Thyroid cancer is a prevalent public health issue with a steadily increasing incidence worldwide in recent decades. According to Global Cancer Statistics 2024, an estimated 44,020 new cases and 2,170 deaths occurred in the United States (1). Over the past four decades, thyroid cancer incidence has risen by 313%, largely due to the enhanced detection capabilities afforded by the extensive use of imaging studies and the advent of fine-needle aspiration biopsies (2). Thyroid cancer is categorized into four primary histological subtypes based on cellular origin, molecular pathogenesis, clinical presentation, and progression: papillary thyroid cancer (PTC), follicular thyroid cancer (FTC), medullary thyroid cancer (MTC), and anaplastic thyroid cancer (ATC). PTC is the most prevalent type, accounting for 80-85% of all thyroid cancer cases, followed by FTC (10-15%), MTC (3-5%), and ATC (<2%) (3). Thyroid cancer development is affected by both genetic and environmental factors. Numerous risk factors for thyroid cancer have been recognized, including exposure to ionizing radiation (4), iodine imbalance, familial thyroid cancer history, and specific genetic syndromes. Additionally, gender may influence thyroid cancer susceptibility, with a notable predominance in females who are three times more likely to develop the disease compared to males (5) and involve a multifaceted interplay of genetic and epigenetic alterations. Thyroid cancer development and progression have been linked to mutations in key genes including BRAF, RAS, RET, and P53 (6, 7). These mutations activate oncogenic signaling pathways such as MAPK and PI3K/AKT, facilitating cell proliferation, survival, and invasion (6, 8). Epigenetic alterations, such as DNA methylation (9) and histone modifications (10), have been implicated in thyroid carcinogenesis. Surgery remains an effective treatment for patients with a suspected or cytologically confirmed differentiated thyroid cancer (DTC) which consistently exhibits high survival rates. The administration of radioactive iodine post-surgery enhances overall survival among patients at elevated risk of recurrence. The utilization of antiangiogenic multikinase inhibitors (eg, sorafenib, lenvatinib, cabozantinib) and therapies targeted at genetic mutations responsible for thyroid cancer is on the rise in the management of metastatic disease (11). The discovery of new biomarkers for thyroid cancer has significantly advanced understanding of its molecular pathogenesis, enabling the creation of more personalized treatment approaches for affected patients. Despite advancements in molecular testing and the discovery of promising therapies, effective treatments for advanced metastatic iodine-refractory thyroid cancer are still lacking. Thus, its diverse subtypes and complex pathogenesis necessitate a comprehensive understanding to optimize management and improve patient outcomes.

Humans harbor a vast and diverse community of microorganisms including bacteria, fungi, viruses, and archaea collectively known as the human microbiota (12). These microbes reside on our skin, in our digestive system, respiratory system, and reproductive and in various other body sites, which is increasingly recognized as a key factor in health and disease (13). Recent decades have seen substantial advancements in comprehending the microbiota-cancer interactions, uncovering intricate relationships that affect cancer development, progression, and treatment outcomes (14, 15). Research on the causal link between microbiota and cancer mainly concentrates on gut microbiota (16, 17). With the improvement of genome sequencing over the past decade, intratumor microbiota have been detected within the microenvironment of various solid tumors, challenging the traditional view of tumors as sterile entities (18, 19). Recent research has illuminated the presence and potential impact of intratumor microbiota and this emerging field of study is rapidly gaining momentum, revealing the intricate interplay between these microbial inhabitants and cancer. Intratumor microbiota, integral to the tumor microenvironment (TME), significantly influence cancer initiation, progression, and therapeutic responses by modulating immune responses and metabolic pathways (20, 21). Hitherto, intratumor microbiota have been identified in a variety of cancers, such as colorectal, pancreatic, bladder gastric, breast, lung, ovarian, prostate, and thyroid cancers (22–25). However, research examining the relationship between thyroid cancer and intratumor microbiota remains limited. Herein, we review the origin and diversity of intratumor microbiota, summarize current findings on their role in thyroid cancer, and explore the mechanisms by which they may influence cancer development. This review aims to inform and inspire future research in this emerging field.

The investigation of intratumor microbiota has a protracted history, originating with early documentation of tumor-associated infections in antiquity (20). Bacteria were initially identified within human tumors over a century ago (26); however, the comprehensive characterization of the intratumor microbiota has been impeded by its low biomass and constrained diagnostic tools. Advances in sequencing technologies, particularly next-generation sequencing and more recently third-generation sequencing (e.g., Nanopore and PacBio) in the 21st century have facilitated a greater recognition of the presence and significance of microbiota within tumors at an unprecedented depth. This progress has enabled extensive studies that have delineated the diversity, spatial distribution, and potential roles of these microbiota in cancer diagnosis and prognosis (18, 19). In 2020, Poore et al. performed a comprehensive study on intratumor microbiota across over 30 cancer types, revealing significant associations between specific microbiota and various cancers. Their findings suggested that microbial-based cancer diagnostics may offer significant future value to patients (18). Simultaneously, another comprehensive study conducted in 2020 demonstrated that intratumor microbiota predominantly reside intracellularly, being located within both cancerous and immune cells (19). It has been observed that the microbial community within a tumor exhibits a non-random distribution; rather, it was precisely structured into microniches linked to immune and epithelial cell functions that influenced cancer progression (27). A comprehensive analysis of 17,401 samples spanning 35 cancer types has identified the presence of low-abundance fungal DNA and cells across various malignancies (28). Additionally, the findings indicated that these fungal communities coexisted with bacterial populations and immune cells within TME, potentially influencing these niches (28). Intratumor microbiota may actively contribute to tumorigenesis and cancer progression through mechanisms such as direct genotoxicity, immune response modulation, and metabolic reprogramming (20, 29). For example, certain microbiota produce genotoxins that damage host DNA, potentially leading to mutations that drive cancer development (30). Additionally, some microbiota could alter the TME to favor tumor growth (31). Research is ongoing into how intratumor microbiota influence the effectiveness of cancer treatments.

Despite the growing body of evidence supporting the presence of intratumor microbiota, this field faces substantial methodological challenges, particularly concerning the risk of contamination. The detection of microbial DNA in tumor tissues—especially those with low microbial biomass—raises important questions about the authenticity of these findings. Contamination can arise from multiple sources, including laboratory reagents, environmental exposure during sample handling, and sequencing platforms themselves. This issue is especially pronounced in studies utilizing formalin-fixed paraffin-embedded tissues, where both the degradation of nucleic acids and the introduction of exogenous microbial DNA during processing can confound results. Recent studies have underscored the critical importance of implementing robust contamination control measures (32). These include the use of appropriate negative controls (e.g., blank extractions, reagent-only controls), rigorous sterilization procedures during sample collection and processing, and the application of bioinformatic techniques to distinguish true microbial signals from background noise. Moreover, low-biomass microbiome studies demand specialized protocols to minimize and monitor contamination at every step-from DNA extraction to sequencing and data analysis (33). In conclusion, while the presence of intratumor microbiota has been supported by multiple independent studies employing diverse methodologies, continued efforts to standardize protocols and improve contamination control are essential to advance this emerging field and validate its translational potential.

The “thyrogastric syndrome” concept, introduced in the 1950s, is based on the embryological and physiological links between the thyroid gland and the gastrointestinal tract (45). The thyroid gland originates from the endoderm, specifically from the floor of the primitive pharynx, which constitutes a part of the foregut. Thyroid follicular cells and gastric mucosal cells originate from the same endodermal embryonic layer (46). This shared developmental origin has prompted researchers to investigate potential similarities and interactions, including the possibility of microbial colonization within the thyroid gland. Based on this connection, it is reasonable to hypothesize the thyroid gland may harbor microorganisms essential for various physiological functions. Although research on intratumor microbiota in thyroid cancer is less extensive than in other cancers, this emerging field is showing promising progress (Table 1). Recent studies reveal distinct microbial compositions in thyroid cancer tumors compared to nearby peritumor tissues, suggesting a potential role of microbiota in the development and progression of thyroid cancer. In a groundbreaking study, Dai et al. examined microbiota changes in different thyroid microhabitats in thyroid cancer patients (47). The study identified significant variations in microbiota composition and diversity between tumor and peritumor tissues. Specifically, the core microbiota of the thyroid comprised Sphingomonas, Comamonas, Acinetobacter, Pseudomonas, Microvirgula, and Soonwooa. The study demonstrated a notable rise in Sphingomonas and Aeromonas in tumor tissues, whereas Comamonas, Acinetobacter, and Peptostreptococcus were more common in peritumor tissues (47). In a recent study, 109 microbial species were found to be significantly altered when comparing tumor and adjacent normal tissues in PTC. Among these, 14 fungal species were predominantly found in tumor tissues, while 94 fungal species and one archaeal species were more prevalent in normal tissues. Fungal species such as Metarhizium acridum CQMa 102, Saccharomyces cerevisiae YJM1338, and Phaffia rhodozyma were notably more abundant in tumor tissues. The archaeal species Anomalluma dodsoniana was found to be more abundant in tumor tissue than in normal tissue. Interestingly, Candida albicans, Microallomyces dendroideus, and the archaeal species Anomalluma dodsoniana were predominantly observed in normal tissues (25). Proteobacteria were identified as the most abundant phylum among thyroid malignant tumor patients in another study (48). PTC encompasses a spectrum of histologic subtypes, including classical PTC (CPTC), follicular variant PTC (FVPTC), and tall cell PTC (TCPTC). The mycobiome analysis of PTC subtypes identified 63 fungal species with increased abundance relative to normal thyroid tissue, with FVPTC exhibiting the most significant microbial dysregulation, followed by TCPTC and CPTC (25). Among the identified fungal species, Botrytis cinerea, Pichia cephalocereana, and Trematosphaeria pertusa were consistently enriched in CPTC, FVPTC, and TCPTC (25). Differential archaeal abundance was observed across PTC subtypes. TCPTC exhibited enrichment of the uncultured euryarchaeote Alv-FOS5 relative to normal tissue. FVPTC demonstrated overabundance of uncultured marine archaeon and uncultured Pyrobaculum sp. compared to normal samples. In contrast, Halovivax ruber XH-70 and Methanosarcina sp. WH1 showed reduced abundance in CPTC tumors compared to normal tissue (25). A group of microbial species, such as Micrococcus luteus, Frankia sp., Anabaena sp. K119, and uncultured Gammaproteobacteria were observed to be overabundant in normal tissues of these PTC subtypes (49). Notably, Trueperella pyogenes and Stenotrophomonas maltophilia K279a displayed a pattern of dysregulation that was consistent between CPTC and FVPTC (49). Each subtype revealed a distinct microbial signature: Rhodococcus fascians-D188 was prevalent in normal CPTC samples, Acinetobacter baumannii AB0057 in normal FVPTC samples, and Bradyrhizobium sp. BTAi1 in normal TCPTC samples, underscoring the subtleties in microbial ecology that distinguish these cancer variants (49). Yuan et al. found Pseudomonas was the dominant bacterium in PTC, followed by Rhodococcus, Ralstonia, Acinetobacter, and Sphingomonas. The study examined microbiota alterations in PTC tumors across different stages, identifying stage-dependent variations in the abundances of the genera Pseudomonas, Rhodococcus, and Sphingomonas. Pseudomonas spp., the predominant genus across all groups, showed higher abundance in early-stage tumors (T1 and T2) than in advanced stages (T3 and T4). They also found Rhodococcus was significantly more abundant in patients with T1 PTC compared to those with T3 PTC, while Sphingomonas showed higher abundance in T1 and T2 than in T3. The T1_2 tumors predominantly featured the genera Pseudomonas, Rhodococcus, and Sphingomonas.T3_4 tumors predominantly featured the genera Streptococcus, Granulicatella, Haemophilus, and unclassified Rhizobiales, along with unranked Coriobacteriales (48). Notably, the number of microorganisms diminished as the distance from the cancerous tissue increased (39). Sex-specific differences in intratumor microbiota were also observed. Synechococcus sp. CC9311 was found to be overabundant in normal samples among males, whereas it was overabundant in tumor samples among females (49). The term alpha diversity(α-diversity) characterizes the richness and evenness of microbial populations in a specific ecological environment (50). The α-diversity of the intratumor microbiota was significantly lower in males compared to females, although no significant differences in β-diversity were found between sexes (48). The genera Rhodococcus, Ralstonia, Chryseobacterium, and Burkholderia-Caballeronia, Paraburkholderia were observed to be more common in females compared to males (48). A separate study found a significant alteration in the abundance of 88 fungal species in females, whereas only 11 fungal and archaeal species showed differential abundance in males (25). These findings collectively underscored the intricate and dynamic characteristics of intratumor microbiota in thyroid cancer. Distinct microbial profiles associated with different PTC subtypes and stages, along with sex-specific differences, suggest that intratumor microbiota could be crucial in thyroid cancer development and progression. Additional studies are required to elucidate the functions of these microbial communities and assess their potential as diagnostic biomarkers or therapeutic targets.

Growing evidence indicates that the intratumor microbiota may influence thyroid cancer development and progression. Recent research suggested that certain bacterial species present in tumors could act as biomarkers for diagnosing and predicting thyroid cancer outcome. A study conducted by Dai et al. served as an example. The combination of Comamonas and Sphingomonas had been identified as an effective biomarker for differentiating between tumor and peritumor tissues. A greater presence of Sphingomonas was associated with lymph node metastasis, indicating its potential as a prognostic marker in thyroid cancer patients. These findings strongly suggested that Sphingomonas may be actively involved in promoting thyroid cancer progression (47). In addition to serving as potential biomarkers, the intratumor microbiota has also been linked to various clinical parameters that are commonly used to assess tumor aggressiveness, surgical outcomes, and risk stratification in patients with DTC. These parameters, collectively known as the distant metastasis, patient age, completeness of excision, invasion, and tumor size (MACIS) classification, provide valuable information for guiding treatment decisions and predicting patient outcomes (51). Notably, certain intratumor microbiota species has demonstrated significant correlations with MACIS scores and pathologic M stage in patients with PTC. Specifically, Frankia sp. and uncultured Gammaproteobacteria bacterium, which predominated in all PTC normal tissue samples, was linked to lower MACIS scores, indicating a possible protective function. In contrast, Bradyrhizobium sp. BTAi1, uniquely in TCPTC normal tissue, had been correlated with higher MACIS scores, indicating an association with more aggressive disease. Moreover, Fran kia sp. and Anabaena sp. K119, overabundant in normal tissue samples of all PTC subtypes, negatively correlates with pathologic M stage, suggesting a protective role against metastasis. Conversely, Stenotrophomonas maltophilia, found to be dysregulated exclusively in CPTC and FVPTC, is positively associated with pathologic M stage, suggesting a potential connection to enhanced metastatic potential (49). John et al. identified correlations between specific fungal abundances and pathological staging in PTC. The abundance of Chaetomium globosum CBS 148.51 was positively correlated with advancing pathological stage. Furthermore, they found 18 fungal species, including Candida albicans, Eremascus albus, and Thanatephorus cucumeris, to be associated with an elevated pathological M stage. Wickerhamiella pararugosa, uncultured Cryptomycota, and Spiromyces aspiralis were also linked to a higher pathological N stage (25). The interaction between the thyroid microbiota and hormonal regulation appeared to be another critical factor in thyroid carcinogenesis. Elevated levels hormones, particularly thyroid-stimulating hormone (TSH) and thyroid hormones, had been implicated in the development of thyroid cancer (52). Specifically, in conditions such as primary hypothyroidism, compensatory increases in TSH levels can lead to thyroid hyperplasia, potentially elevating the risk of malignancy (52, 53). An intriguing discovery demonstrated that Neisseria perflava engages in closely related interactions with species of Roseburia, Amaricoccus, and Streptomyces, potentially coordinating a series of sequential biochemical processes involving TSH and triiodothyronine (39). In conclusion, the microbiota in thyroid cancer demonstrates multifaceted roles, serving not only as promising biomarkers but also potentially influencing disease progression and metastasis. The significant correlations between specific bacterial species and clinical parameters, including MACIS scores and metastatic stages, suggest that the microbiota plays a crucial role in influencing tumor behavior and patient outcomes. Furthermore, the complex interplay between the microbiota and thyroid hormones adds another dimension to our understanding of thyroid cancer pathogenesis. Future investigations into the functional roles of these microorganisms could potentially revolutionize therapeutic approaches and enhance personalized treatment strategies for thyroid cancer patients. The elucidation of precise mechanisms through which intratumor microbiota influence thyroid carcinogenesis remains essential for integrating microbiota-based diagnostics and interventions into clinical practice.

While the presence and diversity of intratumor microbiota in thyroid cancer are becoming increasingly recognized (47, 49), the precise mechanisms through which they influence tumor development and progression remain largely unexplored. The role of intratumor microbiota in thyroid cancer remains under-researched compared to other cancer types, highlighting a significant gap in understanding. Three primary mechanisms are hypothesized to influence the impact of intratumor microbiota on thyroid cancer ( Figure 2 ).

Recent advancements in detection techniques have significantly improved the rapid identification of microbes residing within cancerous tissues. The role of intratumor microbiota in cancer pathogenesis is a rapidly expanding field, with numerous studies highlighting how these microbial communities may influence cancer initiation, progression and treatments. Microorganisms, once established in tumors, may promote tumorigenesis by enhancing mutation rates, modulating oncogenic signaling pathways, and altering the TME. Targeting specific microbial populations or their metabolic products may enhance the efficacy of existing therapies or pave the way for innovative microbiota-based treatments. Although current evidence supports a correlation between intratumor microbiota and various tumor characteristics, these associations should not be interpreted as causative, and mechanistic insights remain limited without direct functional evidence. Most of the existing studies are observational in nature, and the relationship between microbiota and tumor progression or immune modulation remains to be elucidated.

In thyroid cancer, the discovery of intratumor microbiota presents an intriguing new dimension to understanding its pathogenesis and progression. While research in other cancer types has uncovered valuable insights regarding microbial origin, diversity, and functional roles, much remains to be explored in the context of thyroid cancer, particularly with respect to their therapeutic implications. Disrupting specific cancer-promoting or immunosuppressive microbial populations within tumors could potentially interfere with tumorigenic pathways, modulate the TME, and improve treatment outcomes. Targeting the intratumor microbiota has emerged as a promising strategy for improving treatment efficacy. Probiotics may help restore microbial homeostasis, potentially mitigating pro-tumorigenic effects, while antibiotics could selectively eliminate harmful bacterial species. Additionally, microbiome modulation through dietary interventions or fecal microbiota transplantation may offer novel therapeutic avenues. A deeper understanding of these microbial interactions is essential for optimizing personalized treatment strategies. Future research should focus on identifying specific bacterial signatures associated with thyroid cancer and evaluating the clinical benefits of microbiota-targeted therapies. Integrating microbiome-based approaches with existing treatment modalities may pave the way for more effective and tailored therapeutic options. Additionally, utilizing the intratumor microbiota as a biomarker for cancer diagnosis and prognosis holds promise for advancing personalized medicine.

Moreover, the present literature is largely focused on PTC, with relatively few studies addressing other histological subtypes such as FTC, MTC, and ATC. Future research should aim to explore the microbiota landscape in these subtypes to provide a more comprehensive understanding of the thyroid tumor microenvironment.

Future research on the intratumor microbiota in thyroid cancer could significantly advance new diagnostic and therapeutic approaches. However, several challenges remain. It remains uncertain whether particular microbial changes are a cause or effect of tumor development, or if they occur incidentally. Second, the minimal biomass of tumor-associated microbial communities and the potential for contamination during sample collection and processing highlight the necessity for rigorous controls and standardized protocols to ensure study reproducibility and comparability. Third, a critical consideration in interpreting intratumor microbiota studies lies in the inherent limitations and technical biases of current detection methodologies. While 16S rRNA sequencing remains widely used for bacterial profiling, taxonomic biases still exist. Shotgun metagenomics, though offering strain-level resolution, struggles with low microbial biomass in tumors, where host DNA contamination obscures bacterial signals. To address these challenges, future studies should integrate orthogonal methods. Single-cell RNA sequencing (scRNA-seq) offers unprecedented insights into the cellular heterogeneity of the TME and the specific roles of individual immune cells in relation to microbial presence. This technique allows researchers to dissect the complex interactions between immune cells and intratumor microbiota at a single-cell resolution, providing a clearer picture of how these interactions may influence cancer progression and response to therapy. Additionally, spatial transcriptomics has emerged as a powerful tool to study microbial localization within tumor niches. By mapping gene expression data spatially, this method enables researchers to visualize the precise locations of microbial communities and their interactions with surrounding cells. Understanding the spatial dynamics of intratumor microbiota can reveal new therapeutic targets and improve the design of microbiota-based interventions. Incorporating these cutting-edge methodologies into thyroid cancer research will likely drive significant advancements in our understanding of the microbiota-cancer axis, paving the way for innovative treatment strategies. Fourth, further investigation is necessary to clarify the roles of individual microbial species and their interactions with the TME. Fifth, existing studies on intratumor bacteria’s role in thyroid cancer have mainly concentrated on PTC. In contrast, studies investigating the involvement of intratumor bacteria in other thyroid cancer types are relatively scarce. Finally, translating findings from animal models, particularly mice, to human applications poses a challenge, given the significant interspecies differences that can complicate such efforts. In conclusion, despite significant advancements in understanding intratumor microbiota’s role in thyroid cancer, future research must address existing challenges to fully harness microbiota-based diagnostics and therapies in clinical settings.