An iodide membrane electrode, in conjunction with rat liver microsomes, is used to measure thyroxine at sub-micromolar levels. The microsomes serve as biocatalytic reagents which selectively remove iodide from the hormone. The amount of iodide liberated, as measured by the electrode, is proportional to the initial thyroxine concentration. The electrode can further be employed to directly monitor the rate of deiodination of the hormone with advantages of speed and convenience over existing techniques.