Yukiko Yabu

A new type of albumin with predominantly increased binding affinity for 3,3′, 5-triiodothyronine in a patient with Graves’ disease

A new type of serum albumin, that shows a markedly enhanced binding activity for 3,3′, 5-triiodothyronine (T3), a somewhat increased activity for thyroxine (T4), and a normal activity for 3, 3′, 5-triiodothyronine (rT3) is described. This albumin was found in a patient with Graves’ disease. After successful subtotal thyroidectomy, the existence of abnormal binding activity for T3 was suspected in this patient because of persistently increased total T3 concentrations in spite of elevated thyrotropin levels. Although free T3 and T4 concentrations measured by radioimmunoassay using commercial tracer analogue kits were markedly increased, those measured by equilibrium dialysis were within normal ranges. Electrophoretic studies revealed that these abnormalities were due to the markedly increased T3 binding activity by the serum albumin; that for T4 was also slightly increased. Scatchard plot analysis revealed that the association constant (Ka) for T3 of the patient’s albumin was 5.1 × 106/M (normal pooled albumin; 6.2 × 105/M), and those for T4 and rT3 were 5.2 × 106/M and 2.7 × 106/M, respectively (normal pooled albumin; 2.1 × 106/M for both T4 and rT3). The increased binding of albumin to T3 and T4 was markedly inhibited by barbitone, and 8-anilino-1-naphthalenesulfonic acid. These characteristic features, and erroneously high values of free T3 and T4 concentrations measured by tracer analogue kits were similar to those seen in patients with familial dysalbuminemic hyperthyroxinemia, which have been previously reported. These findings strongly suggest that this albumin is a new variant in various dysalbuminemic syndromes, and the abnormal binding of iodothyronines moieties in these syndromes are not biochemically identical.