Many thyroid diseases can lead to pulmonary problems, including hypothyroidism, hyperthyroidism, nodular goiter, and thyroid cancer. Both hypothyroidism and hyperthyroidism cause respiratory muscle weakness and decrease pulmonary function. Hypothyroidism reduces respiratory drive and can cause obstructive sleep apnea or pleural effusion, while hyperthyroidism increases respiratory drive and can cause dyspnea on exertion. Compression of the trachea, which may be positional, can occur with nodular goiters and thyroid cancer, and the latter can metastasize to the lungs.
Depressed ventilatory drive — Some patients with hypothyroidism have alveolar hypoventilation . In the extreme case of myxedema coma, there can be marked hypercapnia . Severe hypothyroidism is associated with marked depression in hypoxic ventilatory drive and hypercapnic ventilatory drive, whereas less severe hypothyroidism (thyroxine replacement therapy stopped for three weeks) causes a moderate reduction in hypoxic ventilatory drive (figure 1 and figure 2) . In one study, patients with hypothyroidism had little increase in minute ventilation with PO2 values of 40 mmHg. The depression in hypoxic ventilatory drive, but not hypercapnic ventilatory drive, improves with thyroxine therapy.
Respiratory muscle weakness — Skeletal muscle myopathy occurs with hypothyroidism ; in animal studies, the proportion of type 1 fibers of the diaphragm and intercostal muscles decreased four weeks after total thyroidectomy . Respiratory muscle strength is reduced in patients with hypothyroidism, and improves with treatment; the reduction is caused by both a myopathy and neuropathy. In a study of six patients, maximal expiratory and inspiratory pressures were reduced and improved with treatment . In a study of 43 hypothyroid patients, respiratory muscle weakness correlated with the degree of hypothyroidism . The degree of weakness is usually mild to moderate, but there have been case reports of patients with marked weakness . With treatment, respiratory muscle strength improves [7,8].
Pulmonary function — Carbon monoxide diffusing capacity (DLCO) may be low and increase during treatment; in one study, the mean value was 63 percent of the predicted value before and 93 percent during . The reasons for the low DLCO and its improvement are unclear.
Non-obese hypothyroid patients have normal lung volumes, whereas obese hypothyroid patients have moderate reductions in vital capacity and lung volumes. Studies of hypothyroidism and pulmonary function and respiratory muscle strength have not reported DLCO values, but vital capacity improved with thyroxine treatment [7,8].