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A competitive binding technique is described for the estimation of the thyroxine (T₄)-binding properties of serum proteins in dilute blood serum and lymph. When used in conjunction with an assay for total T₄ the following parameters can be estimated: the number of functionally different T₄ binding proteins, their individual association constants and binding capacities for T₄, the amount of T₄ which is bound to each binding species, and the concentration of unbound (free) T₄.

Both human and sheep serum have three functionally different T₄-binding proteins. The association constants for the three human proteins were 9·5 x 10⁹, 1·6 x 10⁸ and 3·1 x 10⁵ 1/mol for T₄-binding globulin (TBG), T₄-binding prealbumin (TBPA) and serum albumin, respectively. The corresponding sheep proteins, TBG, TBP-2 and albumin, had association constants of 8·9 x 10⁹, 1·4 x 10⁸ and 3·5 x 10⁵1/mol. Human TBG had a mean binding capacity of 21·3 µg/100 ml and that of ovine TBG was 12·8 µg/100 ml. The other specific binding proteins (TBPA in man and TBP-2 in sheep) had mean binding capacities of 307 and 359 µg/100 ml respectively.

Two functionally different T₄-binding proteins were identified in rat serum.

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				J. E.M. Midgley Direct and Indirect Free Thyroxine Assay Methods: Theory and Practice Clin. Chem., August 1, 2001; 47(8): 1353 - 1363. [Abstract] [Full Text] [PDF]