The uptake of [125I]T3 and [125I]T4 by human erythrocytes was studied. The erythrocytes were obtained from adult subjects (28-41 yr old) and suspended in a protein-free medium. The half-times of equilibration for both T3 and T4 were 6 min. At equilibrium, T3 was concentrated 55-fold inside the cells, while T4 was concentrated 40 times, but these accumulations were not dependent on either cellular ATP or the transmembrane Na+ gradient. The amounts of cell-associated thyroid hormones were 20 times (T3) and 17 times (T4) higher than the amounts of free extracellular hormones at 5 X 10(9) erythrocytes/mL (the blood concentration). Oligomycin and phloretin inhibited T3-saturable transport (but not T4 transport) independently of cellular energy. We suggest that thyroid hormones are concentrated by intracellular trapping. The rates of T3 and T4 efflux from preloaded erythrocytes were similar to the influx rates. The initial velocities of T3 (but not T4) uptake and efflux were 70% saturable. The uptake was specific because the unlabeled analogs T4, triiodothyroacetic acid, rT3, D-T3, and D,L-thyronine inhibited [125I]T3 uptake 60, 125, 160, 190, and 1600 times less, respectively, than did unlabeled T3. The kinetic parameters of T3-saturable uptake, Km, and maximum velocity were determined for three groups of subjects: newborns, 28 to 41-yr-old adults, and 76 to 90-yr-old adults. The Km (67 nmol/L in 28 to 41-yr-old adults) was not age dependent, BUT the maximum velocity was significantly higher in newborns than in adults. We conclude that T3 transport across the human erythrocyte membrane is mediated mainly by facilitated diffusion, whereas T4 transport results from free diffusion. Human erythrocytes might act as a circulating pool of thyroid hormones, especially T3 in newborns.