Studies were conducted to assess the effect of fetal thyroidectomy early in the third trimester on lung growth and surfactant maturation in the sheep. At 95–99 days of gestation, a fetal thyroidectomy was performed. The preparation was killed 36–45 days postthyroidectomy. The mean weights of the cerebrum, cerebellum, and combined heart and lungs were significantly smaller and the pituitary gland significantly greater in the thyroidectomized fetus. The mean tracheal fluid lecithin to sphingomyelin (L/S) ratio was significantly lower in the athyrotic fetus; however, there was no significant difference in the mean cortisol concentration or mean whole lung homogenate phosphatidic acid phosphohydrolase (PAPase) specific activity between the two groups. There was a significant direct correlation between the serum cortisol concentration and tracheal fluid L/S ratio and between the whole lung homogenate PAPase specific activity and serum cortisol level in the euthyroid group, which were not present in the thyroidectomized group. The mean lung DNA concentration was significantly increased and mean lung protein/DNA ratio significantly decreased in the thyroidectomized fetus. On histologic examination, the lung from the athyrotic fetus was hypercellular with thickened alveolar septae. The type II pneumocyte in the hypothyroid lung was less mature with fewer lamellar bodies or its precursors. These results suggest that in the ovine fetus, thyroid hormone deficiency during the last trimester of intrauterine development impairs lung growth and surfactant maturation.Speculation
The present results suggest that thyroid hormones are necessary for normal lung growth and surfactant maturation. In the human preterm infant, idiopathic respiratory distress syndrome is a major cause of morbidity and mortality. Stimulation of the hypothalamic-pituitary-thyroid axis in the fetus at high risk for developing idiopathic respiratory distress syndrome before delivery may accelerate surfactant maturation and decrease the sequelae from this syndrome.