what are the number of patients who received a calculated thyroxine (t4) index lab test during the previous year?
select count( distinct admissions.subject_id ) from admissions where admissions.hadm_id in ( select labevents.hadm_id from labevents where labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'calculated thyroxine (t4) index' ) and datetime(labevents.charttime,'start of year') = datetime(current_time,'start of year','-1 year') )