CD4 and atrial fibrillation: In particular, levothyroxine should be initiated if the free T4 is low or TSH sustained at >10 mIU/L, at an initial dose of 1.6 μg/kg rounded to the nearest 25 μg, unless there are comorbidities such as uncontrolled ischaemic heart disease or atrial fibrillation, or in those over 65 where an initial dose of 25–50 μg daily can be used.