instruction: what was the maximum thyroxine (t4) value for patient 86791?
response: SELECT MAX(labevents.valuenum) FROM labevents WHERE labevents.hadm_id IN (SELECT admissions.hadm_id FROM admissions WHERE admissions.subject_id = 86791) AND labevents.itemid IN (SELECT d_labitems.itemid FROM d_labitems WHERE d_labitems.label = 'thyroxine (t4)')