A radioimmunoassay for 3',5'-diiodothyronine

The present report describes a RIA for 3',5'-diiodothyronine (T₂) that can be performed on unextracted serum and which has a lower limit of detectability of 2 ng/dl. Cross-reactivity with other iodothyronines was negligible, except for rT₃ which began to demonstrate cross-reactivity when rT₃.-t levels were elevated to 180 ng/dl. Employing this RIA for T₂, we have determined that 83 healthy individuals had a mean (±SE) serum T₂ concentration of 5.0 ± 0.3 ng/dl, thyrotoxic subjects (n = 12) had a mean T₂ level that was elevated to 10.8 ± 0.8 ng/dl, and each of 6 hypothyroid subjects had undetectable (<2 ng/dl) concentrations. Athyreotic patients (n = 8), receiving 0.4 mg T< daily, had serum T₂ concentrations of 15.0 ± 3.0 ng/dl. Fasting in obese subjects was associated with an increase in serum T2 to 6.9 ± 0.6 ng/dl from a basal level of 4.4 ± 0.4 ng/dl in the fed state (P < 0.01). Despite the fact that rT₃ levels may be elevated in amniotic fluid and that rT.t is expected to represent the major source from which extrathyroidal T₂ arises, T₂ levels were low in amniotic fluid, being undetectable (<2 ng/dl) in 9 of 19 samples; the mean (±SE) T₂ concentration in the 10 detectable samples was 5.4 ± 1 ng/dl. These data indicate T₂ is a normal component of serum and that the majority of serum T₂ is probably derived from peripheral conversion. Furthermore, these observations suggest that situations associated with elevated rT₃ levels (e.g. thyrotoxicosis and fasting) may also have increased T₂ values.