Thyroid hormones are key regulators of essential cellular processes, including proliferation, apoptosis, and metabolism. Some studies hypothesized that thyroid hormone deficiency may affect cancer outcomes, showing that hyperthyroidism facilitates tumor growth, while hypothyroidism induces an opposite effect [1]. Many preclinical studies have been conducted in recent decades to determine how thyroid hormones exert their tumor growth-promoting effect [2]. Following the discovery of signal pathways for thyroid hormone action in carcinogenesis, such as phosphoinositide 3-kinase (PI3K) or extracellular signal-regulated kinases (ERK) 1/2 pathway activation, the related mechanisms are now better understood [3]. Some clinical studies support an association between thyroid disorders and cancer risk [4-6]. In a large-sized, prospective study of 26,691 people, low thyroid-stimulating hormone (TSH) levels suggestive of hyperthyroid function were associated with increased cancer risk, especially for lung and prostate cancer, compared with the euthyroid group [5].

Treatment for differentiated thyroid cancer is based on thyroidectomy, and postoperative radioactive iodine (RAI) therapy and TSH suppression with supraphysiologic doses of levothyroxine, which is a synthetic thyroid hormone. Many studies have demonstrated that TSH suppression improves disease-free and disease-specific survival, especially in high-risk patients with differentiated thyroid cancer [7,8]. However, because most patients have a favorable prognosis and a long-life expectancy, there is an increasing concern regarding the long-term health consequences in thyroid cancer patients. In this context, questions have arisen about the possibility that high-dose levothyroxine may induce proliferation of subsequent primary cancers (SPCs) [9]. Patients with differentiated thyroid cancer are at greater risk of developing SPCs than the general population [10]. A meta-analysis confirmed that the risk of developing SPCs was 5% to 31% greater than expected [11]. One of the reasons suggested for this increase, though still controversial, is the potentially carcinogenic effect induced by RAI therapy [12-14]. Recently, Mei et al. [14] conducted a study based on the United States Surveillance, Epidemiology, and End Results database, showing that the risk of experiencing hematologic and breast cancers was greater in patients with thyroid cancer who had received RAI therapy compared to those without RAI therapy. While there have been several prior studies evaluating RAI and risk of SPCs in patients with thyroid cancer, no prior studies have assessed the potential independent influence of levothyroxine use on SPC risks. Therefore, we investigated the relationship between levothyroxine dosage and the risk of SPCs in thyroid cancer patients while adjusting for the effect of RAI therapy, using the Korean National Health Insurance Service (NHIS) database, which covers the entire population of Korea.

Table 1 shows participant characteristics according to the dosage of levothyroxine. The mean age at diagnosis of thyroid cancer was 47.8±12.0 years, and females comprised 80.8% of the study population. As for the type of thyroidectomy, most patients in the non-levothyroxine group underwent unilateral thyroidectomy (74.8%), whereas most patients in the levothyroxine group underwent total thyroidectomy (80.4%). No patients received RAI therapy in the non-levothyroxine group, as opposed to 43.6% of patients who did in the levothyroxine group. Especially, increased rates of total thyroidectomy and RAI therapy were observed in the high-dose levothyroxine groups.

During the median follow-up of 7.3 years, 849 (6.3 per 1,000 person-years) and 16,561 (6.9 per 1,000 person-years) SPCs occurred in the non-levothyroxine and levothyroxine groups, respectively. The risk of SPCs was not different between the two groups after adjusting for age, sex, and cumulative doses of RAI therapy, except that the risks of colorectal cancer and liver and biliary tract cancer were higher in the levothyroxine group (adjusted HR, 1.35 and 1.36; 95% CI, 1.04–1.74 and 1.00–1.86, respectively) (Table 2). However, when we analyzed the levothyroxine group according to its dosage, the incidence rates per 1,000 person-years were 5.8, 6.7, 7.1, and 7.6 in the Q1, Q2, Q3, and Q4 groups, respectively, and the risk of all SPCs increased from Q1 to Q4. Compared with the non-levothyroxine group, the adjusted HRs of all SPCs were 0.91 (95% CI, 0.84 to 0.98), 1.08 (95% CI, 1.00 to 1.17), 1.14 (95% CI, 1.05 to 1.24), and 1.27 (95% CI, 1.17 to 1.37) in the Q1, Q2, Q3, and Q4 groups, respectively (Table 2, Fig. 2A). Especially, the risks of digestive system cancers showed a gradual rise as the levothyroxine dose increased, with significant elevations observed in the high-dose levothyroxine groups. Among them, the risks of stomach, colorectal, and liver and biliary tract cancers were increased in the Q3 (adjusted HR, 1.32, 1.51, and 1.64; 95% CI, 1.01–1.73, 1.14–2.01, and 1.17–2.30, respectively) and Q4 groups (adjusted HR, 1.31, 1.60, and 1.95; 95% CI, 1.01–1.71, 1.20–2.12, and 1.40–2.72, respectively), and the risk of pancreatic cancer was increased in the Q4 group (adjusted HR, 2.48; 95% CI, 1.43 to 4.31) (Fig. 2B-D).

These findings remained consistent after further adjustments for comorbidities, as measured by the Charlson comorbidity index (Supplemental Table S1). After additional adjustments for various factors that could influence the occurrence of SPCs, such as obesity, smoking, and alcohol consumption, the levothyroxine group showed a modest increase in the risk of all SPCs compared to the non-levothyroxine group (adjusted HR, 1.12; 95% CI, 1.02 to 1.22). Moreover, this elevated risk was more evident in the high-dose levothyroxine groups (adjusted HR, 1.36 in Q3 and 1.32 in Q4) (Supplemental Table S2). Similar findings were also revealed in the subgroup analysis according to the levothyroxine dosage per body weight, even after adjusting for age, sex, cumulative doses of RAI therapy, Charlson comorbidity index, obesity, smoking, and alcohol consumption (n=260,567) (Table 3, Supplemental Fig. S1).

Next, we compared participants in the lowest quartile of levothyroxine use (Q1) with those in higher quartiles of levothyroxine use. In addition to digestive system cancer, the risks of most cancers such as head and neck, lung, breast, female genital system, brain and central nervous system, and hematologic system cancers increased in the high-dose levothyroxine groups (Supplemental Table S3). Comparisons of the high-dose levothyroxine groups to the Q1 group determined by the levothyroxine dosage per body weight also showed consistent results for increased cancer risks (Supplemental Table S4).

In addition, we conducted additional analyses stratified by the type of thyroidectomy (unilateral lobectomy [n=79,733] and completion or total thyroidectomy [n=260,701]) and using RAI therapy (non-RAI [n=203,320] and RAI [n=139,600]). Regardless of the type of thyroidectomy, the risks for all SPCs and digestive system cancer showed gradually increased risks from Q1 to Q4 and were higher in the highest dose levothyroxine group compared to the non-levothyroxine group (Supplemental Tables S5, S6). In thyroid cancer patients not receiving RAI, the risks for all SPCs and digestive system cancer were increased in the Q3 and Q4 groups (Supplemental Table S7). In thyroid cancer patients receiving RAI, there was no patient in the non-levothyroxine group. Thus, when compared to the Q1 group, the high-dose levothyroxine groups showed the elevated risks of more types of cancer such as head and neck, lung, breast, brain and central nervous system, and hematologic system cancers, in addition to digestive system cancer (Supplemental Table S8). Taken together, these findings of the stratified analyses according to the thyroidectomy type and RAI were consistent with those of the overall analysis.

To address the participants’ adherence issue, we conducted further analysis in patients who had good compliance with the MPR of 80% or higher (n=269,859) (Supplemental Table S9). Similar to the overall analysis results, the high-dose levothyroxine was associated with the increased risks of all SPCs and digestive system cancer. Additionally, the Q4 group showed the increased risk of head and neck, breast, uterine, kidney, and brain and central nervous system cancers.

In the present study, we observed that patients taking high-dose levothyroxine after thyroid cancer operation had an increased risk of developing SPC compared to patients that were never prescribed with levothyroxine, even after adjustment for the RAI therapy. Since levothyroxine dosage can be based on the patient’s body weight, it may be more accurate to analyze HR according to levothyroxine dosage per kilogram of body weight. The subgroup analysis included patients with known body weight information, who were also most patients included in this study (260,567/342,920 [75.9%]) and revealed a consistently positive association between high levothyroxine dosage per body weight and the risk of SPC. Furthermore, we found similar results in models that adjusted for various confounding variables, in the stratified analyses based on thyroidectomy type and RAI therapy, and in an analysis of a subgroup of patients with strong adherence.

TSH suppression therapy with levothyroxine is a mainstay in the management of thyroid cancer following surgery. However, insufficient evidence of benefit has been documented in lowrisk patients, and aggressive TSH suppression therapy with high-dose levothyroxine can cause side-effects. Therefore, the recent treatment guidelines for patients with differentiated thyroid cancer suggest less aggressive TSH suppression for intermediate-risk and low-risk patients than the previous treatment guidelines [20-22]. Moreover, the 2015 American Thyroid Association guidelines introduced a dynamic risk stratification, which is a restaging system based on the response to initial treatment and recommended tailoring the intensity of therapy over time for individual patients [20]. The most frequently documented side-effects of TSH suppression therapy include exacerbation of ischemic heart disease [23], increased risk of cardiac arrhythmias [24], and deterioration of osteoporosis [25]. This study showed that thyroid cancer patients taking high-dose levothyroxine were at a higher risk of developing gastrointestinal tract, and hepato-biliary-pancreatic cancers in a dose-dependent manner. Thus, TSH suppression therapy and optimal dosage of levothyroxine should be decided after weighing the potential risks and benefits.

Previous studies have shown that thyroid hormones play a significant role in the carcinogenesis process via complex mechanisms [26]. Thyroid hormones are known to influence a myriad of oncological events via genomic and non-genomic pathways, controlling transcription factors, altering angiogenesis, and promoting invasiveness, and act in a cell type-specific manner [27,28]. Moreover, the expression of thyroid hormone receptors and iodothyronine deiodinases, enzymes which are involved in the activation and deactivation of thyroid hormones, varies across organs and is also affected by thyroid hormone status [29]. However, the extent to which thyroid hormones influence cancer development in each organ has yet to be determined. This study is noteworthy in this aspect because it identified the effects of levothyroxine on cancer development across organs according to dosage.

In our study, the risk of several cancers was found to be the least in the group of patients receiving the lowest dose of levothyroxine (Q1 group) compared to the non-levothyroxine group. As a result, when compared to the Q1 group, participants in the high-dose levothyroxine groups had a higher risk of more types of cancer when compared to participants in the Q1 group. Based on the results of previous Korean cohort studies that presented TSH levels according to levothyroxine administration after thyroid lobectomy [30,31], the Q1 group may have a lower TSH level than the non-levothyroxine group. Some studies reported that TSH has biological functions in human cells other than thyroid cells [32-35], and it may especially promote angiogenesis via directly stimulating endothelial cells [34]. Thus, the lowest dose of levothyroxine therapy might have a protective effect on the development of certain types of cancers by blocking the cancer-promoting effects of TSH. However, further studies are necessary to confirm this effect.

Levothyroxine is primarily absorbed in the upper gastrointestinal tract ranging from 40% to 80%. Consequently, 20% to 60% of levothyroxine is excreted in the stool, resulting in a supra-physiological exposure of the colonic epithelium to thyroid hormone [36]. The thyroid hormone status affects the growth and homeostasis of gastrointestinal organs through binding to thyroid hormone receptors in the gastrointestinal epithelium [37]. Therefore, our results might show that thyroid cancer patients taking high-dose levothyroxine are at a higher risk of developing gastrointestinal tract cancers. However, the effect of levothyroxine on carcinogenesis remains controversial as there is little or no evidence regarding its association with this type of cancer [38-43]. In addition, this study showed that thyroid cancer patients taking high-dose levothyroxine had a higher risk of developing hepato-biliary-pancreatic cancers in a dose-dependent manner. The thyroid hormone status might influence the liver tumorigenesis, but the effects are complex. Some studies suggest that this may depend on the thyroid hormone receptor expression status, cancer stage, or other co-effectors present in the tumor microenvironment [26,44].

The main strength of this study is its novelty. To the best of our knowledge, this is the first study that evaluated the risk of SPC in thyroid cancer patients according to the dose of levothyroxine received. Another strength is the assurance of consistent results through meticulous control of potential influencing factors. This involved employing methods such as models that adjusted for multiple confounding variables and various subgroup analyses. In addition, the study benefits from the availability of comprehensive medical records for each participant without any recall bias. Patients’ medical records were extracted from the NHID; hence all recorded data were not distorted by individual and subjective memories, and no patient was overlooked during the study period. Finally, the results of our study are nationally representative findings of the entire Korean population.

However, our study had several limitations. First, we used health insurance claims data, which could not assess the actual patients’ compliance with levothyroxine. However, we applied the MPR, which is the most commonly used method for calculating adherence [18], and validated the results by conducting a subgroup analysis on patients who had good adherence with the MPR of 80 or higher [19]. Moreover, medical data are even more accurate compared to survey or subjective data which are susceptible to recall bias. Second, our claims data do not include results of thyroid function tests, including thyroid hormone or TSH levels. We considered the high-dose levothyroxine group to be on relatively aggressive TSH suppression therapy, and thyroid hormone or TSH levels may be associated with the risk of SPCs than levothyroxine itself. However, the dosage of levothyroxine can be affected by several factors, such as age, sex, body weight, and the type of thyroidectomy. Thus, we cannot exclude the possibility of bias in determining the degree of TSH suppression with levothyroxine doses. Nevertheless, we evaluated the risk of SPCs according to levothyroxine dosage per body weight and performed a stratified analysis of unilateral lobectomy and completion or total thyroidectomy. The findings of these analyses were consistent to the results of the overall analysis. Finally, as the limitations of health insurance claim data, we did not include information such as tumor stage, which could lead to residual confounding. Nevertheless, we adjusted for cumulative doses of RAI therapy, the most well-known factor influencing SPC risks in thyroid cancer patients. Furthermore, we demonstrated comparable positive associations between levothyroxine dosage and the risk of SPCs in the stratified analyses according to the thyroidectomy type and RAI therapy.

In conclusion, this nationwide population-based study identified a link between high-dose levothyroxine use and an increase in the incidence of SPC among thyroid cancer patients. Given the relationship, physicians should carefully tailor levothyroxine dosage to patients before and during TSH suppression therapy.