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.

Respiratory Manifestations of Hypothyroidism: A Systematic Review

“…Another study failed to reproduce these results (21), may be due to a sample size half of that used in the previous study. In a non-blinded study, including patients with overt or subclinical hypothyroidism, the end-tidal CO 2 was reduced (22). This finding is surprising, but most likely explained by hyperventilation at the time of examination.…”

Spirometric Evaluation of Pulmonary Expiratory Flow Volume and Flow Rate in Patients with Clinical Hypothyroidism at a Tertiary Care Hospital, West Bengal, India: A Case-control Study

“…Diaphragmatic dysfunction may be experienced by the patients of hypothyroidism that can range from mild forms having decreased tolerance to physical effort to severe forms of diaphragmatic weakness that may mimic diaphragmatic paralysis [6]. In hypothyroid patients, the respiratory rate is reduced, resulting in hypoventilation and mild hypercapnia [7,8]. Hypercapnia stimulates the respiratory centre, which is unable to respond to an increased need for ventilation due to low thyroid hormone levels.…”

Evaluation of sleeping energy expenditure using the SenseWear Armband in patients with overt and subclinical hypothyroidism

“…It occurs in two forms: overt and subclinical. Overt hypothyroidism is characterized by a marked elevation of the thyroidstimulating hormone (TSH) level with decreased levels of free throxine (f T4) and free triiodothyronine (f T3), whereas subclinical hypothyroidism (sHT) is de ned as having serum f T4 levels within the normal range and elevated supra normal serum TSH levels [1][2][3]. yroid activity is related to energy expenditure (EE), and increased thyroid activity is able to increase basal metabolism.…”