The present invention relates to a process for the preparation of the mono sodium salt of the derivative 3,5-diiodo-O-[3-iodo-4-(sulphooxy)phenyl]-L-tyrosine (T3S) by starting from the corresponding phenolic compound, in the presence of chloro-sulfonic acid and dimethylacetamide as a solvent. The so obtained T3S compound may conveniently be isolated in a pure form as a solid in good yields. The present invention further relates to the process for T3S preparation, wherein the starting reagent is T2 and further comprising the formulation of such compound in tablets. Furthermore, the invention discloses non-radioactive immunoassays based on T3S derivatives.