Partition of thyroid hormone among erythrocyte cytosol, erythrocyte membrane and human plasma binding sites.

The partition of thyroid hormone among binding sites on plasma proteins, erythrocyte (RBC) membranes and cytoplasmic proteins has been examined in the context of recent direct estimates of intracellular RBC thyroid hormone content. Isotopic T3 and T4 uptakes by washed RBCs are identical (greater than 80% of tracer at 4 hr), but RBC binding when RBCs are suspended in plasma favors T3 (10% uptake vs. 2.5% for T4). The RBC cytosol: plasma T3 ratio is 1 : 21.