Determination of triiodothyronine in red blood cells by radioimmunoassay.

This study was undertaken to determine T3 content in red cells by radioimmunoassay. T3 in red blood cells was solubilized fairly from the stroma by hemolysis and red-cell T3 content could be determined directly by radioimmunoassay of the lysate. After hemolysing red cells with an equal volume of distilled water, 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, the 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 to 0.25 ng/ml in the content of red-cell T3 in hypothyroid and euthyroid subjects. The 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). From these experiments, it is suggested that the red-cell T3 is low in comparison with the serum T3 levels, and depends on two factors; serum T4 and serum T3 levels.