Method for assessing thyroid function

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. By comparing the percent labeled thyroxine retained in the column in testing the serum sample to the percent retained in testing a reference sample representing a known thyroid condition, a free thyroxine equivalent value is obtained which is useful in differentiating hyperthyroid, euthyroid, and hypothyroid conditions. The method can be modified to provide a means for determining the total thyroxine concentration of the serum sample, as well as its free thyroxine equivalent.