We compared serum Cu levels of patients with PTC, nodular goiter and normal healthy subjects. Zhang[21] and Shen[25] reported that gender and ethnicity influence serum Cu levels and so we investigated Thai women, the gender most often affected by this disease. The mean serum Cu levels of our reference group were comparable to those in other studies (Table 3), and very close to the mean value of the biggest dataset in a Chinese population (1.00 ± 0.30 vs 1.01. ± 0.24 µg/ml) [21]. We did not identify any statistically significant differences in the serum Cu levels of the PTC, nodular goiter, and reference groups. The serum Cu levels were measured using atomic absorption spectrometry, as was done by many previous studies [17, 21, 26]. Moreover, the quality of the measuring techniques used by the Clinical Toxicology Laboratory is also attested by its ISO-accreditation.
Table 3
Serum copper levels in healthy subjects
Studies | Year | Country | Measurement technique | N | Sex | Serum copper levels (mean ± SD; µg/ml) |
Maneeprasopchoke et al. | 2022 | Thailand | AAS | 100 | Female | 1.00 ± 0.30 |
Zhang et al.[21] | 2009 | China | AAS | 890 | Female | 1.01 ± 0.24 |
Baltaci et al. [18] | 2017 | Turkey | AES | 15 | Female | 0.74 ± 0.24 |
Przybylik-Mazurek et al.[26] | 2011 | Poland | AAS | 20 | All | 1.11 ± 0.19 |
Kosova et al. [17] | 2012 | Turkey | AAS | 37 | All | 1.06 ± 0.11 |
Leung et al. [16] | 1996 | China | AES | 50 | All | 0.74 ± 0.19 |
Kucharzewski et al.[15] | 2003 | Poland | TRXRF | 50 | All | 0.69 ± 0.06 |
Abbreviations: AAS, atomic absorption spectrometry; AES, atomic emission spectrometry; TRXRF, total reflection fluorescence |
The use of Cu to support the diagnosis of thyroid cancer remains inconclusive. Baltaci et al. [18] showed that serum Cu levels of women with thyroid cancer were significantly higher than those of healthy controls (0.87 ± 0.33 µg/ml, and 0.74 ± 0.24 µg/ml, respectively; P < 0.05). Additionally, the serum Cu levels of female patients with thyroid cancer significantly decreased to levels close to those of the control group within two weeks after surgery (0.74 ± 0.30 µg/ml). Vesna and colleagues [14] compared 35 cases of PTC and 13 cases of papillary thyroid microcarcinoma with 82 cases of benign thyroid tumor. The serum Cu levels of patients with PTC and microcarcinoma were significantly higher than patients with benign thyroid tumor. However, because incidental microcarcinoma was included in the PTC group, their findings are challenging to interpret and to compare with our study.
In 2015, Shen and colleagues published a meta-analysis of five case-control studies investigating serum Cu levels [25]. One study was carried out in China (n = 100), three in Poland (n = 71, n = 45, and n = 33), and one in Turkey (n = 84). Overall, patients with thyroid cancer had higher serum Cu levels than healthy controls (standardized mean difference, 2.372; 95% CI, 0.945–3.799; P = 0.001). However, consistent with our results, the Polish studies did not find higher serum Cu levels in patients with thyroid cancer relative to their controls. A 2004 study from Kuwait also reported that serum Cu levels in thyroid cancer patients were not different from healthy controls and rose significantly after thyroidectomy [19]. Hence, ethnicity can influence serum Cu levels. Normally, Cu is actively recycled in the digestive tract, body fluids and tissues, and is mainly excreted from the body via bile. Copper levels are primarily controlled by recycling and resorption, and dietary Cu represents only a small proportion of total Cu resorption [15]. Therefore, dietary intake of Cu has an insignificant affect on serum Cu levels and does not need to be controlled.
In the post hoc subgroup analysis of PTC, we found significantly higher serum Cu levels in patients with blood vessel invasion. This finding suggests a relationship between serum Cu and the aggressiveness of PTC. Moreover, the mean serum Cu levels were also higher in the presence of adverse features such as positive capsular invasion, extrathyroidal extension, lymph node involvement, distant metastases and high stage. While the difference was not statistically significant, we can infer that high serum Cu may be related to the invasiveness of PTC.
The potential relationship between serum Cu levels and gene mutation in humans has not been studied. Since copper regulates the function of follicular cells, aberrant levels of serum Cu may be associated with molecular alterations. Currently, there are several genetic mutations reported in thyroid cancer and the BRAFV600E mutation is the most common biomarker for PTC. Brandy et al. demonstrated that Cu is required for BRAF signaling and tumorigenesis. A reduction in serum Cu levels caused the size of BRAFV600E-driven melanomas to decrease in laboratory animals [27]. A recent investigation by Baldari et al. also found that Cu-chelating agents reduced the proliferation, survival, and migration of human colon cancer cells carrying the BRAFV600E mutation [28]. We hypothesized that in thyroid cancer, the BRAFV600E mutation would be associated with increased serum Cu levels, as is seen in melanoma and colon cancer. We did observe a significant elevation of serum Cu levels in PTC with BRAFV600E mutation, suggesting that serum copper may indicate the severity of PTC. The role of Cu-chelating agents as an adjuvant therapy in BRAFV600E mutation positive PTC merits further investigation.
To our knowledge, this is the first study to report serum Cu levels in terms of histopathological aggressiveness, risk of recurrence, staging, and molecular status in PTC. In addition, we screened all healthy subjects with ultrasonography of the thyroid gland to avoid unexpected thyroid nodules in the control group. This ensured that the reference serum Cu values of the healthy Thai women were reliable and could be used as a standard for further studies. On the other hand, our analyses suggests that serum Cu levels are not appropriate for diagnostic purposes, but may have value to predict the severity of PTC.
Our study has some limitations. Our subjects were Thai women with PTC and nodular goiter. We did not address the role of serum Cu levels in men, other types of thyroid cancer, and in advanced-stage thyroid cancers such as tracheal or recurrent laryngeal nerve invasion.