Serum protein binding of propylthiouracil.

Serum protein binding of propylthiouracil (PTU) was measured by ultrafiltration in healthy and hyperthyroid patients. The serum protein binding in 12 euthyroid subjects was 76.2 +/- 1.2% (mean +/- s.d.), not significantly different from the values in 10 hyperthyroid patients: 76.6 +/- 1.3% Binding was unaffected by incubation time and temperatures between 25 and 37 degrees C, but increased from 76.5 to 79.1% when pH changed from 7.4 to 7.9. PTU is predominantly bound to albumin with two classes of binding groups with different number of binding sites and affinity. Displacement experiments showed interaction with acetylsalicylic acid, warfarin and phenylbutazone, but not with antipyrine and nortriptyline or other basic drugs.