Serum Ctn has always been regarded as a sensitive tumor marker, which suggests that the thyroid nodule could be MTC. There is much evidence that the measurement of serum Ctn concentrations in patients with thyroid nodules can lead to an earlier diagnosis of MTC[17]. In our research, we chose all the hypercalcitoninemia patients as the research subject, and aimed to find the best cutoff value of basal serum Ctn to discriminate MTC from other conditions of hypercalcitoninemia. It’s because that from the data of all the MTC patients diagnosed and treated in West China Hospital of Sichuan University, the lowest level of serum Ctn was 10.50 pg/mL, which is higher than the upper limit for our laboratory testing (> 9.52 pg/mL for males and > 6.40 pg/mL for females), so the negative predictive value (NPV) of the serum Ctn levels lower than our laboratory’s lower limit of detection was 100%.
According to our statistics, in addition to MTC, many other common clinical diseases can cause the elevation of serum Ctn, such as non-MTC thyroid diseases, kidney diseases, inflammation, lung and intestinal malignancies, which reduces the diagnostic specificity of serum Ctn for MTC. This result is consistent with previous literature[18], but the proportion of these diseases with hypercalcitoninemia has rarely been reported. Most patients who have non-MTC thyroid diseases and elevated Ctn levels (male > 9.52 pg/mL, female > 6.40 pg/mL) are diagnosed with papillary thyroid carcinoma (PTC), accounting for 40.85% of all non-MTC diseases. The reasons may be as follows: 1) PTC tumors may release some substances that possess a paracrine stimulatory action on C-cells to ultimately raise serum Ctn levels[19]; 2) PTC accounts for the vast majority of thyroid malignancies, and clinicians tend to prescribe several serum Ctn tests for patients with thyroid nodules to aid in determining the nature of the nodules. In addition to PTC, PTC combined with Hashimoto's thyroiditis (HT) or only HT (accounting for 7.23% of the total) could also cause elevated levels of serum Ctn, and this phenomenon may be a consequence of progressive C-cell destruction following the nonspecific follicular and parafollicular cell damage caused by lymphocytic infiltration of the thyroid gland[20]. In addition to thyroid diseases, kidney diseases including uremia, chronic renal disease, triad or secondary hyperparathyroidism, and diabetic nephropathy can also lead to nonspecific elevation of serum Ctn. It has been reported that 30–46% of chronic kidney disease (CKD) patients have elevated serum Ctn levels[21]. The mechanisms by which serum Ctn is elevated in these CKD patients are not yet fully understood. It is assumed that CKD patients’ C-cells are triggered by disturbances in mineral metabolism due to renal failure; thus, C-cells reactively secrete Ctn into the peripheral blood. Patients with neither thyroid disease nor kidney disease can also have elevated levels of serum Ctn due to inflammation, lung/mediastina/bronchial malignancies, intestinal malignancies and prostate cancers. In our study, inflammation accounted for the majority of this group (Group B3, other diseases), which may be due to the release of procalcitonin in the circulation by tissues that do not normally express the Ctn gene during sepsis or other inflammatory conditions[22].
Therefore, physicians must keep in mind that elevation of serum Ctn does not necessarily indicate MTC because many physiological, pharmacological, and pathological factors can influence the level of serum Ctn. An accurate medical history, physical examination and more in-depth investigation can be useful in identifying other reasons for the hyperproduction of serum Ctn[18]. However, we found that non-MTC diseases and conditions usually cause relatively lower Ctn elevation than MTC, and we thought a serum Ctn cutoff value higher than the reference range may better help to discriminate real MTC from other conditions of hypercalcitoninemia. For this reason, we included all MTC patients in West China Hospital of Sichuan University from the past decade as the test group and all non-MTC patients whose serum Ctn level was > 9.52 pg/mL (males) and > 6.40 pg/mL (females) in West China Hospital of Sichuan University from the past five years as the control group. It is recognized that serum Ctn levels are usually higher in males than in females because men have twice as many C-cells as women. Therefore, we drew different ROC curves for males and females to find sex-specific Ctn cutoffs. Finally, we determined 38.24 pg/mL (AUC: 0.964, 95% CI: 0.898-1.000) and 26.00 pg/mL (AUC: 0.998, 95% CI: 0.994-1.000)) to be the best cutoff values to diagnose MTC for males and females, respectively.
Among males, there was only one MTC patient whose serum Ctn level was less than 38.24 pg/mL (Ctn: 10.5 pg/mL), and according to his postoperative pathological results, the largest tumor diameter was 1.5 cm, without lymph node metastasis, which suggested that this patient had early-stage disease. There were five non-MTC male patients whose serum Ctn levels were higher than 38.24 pg/mL. Two of them had uremia complicated by triad hyperparathyroidism (Ctn: 45.91 pg/mL and 47.8 pg/mL). The other three were diagnosed with CKD4 (Ctn: 55.2 pg/mL), severe pneumonia (Ctn: 41.9 pg/mL), and pheochromocytoma (Ctn: 51.1 pg/mL). The pheochromocytoma patient had no imaging evidence of thyroid nodules, thus excluding the diagnosis of MEN2. Among females, there were no MTC patients whose serum Ctn level was less than 26.00 pg/mL. However, there were 2 female non-MTC patients whose serum Ctn levels were higher than 26.00 pg/mL. One of them was diagnosed with CKD4 accompanied by secondary hyperparathyroidism (Ctn: 44.8 pg/mL), and the other had a bronchial/pulmonary malignancy (Ctn: 77.73 pg/mL). It is known that neuroendocrine tumors such as tumors predominantly of bronchial or pancreatic origin can result in inappropriate calcitonin release, which can reach > 100 pg/mL[23]. Our thresholds of serum Ctn to diagnose MTC (male:≥38.24 pg/mL, female ≥ 26.00 pg/mL) are comparable to the Ctn cutoff values reported in other studies (male: ≥46 pg/mL, 68 ≥ pg/mL, > 34 pg/mL, > 43 pg/mL; female: ≥35 pg/mL,≥18.7 pg/mL, ≥ 26 pg/mL, > 30 pg/mL, > 23 pg/mL)[24–27, 10]. The cutoff value for males in our study was slightly lower than that in the other studies mentioned above, which may be due to the following reasons: 1) We chose all patients with non-MTC diseases for the control group, whereas the other studies chose patients with C cell hyperplasia (CCH) or thyroid nodule diseases for the control group, and these diseases may show higher levels of serum Ctn. 2) Different races were studied: our patients were all of Han nationality in China, while the research subjects of the other studies were mainly Caucasian or Indian. 3) Different laboratories and the use of different instruments and reagents may cause slight differences in the test results for Ctn. The method used for serum Ctn testing in our laboratory is electrochemiluminescence immunoassay (ECLIA), and the instruments and reagents are purchased from Roche Diagnostics Corp.
The preoperative serum Ctn level not only has diagnostic value but also is closely associated with primary tumor size and the number of lymph node metastases (LNMs)[28]. In our study, we found that the level of preoperative serum Ctn positively correlated with the largest tumor diameter (r = 0.702, P < 0.0001), which was consistent with the conclusion of Park H et al[28]. However, the relationship between the largest MTC tumor diameter and serum CEA level was not as strong as the largest tumor diameter and Ctn (r = 0.377, p = 0.0025). We also found that the serum Ctn level was more valuable than the serum CEA level when the largest tumor diameter was less than 2 cm, indicating that Ctn had more diagnostic value in the early stage of MTC. In a study[29] that compared the diagnostic value of four selected markers, Ctn, procalcitonin, chromogranin A, and CEA, the diagnostic value of CEA was much lower than that of the other markers and could be within the normal range even in patients with advanced metastatic MTC. Additionally, the serum CEA level was not significantly different between MTC patients with lymph node metastasis and those without lymph node metastasis (P > 0.05) in our study. However, the serum Ctn level in MTC patients with lymph node metastasis was significantly higher than that in patients without lymph node metastasis (P < 0.05). These results can be explained by the fact that Ctn is specifically secreted by thyroid parafollicular C-cells, but CEA is a broad-spectrum tumor marker that is not specific to MTC. For MTC patients, the diagnostic value of CEA alone is low, so CEA should serve as an auxiliary indicator. However, it has been suggested that the determination of serum CEA levels is useful for evaluating disease progression in patients with clinically evident MTC and for monitoring patients following thyroidectomy[1]. Recently, Chen Li et al reported one case in which the CEA level continued to increase and the Ctn value was normal after total thyroidectomy and central lymph node dissection in an MTC patient[30]. Thus, we think that CEA is more valuable for the prognosis of MTC than for its diagnosis. In the study of Meijer JAA et al[31], the CEA doubling time had a higher predictive value than Ctn for MTC recurrence and death. Therefore, more follow-up studies are needed to confirm the value of CEA in the prognosis of MTC patients.
Although clinicians in our nation tend to assess serum Ctn in patients suspected of having MTC, the cutoff value of serum Ctn for discriminating real MTC from other conditions of hypercalcitoninemia has not been reported by Chinese scholars. Serum Ctn levels can be influenced by the testing laboratory and ethnicity, so our study provides valuable information for clinical decision-making. West China Hospital of Sichuan University is a large medical center that receives patients from all over the country, and considering the very low incidence of MTC, this is a relatively large sample size that has a certain representativeness among Chinese people. We hope that our results can be a reference for clinicians in China and East Asia and for other laboratories worldwide that use ECLIA for serum Ctn testing and the same instruments and reagents offered by Roche Diagnostics Corp. In addition to calculating the cutoff value for Ctn, we also describe other diseases that can cause elevation of Ctn and have compared the diagnostic effectiveness of Ctn and CEA in MTC. Finally, we discuss the relationship between the level of serum Ctn and the tumor size and lymph node metastasis of MTC.
There are some limitations to our study. Because this was a retrospective study of MTC patients in our hospital over the past decade, the postoperative pathological reports of some patients were not available because of the prolonged study period, so we had to use their intraoperative pathological records and thyroid ultrasound reports to judge the largest tumor diameters and lymph node metastasis instead of the postoperative reports, which may be discrepant with the true pathological results. In most cases, our laboratory reported > 2000 pg/mL rather than specific values when the level of serum Ctn exceeded 2000 pg/mL, and dilution was performed only at the specific request of the clinician. For statistical convenience, we defined all values over 2000 as 2000. This may influence the relevance analysis of Ctn and the largest tumor size and the degree of lymph node metastasis when the level of Ctn > 2000 pg/mL but had no influence on the determination of the cutoff value of Ctn when the value was at a lower level. Finally, because of the very low incidence of MTC, we were unable to find a suitable validation group in our hospital to verify the cutoff values, but subsequently, we have begun to collaborate with other hospitals to collect more MTC patients and verify the reliability of the cutoffs.