Thyroid hormones in the adult rat lung

Thyroid hormones have been postulated to be involved in lung development and the regulation of the surfactant system. This study was designed to determine the concentration of thyroxine and of triiodothyronine in adult rat lung. Tissue-to-plasma ratios of thyroxine and triiodothyronine were determined in adult rats by the equilibrium distribution of radiolabeled hormones. Extracellular thyroid hormone space was estimated by equilibrium distribution of radiolabeled bovine serum albumin. The mean +/- SE ratio of lung triiodothyronine to plasma triiodothyronine, corrected for extracellular space, as 2.57 +/- 0.40, indicating appreciable concentration of this hormone. In contrast, lung thyroxine ratios indicate that this hormone is virtually excluded from adult lung parenchyma. Such results support the hypothesis that thyroid hormone may have a significant role in the regulation of the surfactant system.