Pressure-volume characteristics of whole lungs were measured in euthyroid rats and in rats fed 0.4% desiccated thyroid for eight weeks. The lungs were degassed by incising the diaphragm after the animals had breathed 100% oxygen for ten minutes. The pressure-volume characteristics were measured by inflating and deflating the lungs at a rate of 3.5 cc/min. Total lung capacity (TLC) was considered to be that volume of air required to produce a transpulmonary pressure of 30 cm H2O. At TLC there was 35% greater lung volume in the thyroid-treated animals than in their littermate controls. Similar results were observed in saline-filled lungs. Alveolar surface are (Sa) increased from 0.28m2 in the lungs of control animals to 0.75m2 in lungs of thyroid-treated animals. There was an 85% increase in the alveolar surface density (SVa) in the thyroid-treated animals. These results, obtained by morphometric analysis, suggest that greater lung volume in the thyroid-treated animals resulted from alveolar hyperplasia or "partitioning."

Studies were conducted to assess the effects of thyrotoxicosis on the lung of the adult adrenalectomized rat. The animals were divided into four groups of seven animals each: Adrenalectomized animals receiving desiccated thyroid in their food for 8 weeks, adrenalectomized animals, and sham-operated animals receiving regular laboratory chow. Unoperated animals were used as controls. There were no differences in the pressure-volume curves or morphometric parameters between unoperated-control and sham-operated animals; therefore they will be collectively reported as control animals. Lung volume in the thyroidtreated animals showed an increase of 19% in pressure-volume studies when compared with the other groups. Lung volume determined morphometrically demonstrated a 19% increase in volume when adrenalectomized thyroid-treated rats were compared with control animals, and a 29% increase compared with adrenalectomized animals. The thyroid-treated animals also showed an alveolar surface density (SJ increase of 58% over control and 28% over adrenalectomized animals. In addition, alveolar surface area (S,) of the treated animals was 89% greater than control and 65% greater than adrenalectomized animals. The marked difference in the surface-to-volume ratio indicates that alveoli were smaller and more numerous in the lungs from the thyroid-treated animals. These results are readily apparent in examination of light microscopic sections of the lung specimens from the four groups of animals. These findings indicate that chronic thyroid administration results in a significant increase in lung volume, which we attribute to increased alveolar partitioning.