Will Botox worsen pre existing Hashimotos thyroiditis or cause the body to develop additional antigens?

Botox, Xeomin and Dysport have no effect on autoimmune disorders. When given in large doses for medical disorders, antibodies have developed, but this has not been reported with the BTX-A medications for cosmetic issues.

Botox, Dysport and Xeomin have no known affect on autoimmune diseases.  Rarely they can illicit antibodies to their binding proteins but this does not have any clinical effect and they are not contraindicated in patients with "overactive" immune systems.