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The pH-dependence of the binding of L-thyroxine to the serum proteins has been investigated by means of a dialysis procedure. A moderate increase in the binding of T4 was found when the pH varied from about 7 to 8.5. At more alkaline and more acid values a pronounced decrease in the ability to bind thyroxine was observed. The use of barbiturate buffer resulted in a very considerable increase in free non-protein-bound T4, whereas phosphate buffer did not interfere with the binding of the hormone. Tris buffer has also a thyroxine-releasing effect but this is far less pronounced than for barbiturate. Structural changes in the protein molecule were found to change its ability to bind T4. Urea denaturation of bovine serum albumin resulted in a decreased binding capacity, whereas denatured egg albumin and lactoglobulin exhibited an increase in binding property.

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It is known from paper electrophoresis experiments that triiodothyronine is bound to plasma proteins with considerably less affinity than is thyroxine. By a dialysis procedure it has been demonstrated that the percentage of the total free plasma triiodothyronine, i. e., triiodothyronine not bound to protein, is about fifteen times the corresponding percentage of free thyroxine.