I have been taking LDN for 3 years for Hashimoto's thyroiditis, how do I find my optimal dose?  I have taken 3 mg and 4.5 mg for extended periods and feel no difference. 

I would say this is a gray area. It does take a long time, up to 18 months to see any changes with Hashimotos thyroiditis. However, the difference between 3mg and 4.5mg shouldn't be an issue. It is likely that there are other deeper issues to sort in your health journey -- think co-infections, gut health/dysbiosis, hormones, trauma, chronic stress...the list goes on and on. I would work with a skilled clinician to regroup and look at the entire picture.