Measurement of free thyroid hormones in serum by dialysis and gas chromatography

We describe a simultaneous, absolute analysis for free (dialyzable) triiodothyronine and thyroxine in serum by the sequence: dialysis, internal standard addition to the dialysate, cation-exchange chromatography, derivatization, and gas chromatography with electron capture detection. The detection limit is 0.2 pg and the precision (repeatability) in the range of 4 to 20 ng/liter is 1.6%. Mean values for free T3 and T4 in 29 euthryoid sera were 4.2 and 18.9 ng/liter, respectively, in close agreement with accepted values. Correlation coefficients for the two compounds were 0.99 and 0.97, respectively, when this procedure was compared with a method involving tracer dialysis and radioimmunoassay. A chromatographic peak was seen for free 3,3',5'-triiodothyronine in the analyses of two of eight hyperthyroid sera, and, with a larger sample volume, from a euthyroid serum. The new method is a potential reference and research procedure for analysis of free iodothyronines.