Serum triiodothyronine: clinical experience with a new radioimmunoassay kit

A new kit for measuring total serum triiodothyronine (T3) by radioimmunoassay ('T3 RIA'; Radiochemical Centre, Amersham) was evaluated using sera from 1114 patients and normal controls. The kit performed reliably with intraassay and interassay variability figures of 3.9% and 9.3%, respectively, at 'medium' concentrations of T3. A T3 measurement could be obtained conveniently within 18 h after overnight incubation at room temperature. There were no critical steps dependent on time or temperature. Serum T3 values showed no significant sex difference. There was no significant change in mean serum T3 between the ages of 15 and 69 years, but it fell by 0.15 nmol/l for every 5 years beyond the age of 70. Mean serum T3 (+/-SD) for 335 normal euthyroid subjects ages 15-69 years was 2.11+/-0.46 nmol/l (range: mean+/-2 SD = 1.19-3.03 nmol/l). 64% of pregnant euthyroid women and 12% of those taking oral contraceptives had elevated serum T3 levels, as did all hyperthyroid patients, apart from one with T4 toxicosis. Overlapping T3 values from hyperthyroid patients and euthyroid subjects with elevated concentrations of thyroid binding proteins could be separated completely by two correction techniques which related total serum T3 to the corresponding T3 resin uptake test, viz an 'augmented free T3 index', or a map plot of T3 vs T3 resin uptake. A 25% incidence of T3 toxicosis was observed. One hyperthyroid patient with T4 toxicosis, and seven euthyroid patients with 'biochemical T4 toxicosis' were investigated. 73% of moderately and 30% of severely hypothyroid patients had normal serum T3 levels. This overlap was not reduced by applying correction techniques. Our studies demonstrate the value of serum T3 measurements in screening for and diagnosing hyperthyroidism. As T3 measurements become more widely available, it would seem reasonable to subclassify hyperthyroid patients into three types: those with 'T3/4 toxicosis', 'T3 toxicosis' and 'T4 toxicosis'.