[en] The concentrations of the serum free T₃(FT₃), free T₄(FT₄), total triodothyronine (TT₃), total thyroxine (TT₄) and thyrotropin (TSH) are determined for 355 cases of normal persons, pregnant women and various thyropathetic patients. The normal values of FT₃ and FT₄ are 2.0-8.5 pmol/l, and 9.5-26.5 pmol/l, respectively. Neither FT₃ nor FT₄ is affected by the thyroxine combined with globulin (TBG), which is of unique diagnostic value for those with variable TBG (such as pregnant women hyperthyroidism, hypothyroidism, etc.), FT₃ and FT₄ are the most sensitive indices for diagnosis of hyperthyroidism, and hypothyroidism, respectively. In addition, FT₃ and FT₄ can greatly contribute to the observation of curative effectiveness under treatment