Propylthiouracil Other names: 6-n-Propylthiouracil

Propylthiouracil is an antithyroid drug that depresses the formation of thyroid hormone. This is effected by interference both with the incorporation of iodine into tyrosyl residues and the coupling of such residues to form iodothyronines. Propylthiouracil achieves these actions by the inhibition of the enzyme peroxidase.

Its effects are only manifest after a latent period of up to 3 or 4 weeks because all the preformed hormone has to be used up before circulatory concentrations will fall.

Propylthiouracil is rapidly absorbed from the gut with average peak blood levels about one hour after administration of an oral dose. Between half and three quarters of the oral dose is bioavailable, due to incomplete absorption or rapid first pass metabolism by the liver. Most is excreted as the glucuronic acid conjugate in the urine. Plasma half life is 1-3 hours, the volume of distribution approximately 30l, with about 80% plasma protein binding.

Propylthiouracil crosses the placenta and is secreted in breast milk reaching about 10% of the serum concentration.

There have been no systematic long term animal toxicology studies performed. Some short term studies carried out when this class of drugs was introduced (approx 45 years ago) show that rats and rodents treated with high doses of propylthiouracil and made markedly hypothyroid will frequently develop thyroid hyperplasia, adenomas, carcinoma, pituitary adenomas and parathyroid hyperplasia.