Acute hypothyroidism has no effect on pulmonary vascular resistance

The effect of acute hypothyroidism on the pulmonary circulation was studied in 9 nonobese athyreotic patients by right heart catheterization at rest and during exercise. The patients were studied while they were hypothyroid 2 weeks after ceasing triiodothyronine treatment and while they were euthyroid on replacement therapy. At rest, pulmonary blood flow [4.0 +/- 0.6 l/min vs 5.8 +/- 1.0 l/min, p less than 0.01] and systolic pulmonary artery pressure [18 +/- 3 mmHg vs 23 +/- 2 mmHg, p less than 0.01] were lower when the patients were hypothyroid than when they were euthyroid. The mean and diastolic pressures in the pulmonary artery and the pulmonary capillary pressures were not different among the groups. Likewise, thyroid hormone levels had no significant effect on pulmonary vascular resistance [100 +/- 25 dyn-s-cm-5 vs 90 +/- 23 dyn-s-cm-5]. With supine exercise, pulmonary blood flow [10.1 +/- 1.6 l/min vs. 13.2 +/- 2.0 l/min, p less than 0.01], mean pulmonary artery pressure [25 +/- 6 mmHg vs 30 +/- 6 mmHg, p less than 0.02], and systolic pulmonary artery pressure [36 +/- 6 mmHg vs 44 +/- 8 mmHg, p less than 0.01] were lower when the patients were hypothyroid. The diastolic pulmonary artery pressure and the pulmonary capillary pressure were similar in both thyroid states. Again, thyroid deficiency had no effect on pulmonary vascular resistance [81 +/- 23 dyn-s-cm-5 vs 76 +/- 24 dyn-s-cm-5]. The lower systolic pressures in the pulmonary artery seen in hypothyroidism are probably due to the decreased systolic volume load of the pulmonary circulation.