Newly diagnosed neonates will be randomly assigned to start therapy with LT4 at the initial dose recommended by the Standard of Care (SOC). Infants already on LT4 therapy will continue at the same daily dose within the randomly assigned treatment group (dose adjustments are allowed, if needed based on laboratory parameters and clinical response). Once enrolled, subjects will be treated and followed for 12 months (±1.5 months), participating in 7-8 study visits, consisting of 6-7 inclinic and 1-2 (or more if follow-up visits are required) telemedicine (TM) visits. The total number of visits depends on the age at inclusion.
