OUR GENES can partly explain our body’s individual response to T4 in thyroid meds like Synthroid / Levothyroxine. It may explain why some thyroid patients do just fine on Synthroid / Levothyroxine, while others continue to have hypothyroid symptoms. Review of: Carlé, A., Faber, J., Steffensen, R., Laurberg, P., & Nygaard, B. (2017). Hypothyroid Patients Encoding Combined MCT10 and DIO2 Gene Polymorphisms May Prefer L-T3 + L-T4 Combination Treatment—Data Using a Blind, Randomized, Clinical Study. European Thyroid Journal, 6(3), 143–151. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5527224/ Reduced T4-T3 conversion A lot of research lately has proven that DIO1 and DIO2 gene polymorphisms can reduce T4-T3 …