Allergic reaction to levothyroxine.
As best as it is understood, my immune system did not accept it to be identical to natural thyroid hormones which triggered a systemic autoimmune response.
I take NDT instead of Levothyroxine.
I have taken magnesium supplements every day for the last year (100 mg - 150 mg) and Omega 3 every second day. Whether or not the supplements have made a difference is anyone's guess, but the perceived amplitude of my tinnitus has unquestionably reduced over time.
I also have made sure I eat more fresh fruit and vegetables than I did.