Abstract: In vitro method for assessing thyroid function based on an indirect estimation of the free thyroxine concentration of a sample of serum. Known amounts of a sample of serum to be tested and a radioactive labeled thyroxine solution are added to an alkaline crosslinked dextran gel column which serves to dissociate the serum thyroxine bound to thyroxine binding proteins. After washing the serum proteins from the column, a portion of the mixture of labeled and serum thyroxine is eluted from the column using a known amount of the serum sample and a known amount of eluting liquid containing thyroxine binding protein. The use of a portion of the serum sample normalizes the elution of free thyroxine against the thyroxine binding capacity of the serum sample.