This study was undertaken to determine T
3 content in red cells by radioimmunoassay. T
3 in red blood cells was solubilized fairly from the stroma by hemolysis and red-cell T
3 content could be determined directly by radioimmunoassay of the lysate. After hemolysing red cells with an equal volume of distilled water, 0.4m
l of the hemolyzate was used for the assay. The red-cell T
3 content was expressed as ng/m
l of red-cell volume. The normal T
3 range in red cells was 0.20-0.45ng/m
l, and the Mean ±SD was 0.32±0.10ng/m
l. The limit of detectability was 0.2ng/m
l. In hyperthyroid patients, the red-cell T
3 content was more than 0.50ng/m
l with a Mean±SD of 1.35±0.65ng/m
l. In hypothyroid patients, red cells contained less than 0.25ng/m
l of T
3, and there was an overlap from 0.20 to 0.25ng/m
l in the content of red-cell T
3 in hypothyroid and euthyroid subjects. The patients with T
3 toxicosis showed a high or normal level of red-cell T
3. A positive correlation was noted between the red-cell T
3 content and the serum T
3 level (
r=0.66). The correlation between the red-cell T
3 content and the free T
4 index (expressed as T
7) was also positive (
r=0.67).
From these experiments, it is suggested that the red-cell T
3 is low in comparison with the serum T
3 levels, and depends on two factors; serum T
4 and serum T
3 levels.
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