Ineffective Cardiorespiratory Function in Hyperthyroidism1

Dyspnea on exertion is a common complaint in hyperthyroidism, and this thyroid dysfunction has been implicated as a primary cause of impaired effort tolerance. Using spirometry and spiroergometry, 42 patients with untreated hyperthyroidism were examined, and the condition was controlled 7 days later under propranolol monotherapy, as well as after 6 months in euthyroidism. While hyperthyroid, reduced forced vital capacity and tidal volume at the anaerobic threshold (AT) were observed in comparison to euthyroidism. Decreased oxygen (O2) pulse at AT (7 +/- 0.4 vs. 9.1 +/- 0.4 mL/beat, P = 0.0012) and at maximal exercise was noted in hyperthyroidism and was enhanced under propranolol (8.9 +/- 0.4 mL/beat, P = 0.0001). During exercise, the increment of minute ventilation (16.1 +/- 0.7 vs. 20.2 +/- 1.0 L/min, P = 0.0015), O2 uptake (9 +/- 0.5 vs. 11.4 +/- 0.5 mL/min/kg, P = 0.0022), O2 pulse (4.0 +/- 0.3 vs. 5.6 +/- 0.3 mL/beat, P = 0.0001), and heart rate (53 +/- 2 vs. 65 +/- 3 beat/min, P = 0.0004) was markedly lower in hyper- vs. euthyroidism. Work rate at AT and at maximum was reduced in hyper- vs. euthyroidism (107.4 +/- 3 vs. 141.1 +/- 4 watt, P = 0.0001). Negative correlations between free T3 and O2 pulse at AT (r = -0.59, P = 0.0005), delta O2 uptake (r = -0.54, P = 0.0007), delta minute ventilation (r = -0.48, P = 0.0007), and maximal work rate (r = -0.62, P = 0.0001) were noted. In hyperthyroidism, analysis of respiratory gas exchange showed low efficiency of cardiopulmonary function, respiratory muscle weakness, and impaired exercise capacity, which were reversible in euthyroidism.