Picogram analysis of free triiodothyronine and free thyroxine hormones in serum by equilibrium dialysis and electron capture gas chromatography.

A methodology involving equilibrium dialysis and gas chromatography has been developed to measure concurrently the concentration of the dialyzable (i.e., free) fractions of thyroxine and 3,5,3'-triiodothyronine in serum. The electron capture response of the N,0-diheptafluorobutyryl methyl ester derivatives of these hormones allows their detection and quantitation in the picogram range (detection limit approx. 0.2pg). Purification of the dialysate is achieved using a cation-exchange column which is pretreated with diiodothyronine. Adsorption problems on the glassware are eliminated by silanization. The methodology also resolves 3,3',5'-triiodothyronine (reverse T3).