Background: Patients with differentiated thyroid cancer (DTC) are usually managed with total thyroidectomy and subsequent radioiodine ablation of the remnant thyroid tissue. L-thyroxine (L-T4) therapy (about 2 µg/kg) is required for life, with a wide variation in patient dose requirements. The iodothyronine deiodinase types I, II, and III ( DIO1 , DIO2, and DIO3 , respectively) regulate the activity of the thyroid hormone via removal of specific iodine moieties from the precursor molecule T4. During LT4 replacement, the active hormone triiodothyronine (T3) levels strictly depend on DIO2