Probably the most common reason for an increased number of open binding sites is the increase in circulating thyroid binding globulin (TBG) that is seen in pregnancy or oral contraceptive use. The latter is the most probable cause for a low T3 resin uptake in a 28-year-old woman like this patient. In both these situations circulating T4 increases to match the increase in TBG, and levels of free T4 (and the calculated FTI) remain normal.