[Thyroxine and triiodothyronine]

The value of measurements of serum thyroxine (T4) and triiodothyronine (T3) concentrations in the clinical evaluation of thyroid function is well established. In the present report the properties of the methods currently available for the assessment of circulating thyroid hormones are examined. Recent data indicate that results of measurements of serum T4 by radioimmunoassay (RIA) or by displacement analysis are qualitatively and quantitatively similar. Evidence has been provided from this and other laboratories that elevated or subnormal serum levels of total tt4 may be observed in several euthyroid physiologicmal serum levels of total T4 may be observed in several euthyroid physiological or pathological conditions associated with altered capacity of the thyroxine-binding protein; in these cases a normal free thyroxine index is usually found indicating its importance in the evaluation of thyroid function. Several physiological observations and the recent recognition of clinical conditions associated with changes in the T4/T3 ratio, justify the present interest in the measurement of serum T3. The advantages of RIA methods with respect to displacement techniques in the determination of this hormone are well documented. The AA. report here their experience with a simple RIA method for total T3, using a single Sephadex G25 column for extraction of T3 from serum and for separation of bound from free hormone. The mean (+/-SD) values of serum T3 found in 78 normal, 23 hyperthyroid and 25 hypothyroid subjects were as follows:160+/37ng/dl, 604+/195ng/dl and 35+/23ng/dl respectively. High values were found in 19 pregnant women at delivery (236+/29ng/dl), but not in 5 subjects on contraceptives (156+/42ng/dl). A relatively large variability is noted when normal values reported from different laboratories are compared. This may be related at least in part to geographical and/or ethnical factors, but methodolgical differences may also be involved. The normal range of circulating T3 should be established in each individual laboratory.