A method for quantitative analysis of low molecular weight components which are contained in a protein-containing liquid sample originating from body fluids and are capable of binding to said protein, characterized by incorporating into the liquid sample a compound having the formula (I):X--(Y).sub.p --SO.sub.3 M (I)in which M is hydrogen ion, an alkali metal ion, or ammonium ion; X is a straight or branched alkyl, alkenyl, fluorinated alkyl, or fluorinated alkenyl group containing at least 6 carbon atoms, Y is a divilent organic residue; and p is 0 r 1. The compound represented by formula (I) liberates thyroid hormone from bound protein to facilitate analysis.