Increase of serum free thyroxine after propylthiouracil in vitro or its peroral administration to rats

After the addition of 6-propyl-2-thiouracil (PTU) (2.4 mg/ml) in vitro to the normal human serum a two fold increase of dialyzable free thyroxine fraction (%FT4) was found. The addition of the same PTU dose to the serum of intact Wistar rats caused up to 4 fold increase of this fraction. After the addition of increasing doses of PTU (40--400 ng/ml) a linear increase of FT4 was found in human serum, while in rat serum a nonlinear dose-response relationship was observed. After a single peroral administration of PTU to rats (42 mg PTU per animal) the serum PTU level reached the maximum (55 ng PTU/ml) after one hour and then decreased very slowly. The free thyroxine fraction also increased up to 150% of control value, but this maximum increase was observed one hour to later then the maximum PTU level. Then the values of %FT4 rapidly decreased to the initial value. Such effect of PTU in vivo was about twofold greater than that obtained in the rat serum in vitro under the same actual PTU level. The results were compared with similar effect of thiocyanate reported previously and it was shown that the effect of PTU both in vitro and in vivo is greater than that of equimolar levels of thiocyanate.