Abstract: The invention relates to a process for preparing tri-iodo-, di-iodo- and mono-iodothyronines and tri-iodo, di-iodo and mono-iodothyroacetic acids, respectively from thyronine, triiodo or diiodothyro- nine and tetra-, tri- or diiodothyroacetic acid, by electrochemical reduction at controlled potential, in an aqueous medium rendered alkaline by the addition in particular of a tetraalkylammonium hydroxide, whereby either the iodine in 5 position or the iodine in 3 position is selectively reduced, depending on the potential at which operation is carried out, and which is determined previously by plotting a polarograph of the starting substance.