Radioimmunological Measurement of 3,3′-Diiodothyronine in Serum and Amniotic Fluid

A radioimmunoassay for the measurement of 3,3'-diiodo-L-thyronine (rT2) in serum and amniotic fluid is described. Specific antisera to rT2 were produced by immunization of rabbits with rT2 conjugated to bovine serum albumin. The molar cross-reactivity tested for various iodothyronines and iodotyrosines was 0.3% for triiodothyronine and 0.001% for thyroxine. The sensitivity of the assay with a detection limit of 1.8 fmol/tube (0.94 pg/tube) was due to the high avidity of the antiserum and the use of 125I-labelled rT2 of maximum specific radioactivity. In most of 45 normal subjects, serum rT2 levels measured in evaporated ethanol extracts were below the detection limit of 0.018 nmol/1. Mean rT2 concentrations were 0.21 nmol/1 in newborn cord serum and 0.10 nmol/1 in amniotic fluid at 12 to 30 weeks of pregnancy. The molar concentration ratio of 3,3',5'-triiodothyronine (rT3) to rT2 was estimated to be 15.7 in cord serum of newborn. A similar rT3/T2 ratio was found in adults after intravenous application of 500 microgram rT3. Using these data and the known rT3 values, a hypothetical mean serum rT2 concentration in adult normal subjects of 0.015 nmol/1 was calculated. The radioimmunoassay described may be a useful analytical tool in studies of the synthesis as well as the metabolism of thyroid hormones.