CT is the most common imaging modality in which ITNs are detected (7). Mainly because of the wide availability of thoracic CT to evaluate various medical conditions, the discovery of ITNs is steadily increasing. At the same time, the Covid-19 pandemic undoubtedly further contributed to this increment. Especially in the early days of the pandemic, the use of CT had significantly increased since PCR diagnostic tests did not have sufficient sensitivity, and chest X-rays did not make an early diagnosis. Theoretically, increasing CT scans may be associated with an increased incidence of ITNs. However, in practice, due to the nature of the pandemic, thoracic CT scans are evaluated by different radiologists and reported quickly. Thus, sufficient information about the thyroid gland may not be given in the CT reports. Our study showed that the prevalence of thyroid heterogeneity was 1.10%, and ITNs were 3.80% on thoracic CT in the adult population. Our finding is much less than the prevalence of 25.10% reported in 2012 by Ahmed et al. (2), who evaluated 3077 consecutive adult patients in their recent study. They found age as the most critical determinant of the significance of ITNs on thoracic CT. While the mean age of the patients included in their study was 59.30, in our study, the mean age was 48.23. Although approximately 24000 thoracic CT scans were taken in six months in our hospital in the pre-pandemic period, this number reached about 38000 in the first six months of the pandemic period. Another reason for the lower prevalence in our study compared to previous studies might be this increased number of CT scans which needed to be reported faster due to the nature of the pandemic by the same number of radiologists. In addition, technical differences such as the image quality of the CT, using intravenous contrast, and taking thin sections of the image might also cause different prevalences in different studies.
In the study by Ahmed et al., in addition to age, female gender was also associated with thyroid nodules detected in CT (2). However, in the study by Ezzat S. et al., they did not find a correlation between age and nodule frequency (8). Like the Ahmed et al. study, the female gender was associated with our study's nodules and multiple nodules.
Major sets of guidelines for the follow-up and management of ITNs are established by the American Thyroid Association (ATA) and the British Thyroid Association (10, 11). Although it is proposed that patients with ITNs larger than 1 cm undergo US for further evaluation in some guidelines (12), it was reported that utilizing a three-component grading system that stratifies nodules according to risk increases diagnostic accuracy in the study by Nguyen et al. (13). They compared the performance of 2 risk-categorization methods of selecting CT-detected ITNs for workup. Method A was based simply on nodule size. They made the 3-tiered system for ITNs malignancy risk evaluation for method B: Risk category 1 is a nodule of any size with aggressive imaging features of invasive (such as local invasion, suspicious lymphadenopathy) or metastatic disease. Risk category 2 is a nodule of any size in a patient younger than 35 years of age and not meeting the criteria for risk category 1. Risk category 3 is a nodule at or above a cutoff of 15 mm and not meeting the criteria for categories 1 and 2. The 15-mm cutoff is intended to reflect a higher size threshold for the workup of nodules that lack aggressive imaging findings or demographic risk factors. The purpose of having three risk categories is to help the radiologist communicate the risk of malignancy in a CT-detected ITN and the need for further investigation by sonography. Compared with the common practice of a 10-mm-size cutoff, the 3-tiered system reduces excess workup of benign ITNs while capturing the same proportion of thyroid malignancies and is no more likely to miss high-mortality malignancies. While it would be ideal for a categorization method not to miss all incidental thyroid malignancies, it may be more rational and cost-effective to capture all malignancies associated with poor outcomes.
Although most thyroid nodules were benign by cytological and histopathological evaluations, thyroid malignancies were also detected in our study population. Despite relatively low (about 5%) cancer risk, most nodules 1 cm or greater are sampled with US-guided FNAB. This situation might cause a marked increase in papillary microcarcinomas (13). A previous study of adult patients showed a 3.90% prevalence of malignancy among ITNs on thoracic contrast-enhanced CT. That study showed no CT feature that can reliably differentiate benign from malignant thyroid nodules. Therefore, the authors suggested that the US is a functional adjunctive test for evaluating incidentally detected thyroid abnormalities on CT (5). Our study saw 109 patients with one nodule and 129 with multiple nodules requiring biopsy. However, only 115 of 238 patients with biopsy indications had thyroid FNAB. Bilateral total thyroidectomy was performed in 6 patients with a giant nodule and four patients whose thyroid FNAB resulted in follicular neoplasia or was suspected of follicular neoplasia. Histopathologically, papillary thyroid cancer was detected in 3, and a well-differentiated thyroid tumor with uncertain malignant potential was detected in one patient. If we extrapolate from other CT series, we expected that 5% of the nodules we noticed would be cancer. In our study, malignancy was found in 0.30% (4/1343) of ITNs on CT and 3.48% (4/115) of patients who underwent a biopsy. The lower malignancy rate in our study might be related to the lack of FNAB results in nearly half of the patients with biopsy indication. The mean age of the patients who underwent a biopsy was 56 years. In addition, the difference in the mean age in the studies may also be a reason for the different results (9, 14).
Regarding thyroid functional status, being euthyroid was associated with improved survival. In critical situations, low thyroid hormone levels at the hypothalamic and peripheral tissue levels are considered compensatory. In patients with severe systemic disease, non-thyroidal illness syndrome is associated with increased mortality and morbidity and indicates a poor prognosis (15, 16). Our study also showed that mortality was significantly increased in patients with non-thyroidal illness syndrome. Overt hypo/hyperthyroidism and subclinical hypo/hyperthyroidism were not associated with survival. The small number of patients in these groups may have affected the results.
There are certain limitations of our study which should be addressed. First, the study's retrospective nature is not ideal for obtaining a random sample but allows a time-efficient review of many cases. Second, due to the effect of the pandemic on the reports' rapid conclusion, extrapulmonary pathologies may not have been paid attention too. Third, the clinical presentations of the patients were not similar, as the physicians reporting the CT and the US were not fixed. Fourth, no comments regarding thyroid were made on about 90% of the patients in the CT reports. It would have been possible to determine both benign and malignant nodules if more evaluation of thyroid nodules had been made in the CT reports.
Our study is the first study on ITNs during the covid pandemic. Furthermore, it is the first to reach the number of patients examined in this amount. In addition, we evaluated not only the prevalence of ITNs but also the follow-up data of the patients.
In conclusion, increased use of thoracic CT during Covid 19 pandemic probably caused increased detection of ITNs. In this large-scale study, the prevalence of thyroid nodules reported in thoracic CT was 3.82%, and thyroid cancer was detected in 1.30% of patients evaluated in the US. Therefore, Thoracic CT scans taken for different reasons might provide the opportunity for early diagnosis and treatment of thyroid cancers. Consequently, it is essential for those who evaluate CT scans not only to focus on the primary disease but also to check other regions for detecting such pathologies.