Background. Hypothyroidism (H), first of all, primary, is important clinical problem. According to statistical data, H affects up to 5% of the population in Europe, and another 5% of the population may have undiagnosed insufficiency of thyroid (T) function [1]. In the United States, according to the National Health and Nutrition Examination Survey, the prevalence of H is 4.6% [2]. In Ukraine the number of cases of T diseases increased by 5 times during 2015-2020, which led to a high level of disability. The incidence of H in the national population increased by 20.3% [3], and in the period 2007-2017 - increased by 1.7 times [4]. Over the past two decades, the number of scientific works devoted to T pathology has increased significantly [5]. Traditionally, the clinical problems of H are considered as cardiovascular and psychoneurological disorders, renal dysfunction, and disorders of bone mineral density [5]. However, recently the scientific community has also focused on RS disorders in T diseases, proving that H is more associated with increased pulmonary morbidity and overall mortality than hyperthyroidism. H, even in its subclinical variant, has many mechanisms of influence on the function of the respiratory system (RS), which explains the presence of respiratory problems at various physiological levels of the human body.
Purpose: based on the analysis of modern scientific literature, to analyze views on the thyroid hypofunction impact on the development of pathology of the structure and function of the RS and to emphasize the importance of H for the course of RS disorders.
Materials and methods: PubMed and Google Scholar were searched for publications of research results evaluating the impact of H on the occurrence and course of respiratory diseases (RD). Search terms included "hypothyroidism", "thyroid underproduction", "respiratory diseases", "respiratory tract diseases", "respiratory system disorders" and others. The search was limited to studies published in English in peer-reviewed journals from 1990 to the present.
Results: H is a clinical syndrome caused mainly by a decrease or loss of T function, accompanied by a violation of the production of T hormones [6, 7, 8]. Deficiency of thyroxine (T4) and the resulting insufficient activity of triiodothyronine (T3) in body cells lead to a general slowing down of metabolic processes and the development of interstitial edema as a result of the fibronectin and hydrophilic glycosaminoglycans deposition in the subcutaneous tissue, muscles and other tissues [8].
Violation of body functions in H can lead to changes in organs and tissues, including a decrease in metabolism, thermoregulation, activity of enzyme systems, and gas exchange. There may also be a violation of protein breakdown and synthesis, changes in the colloidal structure and hydrophilicity of tissues. An excess of vasopressin can lead to retention of water and sodium in tissues, formation of myxedema, and difficulty in lymphatic drainage [9, 10, 11]. This can cause functional changes in the central nervous system, vascular reactivity, hypothyroid myopathy and neuropathy. H can also reduce tissue resistance to infectious agents through metabolic immune depression. Due to this, changes in the functions of the RS organs are possible.
H affects the maxillofacial system and the upper respiratory tract. Such changes include chronic inflammation of the periodontium [12, 13], a decrease in lysozyme activity and a change in the composition and properties of saliva, as well as mucinous swelling of the mucous membranes of the lips, tongue, oral cavity, and larynx. Patients develop nasal breathing dysfunction, nasal discharge, and vasomotor rhinitis.
Other effects of H can include polypoid dystrophy and swelling of the vocal cords, which affect the voice timbre and cause hoarseness, a dry cough and symptoms of the upper respiratory tract inflammation. The presence of goiter can cause narrowing and compression of the airways, which also lead to shortness of breath [14].
With H, changes in the control of ventilation and breathing rhythm can be observed. There is a decrease in the activity of the respiratory center and the development of restrictive dysfunction of the lungs, which leads to bradypnea. Against the background of the above, the syndrome of obstructive apnea and snoring in sleep may occur due to mucoid swelling of the tongue, pharyngeal muscles and weakening of lung ventilation.
Disturbances in the alveolar zone of the lungs, such as interstitial edema and reduced alveolar ventilation, can cause hypoxia and significant hypercapnia, which lead to the development of respiratory failure, which can progress to hypothyroid coma [15, 16].
Clinical symptoms of RS damage in H can appear already in newborns and in the first months of life as difficulty breathing, "rough cry", stridor and other symptoms. In the future, children may become more prone to respiratory diseases.
In adults, such manifestations as chronic cough, unexplained airflow obstruction, primary pulmonary hypertension, expiratory dyspnea of various degrees, impaired external breathing, and others are possible. Without treatment, these manifestations can worsen and disrupt the patient's life quality [17, 18, 19]. Even in patients with subclinical H, such violations of the RS function as a decrease in Forced Vital Capacity (FVC), Forced Expiratory Volume in one second (FEV1), Peak Expiratory Flow Rate (PEFR), Forced Expiratory Flow (FEV25%–75%) can be detected [17, 20].
It should be noted that H can be disguised as somatic or even mental diseases, which complicates its diagnosis [21].
Timely diagnosis and treatment are important not only for H, that can help reduce respiratory problems, but for RS disorders also.
Chest X-ray remains a valuable diagnostic tool. Research shows that patients with H can develop hydrothorax, a decrease in the size of the lungs, which is clearly visible on an X-ray [17, 19]. Accumulation of fluid in the cavities, including the pleural cavity, can be caused by other pathological conditions, not only H. Nevertheless, the researchers found certain laboratory features of hydrothorax due to H. Effusions caused solely by hypothyroidism turned out to be a real entity. These effusions were usually small, on the borderline between exudates and transudates, and had little evidence of inflammation. According to the researchers, such effusions have little clinical significance, but require further research in order to clarify the diagnosis [22].
There is a rather limited number of studies on bronchoscopy in H. Some studies show that the results may include the following: expansion or decrease of the lungs’ size, which may indicate a change in their structure, an increase in the thickness of the bronchial walls, restriction or complete blockage of the airways, swelling of tissue in the bronchi, which indicates inflammatory processes [23].
When examining patients with hypothyroidism during exercise, a significant decrease in oxygen consumption (VO2), carbon dioxide excretion (VCO2), minute lung ventilation, respiratory volume, and pulse oximetry indicators [17] was found, which indicates deterioration of gas exchange and hypercapnia.
Conclusions:
- hypothyroidism provokes anatomical and functional disorders in all parts of the respiratory system, which are accompanied by significant changes in the depth, rhythm of breathing and gas exchange, which cause shortness of breath, obstructive sleep apnea syndrome and have a negative impact on the patients’ health and life quality;
- the progression of changes in the respiratory tract can end in a myxedematous coma;
- all patients with risk factors for hypothyroidism, including the part of newborns and people over the 40 years, need an examination for its timely diagnosis and treatment;
- the main methods of diagnosis in cases of damage to the respiratory system against the background of hypothyroidism are chest X-ray, bronchoscopy and arterial gas analysis.
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