GOITROGENIC ACTION OF THYROXINE ADMINISTERED WITH PROPYLTHIOURACIL

ABSTRACT Rats were fed chow or a low iodine (standard) diet containing 0.02% propylthiouracil (PTU) for a year or longer. Over a critical range of dosage, 1-thyroxine (T4) potentiated the goitrogenic effect of PTU. The effect was greater with a chow diet than with the standard diet. Part of this difference could be attributed to the presence of more iodide in the chow, as iodide added to the standard diet increased the size of the goitres produced. The maximum effect was produced when 25 μg% or less of T4 was administered, and in high dosage (125 μg% in diet) inhibition of goitrogenesis occurred. Similar degrees of goitrogenesis took place when 0.02%. 0.05% or 0.125% PTU were fed supplemented with one concentration of T4. Thus the potentiation of goitrogenesis is related to the intake of thyroxine itself, and can most likely be explained by the action of the hormone on the adenohypophysis or higher centres, or on functions of the thyroid gland.