The present invention relates to a process for the preparation of the monosodium salt of the derivative 3,5-diiodo-O-[3-iodo-4-(sulphooxy)phenyl]-L-tyrosine (T3S) by starting from the corresponding phenolic compound, inthe presence of chlorosulfonic acid and dimethylacetamide as a solvent. Theso obtained T3S compound may conveniently be isolated in a pure form asa 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 ofsuch compound in tablets.Furthermore, the invention discloses non-radioactive immunoassays basedon T3S derivatives.
