Propylthiouracil in human milk. Revision of a dogma.

Propylthiouracil (PTU) concentrations were measured in blood and milk from nine lactating women after oral administration of 400 mg of PTU. 1 1/2 h after PTU ingestion mean serum-PTU reached 7.7 microgram/ml and the mean concentration of PTU in milk reached only 0.7 microgram/ml. The mean total amount of PTU excreted during 4 h was 99 microgram--i.e., 0.025% of the administered dose. One of the suckling babies was studied for 5 months during which the mother received 200-300 mg of PTU daily: there were no changes in any of the thyroid parameters. PTU is not concentrated in human breast milk and recommended dosages to the mother result in minimal and presumably clinically insignificant doses to the suckling infant.