The present invention relates to derivatives of 3,5,3'-triiodothyronine (T.sub.3) which are useful for the measurement of free T.sub.3 in a liquid sample in which the T.sub.3 is present in both free and bound states.
T.sub.3 is one of the two physiologically important iodinated hormones, the other one being thyroxine (T.sub.4). In serum, most T.sub.3 is bound to thyroxine-binding globulin (TBG), although some binding to albumin and probably thyroxine-binding prealbumin (TBPA) also occurs. Of the T.sub.3 present in serum, approximately 0.2 to 0.4 percent remains free; i.e., unbound to plasma proteins. It is this free fraction that is presumably active in stimulating metabolism. Moreover, it has been estimated that 65 to 75 percent of the total metabolic effects in man is due to T.sub.3. Consequently, the measurement of free T.sub.3 in blood serum or plasma is of clinical interest for the prophylaxis or treatment of a variety of disorders and diseases.