Pulmonary gas exchange in hypothyroidism

Abstract

In 7 normocapnic patients with hypothyroidism pulmonary gas exchange was measured before and after successful hormonal treatment. The results were compared with those of 8 healthy control subjects matched with the patients made euthyroid by treatment in relation to their age, sex, body height, and body weight.

Hypothyroidism in patients with normal arterial pCO₂ was found to be associated with a lung defect consisting of a very low physiological dead space and an arterial hypoxemia (low arterial pO₂ and high alveolar arterial O₂ gradient). This hypoxemia is due in part to an intrapulmonary right-to-left shunt that disappears during a short-term increase in tidal volume by connecting an external dead space. In spite of the fact that the patients were made euthyroid by treatment, a slight degree of arterial hypoxemia was always present. This hypoxemia may be a residual sign of hypothyroid lung disease. The latter may also cause the decrease in dead space volume and the intrapulmonary right to left shunt. It is possible that the hypothyroid lung disease (not recognisible by either clinical or radiological examination of the lungs) is related to the fact thatL-thyroxine plays an important role in the regulation of pneumocytes II and the synthesis of the normally functioning alveolar surfactant.