Control of breathing in patients with short-term primary hypothyroidism

Abstract

In 8 patients (3 men and 5 women) with short-term primary hypothyroidism before and during replacement therapy, and in an age-matched control group (9 men and 7 women), we assessed maximal inspiratory muscle force (Pimax) and the ventilatory control system at neural (EMG), muscular (P0.1), and ventilatory (VE and Vt/Ti) levels. While hypothyroid, patients exhibited a significantly lower Pimax. During a CO2 rebreathing test, hypothyroid patients exhibited similar diaphragmatic electromyographic (EMGd) and ventilatory (VE) response slopes to increasing end-tidal CO2 tension (delta EMGd/delta Pet CO2 and delta VE/delta PetCO2), but significantly less delta P0.1/delta PetCO2 (p less than 0.05) and delta(Vt/Ti)/delta PetCO2 (p less than 0.05) response slopes. During replacement therapy with L-triiodothyronine (L-T3), Pimax (p less than 0.05), delta P0.1/delta PetCO2, and delta(Vt/Ti)/delta PetCO2 response slopes were found to be significantly increased (p less than 0.05 for both) while neither EMGd nor VE response changed significantly. We concluded that short-term hypothyroidism does not seem to be associated with blunted neural inspiratory output (EMGd), the respiratory control system seems to be affected mostly at a peripheral (muscular) level, and L-T3 increases both force (P0.1 and Pimax) and velocity (Vt/Ti) of inspiratory muscle contraction.

Respiratory Manifestations of Hypothyroidism: A Systematic Review

“…One study, with 43 overt hypothyroid patients (23), found that the pulmonary function improved after initiation of LT4, despite the fact that it was not affected at baseline. Other studies with fewer patients could not reproduce this finding (24,(29)(30)(31), which might be due to limited power.…”

“…Nine of the eleven studies have study designs with considerable risk of bias (Table 1). Three studies addressed the impact of overt hypothyroidism on the diaphragmatic and abdominal muscle strength, and offer contradictory results (21,23,24). Two studies, one with 43 patients (23) and another with patients (24), found that the diaphragmatic muscle strength improved by either LT4 or LT3 treatment.…”

“…A third study, including 20 patients, found no improvement after LT4 treatment (21). All three studies included a limited number of participants and can therefore not be the foundation of strong conclusions (21,23,24).…”

“…Apart from these three reports, the majority of studies showed no impact of thyroid dysfunction on pulmonary function when using pulmonary function tests (18,24,(29)(30)(31), most likely because of small sample sizes (21-45 patients). The lack of a control group -with a few exceptions (18,24,30) -is clearly another limitation of these studies. One study, with 43 overt hypothyroid patients (23), found that the pulmonary function improved after initiation of LT4, despite the fact that it was not affected at baseline.…”

CO2 rebreathing: an undergraduate laboratory to study the chemical control of breathing

“…Studies of the chemical control of breathing are also commonly used for research purposes in human subjects (19,20,46), which suggests that they are an appropriate exercise for students to enhance their understanding of the curriculum material and pursue studentdirected learning strategies. Although the attribution of chemoreflex sensitivity and threshold to central or peripheral chemoreceptors is complex (3), rebreathing techniques are widely used to study control in respiratory (10, 51), neurological (57, 60), musculoskeletal (27), endocrine (22,28,34,48,53,61), and highaltitude conditions (54, 55). In addition to providing a novel learning environment for the comprehension of physiological/ pulmonary/neuroscience concepts, a student laboratory also has the potential to expose students to unique experiences [e.g., respiratory sensations (38) associated with elevated arterial CO 2 levels (8, 37)] as well as the challenges associated with experimental protocols.…”