Effect of propylthiouracil dose on serum thyroxine, growth, and weaning in young rats

The antithyroid drug propylthiouracil (PTU) is a potent inducer of hypothyroidism in the rat. To evaluate the effects of PTU on serum thyroxine (T4) concentration, growth, and weaning progression during development, five doses of PTU (0.0001, 0.0005, 0.001, 0.005, and 0.01 g/100 ml) were administered to infant rats via drinking water of the dam. The highest dose, 0.01%, is commonly used in developmental studies. The results indicate that 0.001% PTU creates hypothyroid pups without the debilitating characteristics of pups raised on 0.01% PTU. A second experiment examined the effects of 0.001% PTU on serum T4 concentration, growth, and weaning progression during the fourth postnatal week. Serum T4 concentration was depressed throughout the study period to 25% of controls. The hypothyroid pups continued to grow, although they were significantly smaller than untreated controls. Weaning was initiated by postnatal day 22 and completed on day 29. For normal untreated pups, weaning is initiated by day 17 and completed by day 26. Thus hypothyroidism delays but does not abolish the weaning process.