End-tidal CO2 levels lower in subclinical and overt hypothyroidism than healthy controls; no relationship to thyroid function tests

Abstract

BackgroundHypoventilation is a frequently suspected complication of hypothyroidism.ObjectiveIn this study we examined the hypothesis that changes in alveolar ventilation, as measured by end-tidal carbon dioxide (Et-CO2), differ between patients with mild (subclinical) and overt (clinical) thyroid hormone deficiency, and both differ from healthy control subjects.MethodsA total of 95 subjects, including 33 with subclinical hypothyroidism (an elevated thyroid-stimulating hormone (TSH) level and a normal thyroxin (fT4) level), 31 with overt hypothyroidism (elevated TSH and decreased fT4), and 31 healthy controls. All subjects were female and were evaluated clinically by an endocrinologist for evidence of thyroid disease and categorized on the basis of thyroid hormone levels. Et-CO2 was measured using a capnograph. Et-CO2 levels were measured three times and the mean value was considered as the mean level for the individual.ResultsMean Et-CO2 values of the subclinical hypothyroidism group were significantly lower than those of the healthy controls (31.79 ± 2.75 vs 33.81 ± 2.38; P = 0.01). Moreover, mean Et-CO2 values for the overt hypothyroidism group were significantly lower than those for healthy controls (32.13 ± 3.07 vs 33.81 ± 2.38; P = 0.04). There was a significant correlation between Et-CO2 values and TSH levels (r = −0.24; P = 0.01). However, Et-CO2 values were not correlated with fT4 levels (r = 0.13; P = 0.20).ConclusionsAlveolar ventilation, as inferred from lower Et-CO2 levels, is higher in subjects with subclinical hypothyroidism and overt hypothyroidism (lower Et-CO2) than in healthy controls. Furthermore, Et-CO2 levels have no relationship to the levels of TSH or fT4. The lower Et-CO2 in these patients with hypothyroidism, particularly at the subclinical stage, suggests presence of hyperventilation, which may be related to direct effect of TRH on respiratory center or to local changes within the lung.