The incidence of thyroid cancer has increased dramatically over the past few decades in the world, with an estimated 43,720 new cases a year in the United States (1). As the second most common type of differentiated thyroid cancer (DTC), the incidence of follicular thyroid carcinoma (FTC) is much lower than that of papillary thyroid carcinoma (PTC) (2). Compared to PTC, patients with FTC have a two-fold and ten-fold higher risk of developing lung and bone metastases, respectively, due to its propensity for vascular invasion and hematogenous dissemination (3). Therefore, the outcome of FTC is less favorable than that of PTC (4–6). FTC is defined as a malignant epithelial tumor showing follicular cell differentiation, without the nuclear features of PTC (7). Preoperative examinations, such as ultrasonography and fine needle aspiration (FNA), are usually not sufficient to distinguish FTC from its benign counterpart, follicular thyroid adenoma (FTA) (8–10). The histological differences between FTC and FTA are capsular and vascular invasion (10). Thus, it remains challenging for clinicians to determine appropriate extent of therapy and to avoid unnecessary surgery in patients diagnosed with follicular thyroid neoplasms. Several factors have been reported to be associated with the malignancy of a follicular neoplasm, such as large tumor size, but the diagnosis of FTC still mostly relies on postoperative pathology (11). More research may be required to better evaluate the risk of malignancy in patients with follicular thyroid neoplasm.

As one of the most common thyroid diseases, Hashimoto’s thyroiditis (HT) was first described by a Japanese physician, Haraku Hashimoto in 1912. It is the most frequent autoimmune thyroid disease characterized by enlarged thyroid volume, parenchymal lymphocytic infiltration, and thyroid-specific autoantibodies (12). Currently, the incidence of HT is 0.3-1.5 cases per 1000 people, and it has been constantly increasing in recent years (13–17). Women are 4-10 times more likely to have HT than men (17). Circulating antibodies to thyroperoxidase and thyroglobulin are two serological markers of HT, which are widely used in clinical diagnosis (18). Most HT may cause endocrine disorder and ultimately develop to hypothyroidism, although at presentation patients can be euthyroid or hyperthyroid (15, 19). Although several reports have confirmed that HT could be associated with a higher risk of PTC, PTC combined with HT have less metastasis and better outcomes of PTC than patients without HT (20, 21). As to FTC, less relationship between HT and FTC has been established yet.

In this study, the clinical features of patients with follicular thyroid neoplasms were analyzed to portrait the differences between FTA and FTC. Then, the influence of HT on FTC diagnosis and differentiation was further investigated. Our findings are expected to explore new aspects for the preoperative diagnosis and appropriate differentiation strategies in patients with follicular thyroid neoplasms.

Neck ultrasonography has become the most common imaging tool for patients with thyroid disease (25). According to the TIRADS, thyroid nodules with specific ultrasound features, such as solidity, hypoechogenicity, taller-than-wide shape, irregular margins, and microcalcifications, are of high malignancy risk, which helps to identify PTC (24). However, these features are of limited value in diagnosing follicular tumors, especially when differentiating FTC from FTA. The common ultrasound presentations of these two follicular lesions are basically overlapped, including well defined, isoechoic or hypoechoic and with peripheral halo (9, 26). Although a recent paper reported that FTA were less frequently classified in the high-suspicious category compared with follicular variant of PTC and FTC by ultrasound (27), more clinical data are needed for further verification. Most studies have shown that the TIRADS classification is not helpful in differentiating FTC from FTA (28). Similarly, our research of 326 patients with follicular thyroid tumors also found that there was no significant difference in ultrasound TIRADS classification or other ultrasound features between FTA and FTC patients. These results suggested that it was difficult to distinguish FTA and FTC preoperatively solely by ultrasonography.

FNA is commonly used for preoperative judgment of thyroid nodules, which could be the most effective method for diagnosis of PTC before surgery (29). But in terms of follicular tumor diagnosis, only 2 patients were successfully diagnosed among 28 patients in FTC group, while 14 patients were misdiagnosed as “FTA”. The true positive rate (sensitivity) of FTC was only 0.0714, far below the clinical diagnostic requirements. FTCs are pathologically distinguishable from FTAs based on the capsular and vascular invasion, which can hardly be portraited in cytological specimens (10). Thus, it was challenging to distinguish FTA and FTC cytologically. Molecular testing to detect individual mutations has been used for the diagnosis and prognosis of thyroid cancer (30). Studies showed that the importance of RAS mutations detection in FTC diagnosis, with the prevalence of about 40-50% in FTC and 20-30% in FTA (31–33). However, different from the decisive role of BRAF mutation in PTC diagnosis, RAS mutations may not establish a clear cut-off boundary between FTC and FTA.

Hence, we focused on thyroid function indicators expected to clarify the diagnosis of FTC preoperatively. We found that TG levels were significantly higher in FTC patients compared to FTA, which is consistent with previous studies that the risk of DTC was positively correlated with TG levels (34–36). Furthermore, the association with TG was stronger in follicular cancer than PTC (34). Only synthetized in thyroid, TG is primarily determined by the mass of differentiated thyroid tissue present, physical damage or inflammation of the thyroid, and the magnitude of thyrotropin receptor stimulation (37). Elevated TG levels in DTC are likely due to enhanced secretion in the conversion to malignancy of the thyroid follicular cells (35). Preoperative serum TG level proved to be an effective predictor of cervical lymph node metastasis and distant metastasis in DTC (38, 39), especially in PTC (40, 41). Our data shows that high TG levels were also related with tumor invasion and poor prognosis in the FTC. Therefore, we believe that preoperative serum TG should be measured as a predictor of FTC and reference for surgery selection.

However, in our study, the lack of significant differences in TG levels between FTA and FTC patients with HT demonstrated the uniqueness of FTC related to HT. Studies suggested that the presence of TGAb in HT patients can influence the measurement of serum TG levels, which may result in reduced concentrations of TG levels (42). This is consistent with our study that the serum TG concentration of HT patients was significantly lower than that of antibody-negative patients. Therefore, TG in HT patients may be misty to reflect on the real difference between FTC and FTA. Our results were consistent with previous notion that TGAb limits the application of TG as a reference to predict malignant risk in DTC (36). On the other hand, interestingly, we confirmed that the gaps of serum TSH levels and T3 levels were much more apparent between FTC and FTA in HT patients.

As the major growth factor for thyroid follicular cells, TSH suppression is a mainstay of clinical thyroid cancer management (43). The positive correlation between TSH and tumor invasion in FTC patients was found in our study, which also confirmed the importance of TSH inhibition for patients diagnosed with thyroid cancer. However, no significant differences were observed in TSH levels between FTC and FTA in general investigation of all 326 patients. On the contrary, by more detailed stratification, HT patients with higher TSH levels were significantly associated with lower risk of FTC. The negative relationship between TSH level and thyroid cancer risk was also found in both categorical and continuous analyses and supported by a genetic predisposition research (34, 44). Low TSH levels may cause less differentiation of the thyroid epithelial, resulting in a predisposition to malignant transformation (44). In our opinion, the autoimmune response in HT patients induced thyrocyte destruction and attack benign and malignant cells indiscriminately. During this process, the damage of malignant cells may help reduce the risk of FTC. Unlike the variability of TSH among people, most HT patients go through a process that gradually develop to hypothyroidism with elevated TSH levels, which decrease the other noise to influence TSH levels. In our study, the incidence rate of FTA was higher with preoperative serum TSH >1.736U/L according to ROC curve, which suggested that TSH levels could be served as a predictor to separate benign or malignant thyroid neoplasms in HT patients. The detailed regulatory mechanism still deserves further exploration.

Our study has several limitations, including the relatively small sample size, and the underlying molecular mechanism remains unclear. Besides, to accurately differentiate FTC from FTA, a more comprehensive diagnostic approach involving serologic markers, molecular testing, and other diagnostic techniques is required. To this end, studies including larger sample sizes and multiple diagnostic techniques are still required.

There is still a challenge for Ultrasonography and FNA to distinguish FTC from FTA. Serum TG should be measured as a risk factor of FTC. However, in HT, serum TSH levels can be a more reliable indicator to distinguish FTC from FTA preoperatively.