Determination of Triiodothyronine in Red Blood Cells by Radioimmunoassay

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Abstract
T3 [triiodothyronine] content was determined in [human] red cells by radioimmunoassay. T3 in red blood cells was fairly solubilized from the stroma by hemolysis, and red-cell T3 content was determined directly by radioimmunoassay of the lysate. After hemolyzing red cells with an equal volume of distilled H2O, 0.4 ml of the hemolyzate was used for the assay. The red-cell T3 content was expressed as ng/ml of red-cell volume. The normal T3 range in red cells was 0.20-0.45 ng/ml, and the mean .+-.SD was 0.32 .+-. 0.10 ng/ml. The limit of detectability was 0.2 ng/ml. In hyperthyroid patients red-cell T3 content was more than 0.50 ng/ml with a mean .+-. SD of 1.35 .+-. 0.65 ng/ml. In hypothyroid patients red cells contained less than 0.25 ng/ml of T3, and there was an overlap from 0.20-0.25 ng/ml in the content of red-cell T3 in hypothyroid and euthyroid subjects. Patients with T3 toxicosis showed a high or normal level of red-cell T3. A positive correlation was noted between the red-cell T3 content and the serum T3 level (r = 0.66). The correlation between the red-cell T3 content and the free T4 index (expressed as T7) was also positive (r = 0.67). Red-cell T3 is probably low in comparison with the serum T3 levels and probably depends on 2 factors: serum T4 [thyroxine] and serum T3 levels.