Risk of developing pneumonia associated with clinically diagnosed hypothyroidism: a nationwide population-based cohort study

Background: Hypothyroidism has a detrimental effect on the immune system, which may predispose patients to infection. However, evidence about the risk of developing either community- or hospital-acquired pneumonia in patients with hypothyroidism is scarce. Objective: To evaluate the association between hypothyroidism and the risk of developing pneumonia. Methods: This was a retrospective population-based cohort study from Taiwan's National Health Insurance Research Database. After 1:1 propensity score matching, 9749 patients (age >= 20 years) newly diagnosed with hypothyroidism between 2001 and 2014 and 9749 patients without hypothyroidism or other thyroid diseases were included in the hypothyroidism and non-hypothyroidism cohorts, respectively, and followed up until 2015. The development of pneumonia was defined as the primary outcome. Cox proportional hazards regression models were used to calculate the hazard ratios (HRs) of developing pneumonia between hypothyroidism and non-hypothyroidism cohorts after adjusting for age, sex and baseline comorbidities. To evaluate whether thyroxine replacement therapy (TRT) modified the risk for pneumonia, we divided patients with hypothyroidism into subgroups: patients who received TRT and those who did not. Results: Hypothyroidism was associated with a higher risk of pneumonia [adjusted HR (aHR) 1.38, 95% confidence interval (CI) 1.29-1.49, P < 0.001]. Patients with hypothyroidism who received TRT had a lower risk of pneumonia than patients who did not (aHR 0.85, 95% CI 0.76-0.93, P = 0.001). Similar results were obtained in the age- and sex-stratified analyses. Conclusions: Clinically diagnosed hypothyroidism was independently associated with the risk of pneumonia. In patients with hypothyroidism, TRT was associated with a lower risk of pneumonia. Lay Summary Hypothyroidism has a detrimental effect on the immune system, which may predispose patients to infection. The risk factors for pneumonia include older age, chronic lung diseases and, most importantly, a decreased immune response against respiratory pathogens. However, evidence of the risk for pneumonia in patients with hypothyroidism is scarce. The high prevalence of hypothyroidism, high annual incidence rate of pneumonia and their interrelationship through the immune system emphasize the importance of understanding the association between the two diseases. This study aimed to determine whether patients diagnosed with hypothyroidism conferred a predisposition to the development of pneumonia. In this 15-year population-based retrospective cohort study, we found that patients clinically diagnosed with hypothyroidism might be at increased risk for the future development of pneumonia. Moreover, in patients with hypothyroidism, the risk of pneumonia may be reduced by thyroxine replacement treatment. Additional prospective cohort studies, which include thyroid function tests and a more accurate clinical diagnosis of pneumonia based on radiological findings, are needed to confirm the effects of hypothyroidism on the risk of developing pneumonia.