Radioimmunoassay for 3,3'-diiodothyronine in human serum

A specific radioimmunoassay for measurement of 3,3'-diiodothyronine (T2') is presented. With the method described (ethanol extraction of native serum and lyophilisation of the extract) the application of 400 microliter serum equivalent in the assay is possible. Standards and sera are treated similarly. The detection limit is 0.625 ng/dl, comparison between direct assay and dried extract assay shows good correlation. Mean normal T2' serum concentration in man is 7.2 ng/dl (range 3 to 11 ng/dl), hypothyroid: below 3.0 ng/dl, hyperthyroid: 11-64 ng/dl (range). T2' level in cord-blood of newborns: 16.5 ng/dl. The urinary excretion of free T2' of normal man is 0.49 microgram/24 h (mean), a relatively high excretion rate in comparison to the low serum level.