Abstract: A "single tube" thyroid hormone assay is provided whereby endogenous thyroid hormone is initially separated from a serum sample with an acid reagent and thereafter a tracer amount of radioactive labelled thyroid hormone is allowed to competitively bind with antibodies for thyroid hormone. The resulting free hormone (not bound to antibody) is separated from the hormone bound to such antibodies by contacting the solution with an aqueous solution of water-soluble sulfate salt containing a minor but effective amount of added animal serum to cause precipitation of the hormone bound to the thyroid hormone binding protein material. Thereafter, either the free hormone in supernatant fluid or the bound hormone that has precipitated is counted with a scintillation counter.