SERUM TRIIODOTHYRONINE: CLINICAL EXPERIENCE WITH A NEW RADIOIMMUNOASSAY KIT

Abstract

A new kit for measuring total serum triiodothyronine (T3) by radioimmunoassay 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 15-69 yr, but it fell by 0.15 nmol/l for every 5 yr beyond the age of 70. Mean serum T3 (.+-.SD) for 335 normal euthyroid subjects aged 15-69 yr was 2.11 .+-. 0.46 nmol/l (range: mean .+-. 2 SD = 1.19-3.03 nmol/l). Sixty-four percent of pregnant euthyroid women and 12% of those taking oral contraceptives had elevated serum T3 levels, as did all hyperthyroid patients, apart from 1 with T4 [thyroxine] toxicosis. Overlapping T3 values from hyperthyroid patients and euthyroid subjects with elevated concentrations of thyroid binding proteins could be separated completely by 2 correction techniques which related total serum T3 to the corresponding T3 resin uptake test, i.e., 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 7 euthyroid patients with biochemical T4 toxicosis were investigated. Seventy-three percent of moderately and 30% of severely hypothyroid patients had normal serum T3 levels. This overlap was not reduced by applying correction techniques. The value of serum T3 measurements in screening for and diagnosing hyperthyroidism was demonstrated. As T3 measurements become more widely available, it would seem reasonable to subclassify hyperthyroid patients into 3 types: those with T3/4 toxicosis, T3 toxicosis and T4 toxicosis.