Vitamin D is important to so many different pathways of biochemistry in the body. That question can only be answered by a specialist and/or a medical researcher.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5143473/
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4149948/
https://bmcpediatr.biomedcentral.com/articles/10.1186/s12887-021-02906-7
https://www.nursingtimes.net/students/the-effects-of-vitamin-d-on-epilepsy-29-05-2018/
I also looked for literature relating Hashimoto's with seizures and this is what I found:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8140527/
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6517482/
https://www.seizure-journal.com/article/S1059-1311%2819%2930289-4/fulltext
This is of interest to me, because I am a person who has recovered from Hashimoto's. My Hashimoto's was affected by toxicity from silver dental amalgam fillings that were old. Removing these improved everything and dropped my antibody levels against my thyroid. I take Armor thyroid which is desiccated pig thyroid gland. It has all the components of thyroid hormones, where Synthroid does not, and the body must convert it to make the other components. That may be something you would want to ask your doctor about. Some patients doesn't convert this well, according to my environmental medicine doctor.