<prompt> Long thyroid stimulating agent isAntibody to thyroid globulinAntibody to thyroid cell receptorsAntibody to thyroxineAntibody to thyroid cells  <response>: Answer is option 2, antibody to thyroid receptors. Thyrotropin receptor TSHR antibodies that stimulate the thyroid TSAb cause Graves hypehyroidism and TSHR antibodies which block thyrotropin action TBAb are occasionally responsible for hypothyroidism. Unusual patients switch from TSAb to TBAb or vice versa with concomitant thyroid function changes. We have examined case repos to obtain insight into the basis for switching. 