Lowering the upper limit of TSH from 5 to 3 mIU/L has been proposed by some authorities.12The argument favoring this lower limit is due to higher rate of detection of antithyroid peroxidase Ab (anti-TPO Ab) at TSH values between 3 and 5 mIU/L and the risk of progression to overt hypothyroidism. The gene discussed is TPO; the disease is hypothyroidism.