Thyroid hormone depresses antioxidant enzyme maturation in fetal rat lung

The surfactant system and antioxidant enzyme system of the lung have chronologically similar developmental patterns, and the maturation of both systems is accelerated by glucocorticoid hormones. To investigate whether thyroid hormone might also stimulate the development of the antioxidant enzyme system as well as the surfactant system, we injected pregnant rats with triiodothyronine (T₃) or diluent. Fetal T₃ offspring demonstrated significantly elevated T₃ levels, had significantly increased lung tissue disaturated phosphatidylcholine (DSPC) and total phospholipid content, yet had significantly decreased activities of three lung antioxidant enzymes (AOE) (superoxide dismutase, catalase, and glutathione peroxidase). When dexamethasone was administered in combination with T₃, fetuses demonstrated increases in lung DSPC content but decreases in AOE of magnitude equivalent to or greater than that seen with T₃ alone. These findings indicate that thyroid hormone affects surfactant and AOE development in opposite ways and may have potentially harmful as well as beneficial effects on different aspects of lung development.