Effects of Thyroid Hormone Treatment on Diaphragmatic Efficiency in Subjects With Nonthyroidal Illness Syndrome and on Ventilation

BACKGROUND: Several respiratory abnormalities can be present in primary hypothyroidism and can be reversed with adequate hormone treatment. However, the role of thyroid hormonereplacement therapy on the respiratory system in patients with nonthyroidal illness syndrome is still unclear. This physiologic study evaluated the effect of thyroid hormonetreatment on respiratory muscle function in subjects with nonthyroidal illness syndrome and while on mechanical ventilation. The primary end point was neuromechanicalefficiency, which provides an estimate of the efficiency of diaphragmatic contraction. Secondary end points were the transdiaphragmatic pressure-time product and the swing of the electrical activity of the diaphragm, which reflect the workof breathingand inspiratory effort, respectively. METHODS: Fifteen subjects on mechanical ventilation for >48 h and with a diagnosis of nonthyroidal illness syndrome who had a failed spontaneous breathing trial, received intravenous triiodothyronine. The hormone was administered as an intravenous bolus of 0.4 g/kg triiodothyronine, followed by continuous perfusion at 0.6 g/kg for 24 h. Neuromechanicalefficiency was calculated as the ratio between the drop in airway pressure during an expiratory occlusion and the corresponding electrical activity of the diaphragm peak. Recordings were taken at baseline and after 3, 6, and 24 h. RESULTS: After study completion, free triiodothyronineserum concentrations increased in all the subjects (mean ± SD increase, 0.84 ± 0.34 pg/mL). Neuromechanicalefficiency showed no significant changes throughout the study (mean ± SD baseline, 1.40 ± 0.87 cm H 2 O/μV; 3 h, 1.28 ± 0.64 cm H 2 O/μV; 6 h, 1.33 ± 0.87 cm H2O/μV; 24 h, 1.41 ± 0.96 cm H 2 O/μV). Similarly, no variations in transdiaphragmatic pressure-time product per min (mean ± SD baseline, 238.1 ± 124 cm H 2 O × s/min; 3 h, 242.5 ± 140.3 cm H 2 O × s /min; 6 h, 247.5 ± 161.7 cm H 2 O × s/min; 24 h, 281.2 ± 201.2 cm H 2 O × s/min) or swing of electrical activity of the diaphragm (mean ± baseline, 20.9 ± 13.1 μV; 3 h, 17.2 ± 8.3 μV; 6 h, 17.4 ± 11.3 μV; 24 h, 20.3 ± 13.7 μV) were observed during hormone administration. CONCLUSIONS: In the subjects on mechanical ventilation who were admitted to the ICU with non-thyroidal illness syndrome, thyroid hormonereplacement treatment did not yield any benefit on respiratory muscle function when assessed by neuromechanicalefficiency, which indicated that, in these subjects restoring normal levels of serum thyroid hormonesis debatable. (ClinicalTrials.gov registration NCT03157466.)