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When plasma proteins are diluted with buffer the ionic strength and ionic composition of that buffer affects the interactions between thyroxine (T₄) and its plasma protein-binding sites. Increases in phosphate, chloride or barbiturate ion concentration from 50 to 200 mmol/l caused a significant decrease in the affinity of plasma proteins for T₄, and a concurrent increase in the concentration of unbound T₄. These results cannot be completely accounted for by changes in ionic strength since at the same ionic strength different anions caused quantitatively different effects on unbound T₄ concentration. The degree of depression of T₄ binding by the three anions studied was in the order barbiturate > chloride > phosphate.

The results of a systematic study on the composition of diluent buffer systems indicated that when a 50 mM-sodium phosphate–100 mM-NaCl buffer (pH 7·4) was used as a plasma diluent, there were unlikely to be gross changes in the T₄-binding properties of plasma proteins with dilution.