Effect of thyroxine on permeability of the neonatal rat lung to drugs.

The effect of thyroxine (1-2 microgram/g i.p. on days 1-4 after birth) on development of adult-type permeability characteristics in the lung was investigated in rats 6-18 days of age. In both treated and control rats, the lipid-soluble drug procaine amide was absorbed at similar rates in all age groups. In contrast, in thyroxine-treated animals, pulmonary absorption rates for the lipid-insoluble compounds mannitol and p-aminohippuric acid declined to adult levels at age 6-12 days, 6 days earlier than in controls. Thyroxine treatment thus accelerates development of adult permeability characteristics in the pulmonary epithelium.