Kinetics of triiodothyronine uptake by erythrocytes in hyperthyroidism, hypothyroidism, and thyroid hormone resistance.

We studied the initial uptake of L-T3 in erythrocytes isolated from normal subjects and patients with abnormal thyroid function. The uptake was saturable, with a mean maximum velocity (Vmax) of 3.60 pmol/min.10(8) cells and a Km of 248 nmol/L in normal subjects. Women had a somewhat lower Km but a similar Vmax compared to those in men. The Vmax was increased in patients with hyperthyroidism and decreased in those with hypothyroidism. Successful treatment lowered the initially high Vmax in hyperthyroid patients and raised the low Vmax in hypothyroid patients. One subject with the rare syndrome of thyroid hormone resistance had a Vmax in the upper normal range. The mean Km values of the hyperthyroid and hypothyroid groups and of the subject with thyroid hormone resistance were similar to that of the normal subjects. The results indicate that the initial saturable uptake of T3 by the human erythrocyte is increased in hyperthyroidism and decreased in hypothyroidism.