MAFLD, formerly named as non-alcoholic fatty liver disease, is a serious liver issue worldwide and will be the leading cause of liver transplantation in the forthcoming decades (Méndez-Sánchez and Díaz-Orozco, 2021). A large number of studies reported that TH regulates hepatic triglyceride and cholesterol metabolism (Zhao et al., 2020). Accordingly, TH increases the activity of hepatic lipase, thus enhancing lipid mobilization from fat droplets. Moreover, TR activation triggers free fatty acid transporting into the hepatocytes (Tanase et al., 2020). Considering the aforementioned facts, the TH/TR axis is likely to be intimately correlated with hepatic diseases such as MAFLD. Accumulating evidence has demonstrated that the TH/TR axis is involved in MAFLD. To be specific, MAFLD is positively related with hypothyroidism, elevated TSH, T3, and thyroid peroxidase antibody (TPOAb), and suppressed T4 (Gor et al., 2021; D'Ambrosio et al., 2021).

The fact that whether the TH/TR axis can be a risk factor in MAFLD is not clear. Martínez-Escudé A et al. (2021) reported that TSH, regarded as a risk factor of MAFLD, is involved in obesity, atherogenic dyslipidemia, metabolic syndrome (MetS), hypertransaminasemia, and altered cholesterol and triglycerides levels. Then, a recent research hint that TSH is a MAFLD risk factor but excludes the FT3 and FT4 levels (Tan et al., 2021). Intriguingly, the result in a middle-aged and elderly euthyroid subjects showed that high-normal FT3 and low-normal TSH independently predict the high incidence of MAFLD (Gu et al., 2021). In addition, Chao et al. thought that FT3 and FT4 are independent risk factors to MAFLD. Conversely, although the level of TSH in non-MAFLD and MAFLD subjects who are undergoing health examinations are significantly different, TSH is excluded as an independent risk factor of MAFLD (Zhang X. et al., 2020; Chao and Chen, 2021). As described earlier, there is still controversy to identify TSH and TH as independent risk factors of MAFLD.

Importantly, the TH/TR axis regulate hepatic lipid metabolism such as mitochondrial fatty acid β-oxidation, lipid autophagy, and expression of lipid-related genes (Sinha et al., 2012; Kagawa et al., 2018). Thus, selective TRβ agonists may improve hepatic lipid disorders and MAFLD (Kowalik et al., 2018; Senese et al., 2019; Ritter et al., 2020). These agonists include MGL-3196 (Taub et al., 2013; Raza et al., 2021; Kannt et al., 2021), MB07811 (Erion et al., 2007), KB-141 (Erion et al., 2007), sobetisome (GC-1) (Huang Y. Y. et al., 2013; Ferrara et al., 2018; Saponaro et al., 2020), KB2115 (Eprotirome) (Ladenson et al., 2010a; Senese et al., 2019), and DITPA (Ladenson et al., 2010b; Senese et al., 2019) (Table 1). The side effects of selective TRβ agonists mostly result from TRα-induced dose-dependent cardiac effects, muscle metabolism, and bone turnover (Erion et al., 2007; Kelly et al., 2014). In short, the TH/TR axis may act as a promising treatment method for MAFLD.

HCC is one of the most common malignant tumors. The TH/TR axis is involved in HCC. Some studies have demonstrated that mutations of TR genes are associated with human carcinoma (Anyetei-Anum et al., 2018; Piqué et al., 2020). A clinical study exhibited that hypothyroidism delays hepatocyte growth while hyperthyroidism promotes HCC (Mishkin et al., 1981). Additionally, the level of TR expression in adenomas (83%) and cancer (68%) is significantly lower than that in normal epithelium (96%) (Liu et al., 2019). Moreover, TH-related mitochondrial turnover protects hepatocytes from HBV hepatocarcinogenesis (Chi et al., 2017; Hossain et al., 2020). In addition, the TH/TR axis regulates proliferation, differentiation, metastasis, and drug resistance, autophagy in HCC (Jazdzewski et al., 2011; Rosen and Privalsky, 2011; Jerzak et al., 2015; Liu et al., 2019; Lin et al., 2020a).

Hepatitis of HBV and HCV infection are global issues, which have a risk to develop severe liver disease such as liver cirrhosis and HCC (Jing et al., 2020; Zhang et al., 2021). A study hinted that the FT3 level decreases in HBV patients, while the FT3 and FT4 levels increase in HCV patients (Orságová et al., 2014). More specifically is that along with the increasing inflammatory grade, the level of TT3 primary increased and then decreased, but only the increased level was significantly statistic (Hu et al., 2020). Interestingly, HBV/HCV coinfection elevates the probability of thyroid dysfunction (Ji et al., 2016). Meanwhile, one of the major problems with interferon therapy in hepatitis is the occurrence of aberrant TSH, T3, and T4 values, as well as autoantibodies and thyroid diseases (Ignatova et al., 1998; Orságová et al., 2014; Karwowska et al., 2018). Nevertheless, Huang MJ et al. (1999) indicated that seropositivity of thyroid autoantibodies should not be a contraindication to IFN therapy in HCV-infected patients. Similarly, a recent research reported that the antithyroid antibodies do not cause severe autoimmune disorders in children with chronic HBV infection and merely associated with subclinical hypothyroidism (Kansu et al., 2004).

Acute liver failure (ALF), characterized by elevated liver biochemistry, coagulopathy, and hepatic encephalopathy (HE) but with no underlying chronic liver disease (CLF), is a severe and complex clinical syndrome (Lopes and Samant, 2021). The TH/TR axis may be involved in ALF. On the one hand, type A HE is strongly related to low TSH in ALF patients with a concerning poor survival rate (Anastasiou et al., 2015; Wang et al., 2017). HINAT ACLF has proposed liver failure incorporating TSH into the standard (Feng and Shi, 2018; Wu et al., 2018). Interestingly, type C HE often happens to patients with cirrhosis and lower T3 and T4 levels (Wang et al., 2017). In addition, ALF induced by surgical liver devascularization in female pigs observed a decrease in serum-free T3 and T4 as well as TRα protein levels (Kostopanagiotou et al., 2009). Intriguingly, thioacetamide-induced ALF promotes hepatocyte proliferation in response to T3 in the rat (Malik et al., 2006). On the other hand, hypothyroidism prevents immune-mediated acute liver injury in mice, subsequently elevating TSH levels and survival rates and declining serum liver enzymes, blood ammonia, and prothrombin time. It has been reported that a patient with ALF results from non-controlled hyperthyroidism (Sousa Domínguez, 2015). Clinically, plasma exchange is an effective method to eliminate TH in acute liver failure with thyroid storm (Zeng et al., 2017). Mechanistically, low T3 and T4 levels in hypometabolism-associated hypothyroidism link to inflammation and oxidative stress (Bruck et al., 1998). In general, high TSH levels and low TH/TR functions manifest a protector in ALF.

Liver fibrosis is characterized by chronic inflammation and fibrous scar formation in the liver, finally resulting in hepatocyte deficiency and loss of hepatic function (Erhardtsen et al., 2021). Advanced fibrosis is associated with decreased serum FT3 levels (Du et al., 2021). As an independent risk factor, an elevated TSH level is significantly correlated with the risk of fibrosis (Martínez-Escudé et al., 2021). In a recent study, compared with 12.19% in chronic hepatitis C (CHC) patients without thyroid disease (TD), severe fibrosis is found at 92.85% among CHC patients with TD (Biciusca et al., 2020). However, studies have suggested that hypothyroidism was not highly associated with fibrosis (D'Ambrosio et al., 2021). Treating with TRβ agonist resmetirom in advanced NASH with fibrosis mice have lower α-smooth muscle actin, fibrogenesis-involved genes, and markers of fibrosis, especially including liver stiffness and N-terminal type III collagen pro-peptide (PRO-C3), which indicate that resmetirom can improve fibrosis (Harrison et al., 2021; Kannt et al., 2021). As a result, the TH/TR axis ameliorates liver fibrosis, although the deeper connection between hepatic fibrosis and the TH/TR axis needs more exploration.

Alcoholic liver disease is caused by long-term heavy drinking, initially manifesting as fatty liver, and hepatocyte necrosis, then developing into alcoholic hepatitis, liver fibrosis, cirrhosis, and liver failure (Wang and Mu, 2021). Papineni et al. (2017) indicated that TH-free T3 (fT3) decreases in alcoholic hepatitis and cirrhosis while fT3 and fT4 increase in chronic alcoholic liver disease patients after treatment. More specifically speaking, low fT3 not only probably reflects the severity of liver disease, the degree of liver damage but may also increase the withdrawal effects and craving for alcohol (Nomura et al., 1975; Israel et al., 1979; Burra et al., 1992). However, it has been reported that alcohol and TH also might cause a hypermetabolic state of the liver and liver cell damage. Accordingly, antithyroid drugs can cure alcoholic fatty liver via inhibiting ethanol metabolic rate (EMR) in chronic ethanol-consuming patients (Szilagyi et al., 1983). Overall, the TH/TR axis may aggravate alcoholic liver disease.

Liver injury is caused by multiple factors mainly including some drugs, poisons, or chronic liver, and extrahepatic diseases (Yamamoto, 1995). In CCl4-induced liver injury in rats, the serum T3 level is reduced due to the decreased release of T3 from liver cells rather than a decreased conversion of T4 to T3 (Ikeda et al., 1986). In general, the TH/TR axis protects against liver damage, and thyroid disorder aggravates the development of liver injury. From the protective aspect, T3 replenishment protects against liver injury via improving oxidative stress, cell ferroptosis, detoxification, and increasing drug transport proteins expression, inflammatory factors, autophagy, and lipid metabolism. DNA damage generated by reactive oxygen species upregulates 8-hydroxy-2-deoxyyguanosine (8-OHdG) levels. During diabetes, hypothyroidism, and hypothyroidism with diabetes, the use of TH downregulates the level of 8-OHdG, protein carbonyl content (PCO), protein oxidation, and advanced oxidation protein products (AOPPs) (Altan et al., 2010). In addition, TH synergizes with methylprednisolone (MP) to improve oxidative stress and liver damage and then realizing anti-inflammatory and antioxidant effects (D’Espessailles et al., 2013). Intriguingly, T3 scavenges lipid peroxyl free radicals and improves cell ferroptosis in the LPS/galactosamine-induced liver injury mouse model (Mishima et al., 2020). Moreover, the combined supplementation of T3 and n-3 polyunsaturated fatty acid (n-3 PUFA) in rat decreases ischemia-reperfusion (IR) liver injury and oxidative stress. From the aggravating aspect, hyperthyroidism promotes liver injury. Otherwise, thionamides, methimazole, and propylthiouracil are associated with drug-induced liver injury (LiverTox, 2012; Yan et al., 2017) (Table 2). In a word, the activation of the TH/TR axis can ameliorate liver injury.

In MAFLD, the deiodinase family members, especially including types 1, 2, and 3 iodothyronine deiodinases (Dio1, 2, and 3) and responsible for the activation and inactivation of TH, can modulate the TH/TR axis. As hepatic enzymes, Dio1 and Dio2 convert T4 to T3 and increase T3/T4 levels. Conversely, Dio3 inactivates TH. To be specific, the activation of Dio3 activates hypoxia-inducible factor 1α (HIF-1α), thus inhibiting T3 signaling and the metabolic rate (Bianco and da Conceição, 2018; Luongo et al., 2019; Russo et al., 2021). Increased Dio1 level promotes β-oxidation of fatty acid and oxidative phosphorylation, then preventing hepatocyte steatosis. Moreover, the level of Dio1 mRNA depends on the dietary conditions. When fed a normal chow diet (NCD), Leprdb mice grows up with severe steatosis with only mild inflammation. The depletion of Dio1 in Leprdb mice upregulates hepatic Tnfa and Co1a1 mRNA levels, which are inflammation and fibrosis biomarkers, respectively (Bruinstroop et al., 2021). Genes related to reverse cholesterol transport and lipase activity decrease with the downregulation of Dio2 in rats (Russo et al., 2021). The sites of de novo DNA hypermethylation (H sites) disrupt long-distant chromatin interactions, looping enhancers, and promoters in hepatocytes. TH produced from Dio2 activation depletes H3K9me3 and interferes with the formation of more than a thousand H sites, subsequently maintaining the liver development and function (Fonseca et al., 2021). In general, the TH/TR axis modulated by the deiodinases may delay MAFLD progression. Nevertheless, the regulatory mechanism of the TH/TR axis in MAFLD remains to be further studied (Figure 1).

The TH/TR axis modulates cyclin-dependent kinase (CDK) and cyclins, MicroRNAs (miRNAs), long non-coding RNA (lncRNA), TGF-β signaling, hedgehog (Hh) (relying on the local deiodinase expression), and other tumor-related genes and -proteins to be involved in the growth, proliferation, and metastasis of HCC (Manka et al., 2018).

In addition, CDKs and regulatory subunits cyclins regulate the cell cycle in mammalian. P21, as a CDK inhibitor, halts G1/S and G2/M transitions of cell cycle progression by inhibiting CDK4,6/cyclin-D and CDK2/cyclin-E, respectively (Karimian et al., 2016). The inhibition of HCC cells growth and proliferation is dependent upon the activation of P21 by the TH/TR axis. A recent study reported that the activation of the TH/TR axis upregulates endoglin in HCC cells, thus restraining P21 polyubiquitination-induced cell proliferation (Lin et al., 2013a). In addition, the TH/TR axis inhibits hepatoma cell growth via repressing UHRF1 and relieves UHRF1-mediated P21 silence (Wu et al., 2015). Furthermore, TH induces the miR-214-3p expression, followed by interfering with the proto-oncogene serine/threonine-protein kinase (PIM-1) and activating P21, thus blocking cell proliferation (Huang et al., 2017). Other CDKs and cyclins also involve in the regulatory mechanism in HCC in a TH/TR axis-dependent fashion. Ezequiel et al. (Ridruejo et al., 2021) suggested that hexachlorobenzene (HCB) is an endocrine disruptor and a liver tumor promoter. In the HepG2 cell line, the depletion of HCB by TH leads to the downregulation of the TGF-β1/pSMAD-2/3 signaling pathway, thus increasing Dio1 levels and decreasing p21 and P27, ultimately suppressing cell proliferation. Beyond that, the silence of TGF-β mice promote the proliferation by increasing the expressions of CDK2, cyclin E, and cyclin A, as well as decreasing the expression of CDKn1a/p21 (Baek et al., 2010). It has been reported that depleting forkhead box M1 (FOXM1) by TH interferes with oncogenic expression of cyclin D1, cyclin E, and CDK2, thereby inhibits HCC cells proliferation (Barrera-Hernandez et al., 1999; Wu et al., 2020). These pathways all manifest that the TH/TR axis closely interacts with p21, CDKs, and its regulatory subunits cyclins to affect cell proliferation in HCC.

Moreover, miRNAs, a class of evolutionarily conserved non-protein-coding small RNA, are responsible for regulating gene expression at the translation level (Oura et al., 2020). The TH/TR axis regulates miRNAs, thereby producing various effects in HCC. For example, the downregulation of the TH/TR axis induces nodule regression and the increased expression of targeted microRNA, miR-27a, miR-181a, miR-204a, and miR-181a in the resistance-hepatocyte rat model (R-H model) and human cirrhotic peritumoral tissue (Frau et al., 2015). Beyond that, miR-214-3p, miR-130b, miR-17, miR-21, miR-424, and miR-503 also participate in the regulation of the TH/TR axis-mediated liver cancer (Huang Y. H. et al., 2013; Lin et al., 2013b; Ruiz-Llorente et al., 2014; Lin et al., 2015). Thus, miRNAs have the potential to be targets in TH/TR-involved HCC.

Furthermore, lncRNA, the human major transcriptional genome, is a length greater than 200 nucleotides, which is a non-coding protein (Dang et al., 2015). A recent study has reported that compared to the non-tumor samples, the expression of lncRNA related genes including MSC-AS1, POLR2J4, EIF3J-AS1, SERHL, RMST, and PVT1 are upregulated in tumor samples. Beyond that, lncRNA genes mostly cluster in the TGF-β signaling pathway, internal ribosome entry pathway, granzyme A mediated apoptosis, FAS signaling pathway, calcium signaling by HBx, and p38/MAPK signaling pathway (Gu et al., 2019). It has been reported that lncRNA CRNDE and lncRNA SNHG7 are independent risk factors of synchronous colorectal liver metastasis (SCLM), which also predict a high tumor recurrence rate (Zhang P. et al., 2020). Since lncRNA is associated with the occurrence of tumor, it is plausible that lncRNA may intimately be regulated by the TH/TR axis. Indeed, the TH/TR axis is related to lncRNA in HCC. For instance, brain cytoplasmic RNA 1 (BCYRN1 or BC200) is widely expressed in tumors. BC200 is also inhibited by T3/TR, then downregulating the expressions of CDK2, cyclin E1, and cyclin E2 and upregulating P21, thereby repressing cell growth and tumor sphere formation and preventing the evolvement of HCC (Lin et al., 2018). Otherwise, the downregulation of taurine upregulated gene 1 (TUG1) by the TH/TR axis also cause AFP mRNA, cyclin E, and H3K27me3 silence and cell growth inhibition (Lin et al., 2020b).

Finally, other TH/TR-related genes and -proteins are involved in HCC. Thyroid hormone receptor-interacting proteins (TRIP), the Zyxin family of LIM proteins, is responsible for regulating transcription of TR. Significantly, the transcriptional activation of the TH/TR axis in HCC may depend on TRIP. Specifically, when TRIP6 activates FOXC1, migration, invasion, and proliferation are strongly promoted. It is also found that TRIP6 induces cyclin D1 expression, decreases p21 and p27 activation, and HCC cell proliferation arrest (Lee et al., 1995; Zhao et al., 2017; Wang et al., 2020). Moreover, the downregulation of TRIP13 impairs the NHEJ repair process, increased apoptosis, and cell cycle arrest at the S-phase, ultimately inhibiting the proliferation, migration, and invasion of HCC cells (Ju et al., 2018). In addition, pituitary tumor transforming gene 1 (PTTG1) is silenced by Sp2, which is negatively mediated by T3/TR in Hep3B hepatoma cells (Chen et al., 2008). Ndrg2 is a Myc suppressor gene. The activation of V-erbA leads to the depletion of Ndrg2, thus exacerbating tumor invasion and metastasis (Ventura-Holman et al., 2011). As a tumor-associated protein, lipocalin 2 (Lcn2) can activate the Met/FAK pathway in a TH/TR axis-dependent manner, thus enhancing tumor cell migration and invasion (Chung et al., 2015). Intriguingly, T3/TR/MEK/ERK/NUPR1/PDGFA cascade may play a vital role in hepatocarcinogenesis. Consistently, T3/TR positively regulates nuclear protein 1(NUPR1) via binding to the NUPR1 promoter regions, therefore promoting vascular invasion (Chen et al., 2019). Recently, it has been found that increasing thyroid hormone responsive (THRSP) prevents the silence of the ERK/ZEB1 signaling pathway and inhibits the process of epithelial-to-mesenchymal transition, subsequently preventing hepatocellular carcinogenesis (Hu et al., 2021). A secreted protein named Dickkopf 4 (DKK 4) antagonizes the Wnt signal pathway and inhibits tumor metastasis, which is dependent upon the activation of the T3/TR axis (Chi et al., 2013).

In short, the TH/TR axis have paradoxical role in the growth, proliferation, invasion, metastasis, and migration of HCC. A relevant regulatory mechanism of the TH/TR axis in HCC remains to be further explored (Figures 2, 3).

In liver injury, the studies show that T3 treatment recovers NF-κB activity, signal transducer, and activator of transcription 3 (STAT3), TNF-α and haptoglobin and increases liver GSH depletion and protein oxidation protecting against IR (Fernández et al., 2007; Mardones et al., 2012). A study exhibited that T3 upregulates the liver redox-sensitive nuclear transcription factor erythroid 2-related factor 2 (Nrf2) DNA, detoxification, and drug transport proteins expression, mainly including protein levels of epoxide hydrolase 1 (Eh1), NADPH-quinone oxidoreductase 1 (NQO1), glutathione-S-transferases Ya (GST Ya), GST Yp, multidrug resistance-associated proteins 2 (MRP-2), mrp-3 ,and MRP-4 in male Sprague–Dawley rats, which may indicate the hepatocyte protective mechanism in liver injury attributed to ROS and chemical toxicity (Cornejo et al., 2013). In addition, the inactivation of Kupffer cell by gadolinium chloride (GdCl3) can suppress T3-induced oxidative stress, thus ameliorating the development of liver injury characterized by neutrophil infiltration and necrosis (Simon-Giavarotti et al., 2002). Accumulating evidence has demonstrated that T3 induces liver preconditioning (PC) against IR supported by triggering AMP-activated protein kinase (AMPK), ultimately accelerating the depletion of inflammatory factors such as hepatic NLRP3 and IL-1β (Fernández et al., 2009; Vargas and Videla, 2017). T3 induces hepatocyte proliferation in toxic liver injury (Malik et al., 2006). T3 injection protects liver IR damage by enhancing MEK/ERK/mTORC1-mediated autophagy in male C57BL/6 mice (Yang et al., 2015). In addition, TH-induced monoamine oxidase (MAO) inhibitors inhibit the activity of MAO protecting against liver injury in rats (Obata and Aomine, 2009). In addition to the TH/TR axis also prevents liver damage. It has been discovered that the accumulation of TR mRNA may remove negative influences in fluoride-related liver injury via preventing disruption of lipid metabolism, oxidative damage, and apoptosis (Bo et al., 2018). Particularly, traditional Chinese medicine is also involved in improving the TH/TR axis-induced liver injury. To be specific, the Yinning Tablet restores the expression of antiapoptotic Bcl-2 cytosol cytochrome c protein and downregulates the expression of l-thyroxine-induced overexpressed caspase-9, -8, -3, proapoptotic BAX and Dio1, thus ameliorating TH-induced liver injury in rats via regulating mitochondria-mediated apoptotic signals (Yang et al., 2020).

The TH/TR axis may be correlated with hepatitis. Some studies have demonstrated that ubiquitin-specific protease 18 (USP18), known as UBP43, participates in gene regulations of the TH signaling pathway. Li et al, (2017) discovered that USP18 regulates the signaling of antivirus by the TH signaling pathway, prolactin signaling pathway, insulin resistance and complement, and have crosstalk among them. Also, the thyroid hormone-uncoupling protein (TRUP) gene and thyroid hormone receptor-associated protein 150 alpha gene are associated to the integration of HBV DNA into liver cell DNA, which are the key regulators of cell proliferation and viability (Gozuacik et al., 2001; Paterlini-Bréchot et al., 2003). In recent years, the relationship between the TH/TR axis and hepatitis of HBV and HCV infection has been gradually elucidated. However, understanding of the antiviral mechanism of the TH/TR axis is not entirely clear, which is still being explored.

Furthermore, the TH/TR axis is associated with alcohol-related hepatic alterations. For example, TH has been proven to increase the level of Dio2, thereby elevating susceptibility to hepatic steatosis in a model of alcoholism (Fonseca et al., 2019; Hernandez, 2019). In addition, the mRNA level of TRIP12 is significantly different in alcohol-feed (AF) and control pair-feed (PF) mice (Zhang et al., 2018) (Figure 4).