The feasibility of efficient gas chromatographic analysis of thyroid hormones has recently been demonstrated. Various procedures for the initial extraction of the hormones from serum have been investigated. Solvent extraction followed by adsorption and elution from Sephadex LH-20 columns results in a relatively clean isolation of the hormones. The derivatives employed were the O,N dipivalyl methyl esters which are chromatographed on small packed columns containing OV-17 at the stationary phase. Excellent sensitivity is obtained using electron capture detection.