Hypothyroidism and Nonalcoholic Fatty Liver Disease: Pathophysiological Associations and Therapeutic Implications

MAFLD Pandemic: Updates in Pharmacotherapeutic Approach Development

With around one billion of the world's population affected, the era of the metabolic-associated fatty liver disease (MAFLD) pandemic has entered the global stage. MAFLD is a chronic progressive liver disease with accompanying metabolic disorders such as type 2 diabetes mellitus and obesity which can progress asymptomatically to liver cirrhosis and subsequently to hepatocellular carcinoma (HCC), and for which to date there are almost no approved pharmacologic options. Because MAFLD has a very complex etiology and it also affects extrahepatic organs, a multidisciplinary approach is required when it comes to finding an effective and safe active substance for MAFLD treatment. The optimal drug for MAFLD should diminish steatosis, fibrosis and inflammation in the liver, and the winner for MAFLD drug authorisation seems to be the one that significantly improves liver histology. Saroglitazar (Lipaglyn®) was approved for metabolic-dysfunction-associated steatohepatitis (MASH) in India in 2020; however, the drug is still being investigated in other countries. Although the pharmaceutical industry is still lagging behind in developing an approved pharmacologic therapy for MAFLD, research has recently intensified and many molecules which are in the final stages of clinical trials are expected to be approved in the coming few years. Already this year, the first drug (Rezdiffra™) in the United States was approved via accelerated procedure for treatment of MAFLD, i.e., of MASH in adults. This review underscores the most recent information related to the development of drugs for MAFLD treatment, focusing on the molecules that have come furthest towards approval.

Incidence and predictors of hepatocellular carcinoma in NAFLD without diagnosed cirrhosis: a nationwide real-world U.S. study

BACKGROUND

A substantial proportion of patients with nonalcoholic fatty liver disease (NAFLD)-related hepatocellular carcinoma (HCC) do not have cirrhosis. Data regarding the incidence and predictors of HCC development in NAFLD without cirrhosis are limited. We conducted a large, national study of NAFLD patients without documented cirrhosis to examine the incidence and predictors for HCC development.

METHODS

This retrospective study included 751,603 NAFLD patients (54% female) without documented cirrhosis derived from the deidentified Optum Clinformatics® Data Mart Database. Patients with cirrhosis, platelets < 120,000/µL or FIB-4 values > 2.67 were excluded.

RESULTS

The mean age was 53.7 ± 15.0 years, 45.9% were male, 39.5% had diabetes, 57.6% were White, 18.4% Hispanic, 8.2% Black and 4.9% were Asian. The mean platelet count was 264,000 ± 72,000/µL, and 96.3% of patients had a FIB-4 < 1.30. Over 1,686,607 person-years of follow-up, there were 76 incident cases of HCC, resulting in an HCC incidence rate of 0.05 per 1000 person-years. There was a higher HCC incidence rate among patients with platelets ≤ 150,000/µL, versus those with platelets > 150,000/µL (0.23 per 1000 person-years, vs. 0.04 per 1000 person-years, p = 0.02) but not in subgroup analyses for age, sex, race/ethnicity or diabetes. Using multivariable Cox proportional hazards model adjusted multiple confounders, platelet count ≤ 150,000/µL remained an independent predictor of HCC development (adjusted HR 5.80, 95% CI 1.67-20.1, p = 0.006).

CONCLUSION

HCC incidence in NAFLD without documented cirrhosis was below the threshold for cost-effective HCC surveillance in overall and multiple subgroup analyses. Platelet count < 150,000/µL may be a useful predictor of HCC development in this population.

Thyroid Hormone and Mitochondrial Dysfunction: Therapeutic Implications for Metabolic Dysfunction-Associated Steatotic Liver Disease (MASLD)

Metabolic dysfunction-associated steatotic liver disease (MASLD), formerly termed nonalcoholic fatty liver disease (NAFLD), is a widespread global health concern that affects around 25% of the global population. Its influence is expanding, and it is anticipated to overtake alcohol as the leading cause of liver failure and liver-related death worldwide. Unfortunately, there are no approved therapies for MASLD; as such, national and international regulatory health agencies undertook strategies and action plans designed to expedite the development of drugs for treatment of MASLD. A sedentary lifestyle and an unhealthy diet intake are important risk factors. Western countries have a greater estimated prevalence of MASLD partly due to lifestyle habits. Mitochondrial dysfunction is strongly linked to the development of MASLD. Further, it has been speculated that mitophagy, a type of mitochondrial quality control, may be impaired in MASLD. Thyroid hormone (TH) coordinates signals from the nuclear and mitochondrial genomes to control mitochondrial biogenesis and function in hepatocytes. Mitochondria are known TH targets, and preclinical and clinical studies suggest that TH, thyroid receptor β (TR-β) analogs, and synthetic analogs specific to the liver could be of therapeutic benefit in treating MASLD. In this review, we highlight how mitochondrial dysfunction contributes to development of MASLD, and how understanding the role of TH in improving mitochondrial function paved the way for innovative drug development programs of TH-based therapies targeting MASLD.

Thyroid Dysfunction Induced by Fungicide Famoxadone Exposure Contributes to Nonalcoholic Fatty Liver Disease in Male Mice: In Vivo, In Vitro, and In Silico Studies

Thyroid dysfunction has become a serious public health problem, which is considered a trigger of nonalcoholic fatty liver disease (NAFLD). Pesticide exposure could contribute to thyroid dysfunction and NAFLD, but the relationship between these factors remains unclear. In this study, the effects of subchronic famoxadone exposure on thyroid and liver at no observed adverse effect level (NOEL) related concentrations were investigated using in vivo, in vitro, and in silico models. Famoxadone caused hepatic steatosis, lipid metabolism disorder, and liver oxidative stress and induced NAFLD in male mice. The suppression of hepatic fatty acid β-oxidation was the key factor of NAFLD, which was highly associated with hypothalamic-pituitary-thyroid (HPT) axis hormones disorder. Famoxadone disrupted thyroid hormone biosynthesis by causing thyroid follicle aberrations and abnormal HPT axis-related gene expression. In vitro studies confirmed that famoxadone inhibited the transport of thyroxine (T4) into hepatocytes and the conversion of T4 to triiodothyronine (T3). In silico studies verified that famoxadone interfered with the binding of thyroid hormones to proteins mediating thyroid hormone transport, conversion, and activation. This study comprehensively reported the association between NAFLD and thyroid dysfunction caused by famoxadone, providing new perspectives for the health risk evaluation of pesticides with a similar structure in mammals.

Retrospective Evaluation of Gastrointestinal Signs in Hypothyroid Dogs

Few observations about gastrointestinal (GI) signs in hypothyroid dogs (hypo-T dogs) are available. We aimed to evaluate the prevalence and characteristics of concurrent GI signs in hypo-T dogs, describe clinicopathological, hepato-intestinal ultrasound findings in hypo-T dogs, investigate changes in GI signs after thyroid replacement therapy (THRT). Medical records of suspected hypo-T dogs from two hospitals were retrospectively reviewed. The inclusion criteria were: (1) having symptoms and clinicopathological abnormalities related to hypothyroidism (i.e., mild anemia, hyperlipemia); (2) not being affected by systemic acute disease; (3) not having received any treatment affecting thyroid axis. Hypothyroidism had to be confirmed using low fT4 or TT4 with high TSH and/or inadequate TSH-stimulation test response; otherwise, dogs were assigned to a euthyroid group. Clinical history, GI signs, hematobiochemical parameters, and abdominal ultrasound findings were recorded. Hypo-T dogs were assigned to the GI group (at least 2 GI signs) and not-GI group (1 or no GI signs). Follow-up information 3-5 weeks after THRT was recorded. In total, 110 medical records were screened: 31 dogs were hypo-T, and 79 were euthyroid. Hypo-T dogs showed a higher prevalence of GI signs (44%), especially constipation and diarrhea (p = 0.03 and p = 0.001), than euthyroid dogs (24%) (p = 0.04). Among hypo-T dogs, no difference in hematological parameters between GI and non-GI groups was found. Hypo-T dogs had a higher prevalence of gallbladder alterations than euthyroid dogs (20/25; 80% and 32/61; 52% p = 0.04). The hypo-T GI group showed a significant improvement in the GI signs after THRT (p < 0.0001). Specific investigation for concurrent GI diseases in hypo-T dogs was lacking; however, improvement in GI signs following THRT supports this association between GI signs and hypothyroidism.

Conferring liver selectivity to a thyromimetic using a novel nanoparticle increases therapeutic efficacy in a diet-induced obesity animal model

Optimization of metabolic regulation is a promising solution for many pathologies, including obesity, dyslipidemia, type 2 diabetes, and inflammatory liver disease. Synthetic thyroid hormone mimics-based regulation of metabolic balance in the liver showed promise but was hampered by the low biocompatibility and harmful effects on the extrahepatic axis. In this work, we show that specifically directing the thyromimetic to the liver utilizing a nanogel-based carrier substantially increased therapeutic efficacy in a diet-induced obesity mouse model, evidenced by the near-complete reversal of body weight gain, liver weight and inflammation, and cholesterol levels with no alteration in the thyroxine (T4) / thyroid stimulating hormone (TSH) axis. Mechanistically, the drug acts by binding to thyroid hormone receptor β (TRβ), a ligand-inducible transcription factor that interacts with thyroid hormone response elements and modulates target gene expression. The reverse cholesterol transport (RCT) pathway is specifically implicated in the observed therapeutic effect. Overall, the study demonstrates a unique approach to restoring metabolic regulation impacting obesity and related metabolic dysfunctions.

Hypothyroidism and non-alcoholic fatty liver disease: A coincidence or a causal relationship?

Non-alcoholic fatty liver disease (NAFLD) is a global problem. It may be caused by metabolic and hormonal disorders, including hypothyroidism. However, non-thyroid causes of NAFLD in people with hypothyroidism, including improper eating behavior and low physical activity, should be acknowledged. This study aimed to present the current literature on whether the development of NAFLD is related to hypothyroidism or a typical consequence of an unhealthy lifestyle in people with hypothyroidism. The results of previous studies do not allow for an unequivocal determination of the pathogenetic relationship between hypothyroidism and NAFLD. Important non-thyroid-initiating factors include providing too many calories in relation to requirements, consuming excessive amounts of monosaccharides and saturated fats, being overweight, and maintaining low physical activity levels. The recommended nutritional model for both hypothyroidism and NAFLD may be the Mediterranean diet, which is rich in fruits and vegetables, polyunsaturated fatty acids, and vitamin E.

Low-Normal Thyroid Function Is Not Associated with Either Non-Alcoholic Fatty Liver Disease or with Metabolic Dysfunction-Associated Fatty Liver Disease

BACKGROUND

The association of low-normal thyroid function (LNTF) with non-alcoholic fatty liver disease (NAFLD) or metabolic dysfunction-associated fatty liver disease (MAFLD) is controversial; thus, the aim of this study is to determine this association.

METHODS

NAFLD was evaluated by controlled attenuation parameter of transient elastography. Patients were classified by MAFLD criteria. LNTF was defined as TSH levels of 2.5 to 4.5 mIU/L and were divided into three different cut-off points (>4.5 to 5.0, >3.1, and >2.5 mIU/L). Associations between LNTF, NAFLD, and MAFLD were evaluated by univariate and multivariate logistic regression analyses.

RESULTS

A total of 3697 patients were included; 59% (n = 2179) were male, and median age and body mass index were 48 (43-55) years and 25.9 (23.6-28.5) kg/m², respectively, and 44% (n = 1632) were diagnosed with NAFLD. THS levels of 2.5 and 3.1 showed significant associations with the presence of NAFLD and MAFLD; however, LNTF did not show an independent association with the presence of NAFLD or MAFLD in multivariate analysis. According to different cut-off points, patients with LNTF presented similar risks for NAFLD as the general population.

CONCLUSION

LNTF is not associated with NAFLD or MAFLD. Patients with high LNTF are equally at risk for NAFLD as the general population.

CONSTRUCTION AND VALIDATION OF A BUNDLE FOR EVALUATION OF THYROID FUNCTION IN PATIENTS WITH CIRRHOSIS

Thyroid dysfunction has been reported in association with several chronic diseases, including advanced liver disease. •The bundle discussed here is aimed at proposing systematic assistance according to the best evidence-based practices available. •The process of constructing and validating the bundle was carried out in the following stages: a) bibliographic survey; b) bundle elaboration; and c) content validation. •The bundle was considered valid to facilitate medical decision making, aiding physicians to manage, in a practical and effective manner, the thyroid function of patients with liver cirrhosis. Background - Thyroid dysfunction has been reported in association with several chronic diseases, including advanced liver disease. This disease and its management are often neglected in clinical practice. The bundle discussed here is aimed at proposing systematic assistance according to the best evidence-based practices available. Objective - To construct and validate a bundle to evaluate thyroid function in patients with liver cirrhosis. Methods - The process of constructing and validating the bundle was carried out in the following stages: a) bibliographic survey; b) bundle elaboration; and c) content validation. The bibliographic survey was carried out in an integrative review about evidence related with the thyroid function of patients with liver cirrhosis. The findings from the integrative review were considered as supporting evidence for the elaboration of the bundle. The tool then created used accessible language and was evidence-based, ensuring that information was based on current literature. Results - The bundle was restructured to provide guidance on the management of patients with liver dysfunctions, including: cirrhosis due to general causes, cirrhosis due to hepatitis C, non-alcoholic fatty liver disease, primary biliary cholangitis, and hepatocellular carcinoma. The orientations in the bundle included: exams to be requested to screen for thyroid disorders, and guidance about the treatment of these dysfunctions and their associated complications. We analyzed specialist evaluation of the bundle using the Content Validity Index (CVI). We carried out a binomial test to evaluate consistency and specialist agreement regarding the items in the bundle, considering values >0.61 as a good level. The items in the bundle were considered to be valid (CVI >0.80). The general CVI of the instrument was 0.95 (CI95%: 0.91-0.98). Conclusion - The bundle was considered valid to facilitate medical decision making, aiding physicians to manage, in a practical and effective approach, the thyroid function of patients with liver cirrhosis. This tool should not be used as a replacement for individual, evaluation of the physician providing assistance. We recommend the structured bundle to be added to medical practice, considering its simple application, low cost, and potential to contribute for the management of these patients.

Therapeutic Perspectives of IL1 Family Members in Liver Diseases: An Update

Interleukin (IL) 1 superfamily members are a cornerstone of a variety of inflammatory processes occurring in various organs including the liver. Progression of acute and chronic liver diseases regardless of etiology depends on the stage of hepatocyte damage, the release of inflammatory cytokines and disturbances in gut microbiota. IL1 cytokines and receptors can have pro- or anti-inflammatory roles, even dual functionalities conditioned by the microenvironment. Developing novel therapeutic strategies to block the IL1/IL1R signaling pathways seems like a reasonable option. This mode of action is now exploited by anakinra and canakinumab, which are used to treat different inflammatory illnesses, and studies in liver diseases are on the way. In this mini review, we have focused on the IL1 superfamily members, given their crucial role in liver inflammation diseases, specifically discussing their potential role in developing new treatment strategies.

Lessons on Drug Development: A Literature Review of Challenges Faced in Nonalcoholic Fatty Liver Disease (NAFLD) Clinical Trials

NAFLD is the most common chronic liver disease worldwide, occurring in both obese and lean patients. It can lead to life-threatening liver diseases and nonhepatic complications, such as cirrhosis and cardiovascular diseases, that burden public health and the health care system. Current care is weight loss through diet and exercise, which is a challenging goal to achieve. However, there are no FDA-approved pharmacotherapies for NAFLD. This review thoroughly examines the clinical trial findings from 22 drugs (Phase 2 and above) and evaluates the future direction that trials should take for further drug development. These trialed drugs can broadly be categorized into five groups-hypoglycemic, lipid-lowering, bile-pathway, anti-inflammatory, and others, which include nutraceuticals. The multitude of challenges faced in these yet-to-be-approved NAFLD drug trials provided insight into a few areas of improvement worth considering. These include drug repurposing, combinations, noninvasive outcomes, standardization, adverse event alleviation, and the need for precision medicine with more extensive consideration of NAFLD heterogenicity in drug trials. Understandably, every evolution of the drug development landscape lies with its own set of challenges. However, this paper believes in the importance of always learning from lessons of the past, with each potential improvement pushing clinical trials an additional step forward toward discovering appropriate drugs for effective NAFLD management.

Correlation between thyroid function, sensitivity to thyroid hormones and metabolic dysfunction-associated fatty liver disease in euthyroid subjects with newly diagnosed type 2 diabetes

PURPOSE

To estimate the prevalence of metabolic dysfunction-associated fatty liver disease (MAFLD) and to evaluate the associations between thyroid parameters, MAFLD and liver fibrosis in euthyroid patients with newly diagnosed type 2 diabetes mellitus (T2DM).

METHODS

Overall, 776 patients with newly diagnosed T2DM and 120 subjects without diabetes were included. All the participants were euthyroid, and were categorized as non-MAFLD and MAFLD. Demographic information, biochemical parameters, and serum thyroid hormones were collected. The thyroid hormone sensitivity indices were calculated. MAFLD was defined according to abdominal ultrasound and clinical manifestations. Noninvasive fibrosis indices were calculated to identify advanced liver fibrosis.

RESULTS

The prevalence of MAFLD was significantly higher in patients with T2DM than in subjects without diabetes. Levels of free triiodothyronine (FT3) and FT3 to free thyroxine (FT4) ratio were significantly higher in subjects with MAFLD. In patients with T2DM, levels of thyroid stimulating hormone (TSH), Thyroid feedback quantile-based index (TFQI_FT3) calculated using FT3 and TSH, thyrotroph T3 resistance index (TT3RI) and thyrotroph T4 resistance index (TT4RI) were significantly higher in subjects with MAFLD. The prevalence of MAFLD increased with the rise of FT3, FT3/FT4, TSH, and sensitivity to thyroid hormone indices (TFQI_FT3, TT3RI, and TT4RI). But significant correlations were not found between thyroid hormones, sensitivity to thyroid hormones and MAFLD, after adjustment for BMI and HOMA-IR. The incidence of advanced fibrosis tended to increase as the rise of TSH and sensitivity to thyroid hormone indices (TFQI_FT3, TT3RI, TT4RI, and TSHI).

CONCLUSION

MAFLD was prevalent in euthyroid patients with newly diagnosed T2DM. Higher normal FT3, TSH and impaired sensitivity to thyroid hormones are associated with increased incidence of MAFLD, being dependent on other metabolic factors.

NAFLD and thyroid function: pathophysiological and therapeutic considerations

Nonalcoholic fatty liver disease (NAFLD) is a worldwide rising challenge because of hepatic, but also extrahepatic, complications. Thyroid hormones are master regulators of energy and lipid homeostasis, and the presence of abnormal thyroid function in NAFLD suggests pathogenic relationships. Specifically, persons with hypothyroidism feature dyslipidemia and lower hepatic β-oxidation, which favors accumulation of triglycerides and lipotoxins, insulin resistance, and subsequently de novo lipogenesis. Recent studies indicate that liver-specific thyroid hormone receptor β agonists are effective for the treatment of NAFLD, likely due to improved lipid homeostasis and mitochondrial respiration, which, in turn, may contribute to a reduced risk of NAFLD progression. Taken together, the possible coexistence of thyroid disease and NAFLD calls for increased awareness and optimized strategies for mutual screening and management.

Thyroid Stimulating Hormone Levels Are Associated With Genetically Predicted Nonalcoholic Fatty Liver Disease

CONTEXT

Findings from observational studies indicate an association of thyroid hormone levels with the risk of nonalcoholic fatty liver disease (NAFLD); however, conflicting results remain and reverse causality may be a possibility.

OBJECTIVE

This study aimed to evaluate the associations between NAFLD and both plasma thyroxine (T4) and thyroid stimulating hormone (TSH) at the phenotypic and genetic levels.

METHODS

We included 14 797 participants, aged 20 to 74 years who had undergone abdominal ultrasonography during the Third National Health and Nutrition Examination Survey (NHANES III). Multivariable logistic regression analyses were used to examine the observational associations of TSH and T4 with NAFLD. Mediation analyses were performed to study whether the relationship between NAFLD and TSH levels was mediated via potential confounders. A bidirectional, two-sample Mendelian randomization (MR) analysis was used to determine the potential causal relationship.

RESULTS

Multivariable logistic regression model suggested a "dose-response" relationship between TSH (Q4 vs Q1: OR = 1.29; 95% CI, 1.10-1.52; Ptrend = 0.001) and NAFLD. BMI and ALT partially mediated the association between TSH and NAFLD, while the proportion of the mediation effects of BMI and ALT were 39.1% and 22.3%, respectively. In MR analyses, the inverse-variance weighted method was selected as primary method and suggested a putative causal effect of NAFLD on serum TSH levels (OR = 1.022; 95% CI, 1.002-1.043). The result was further validated in the sensitivity analyses.

CONCLUSION

Circulating TSH levels were associated with the risk of NAFLD. MR analysis suggested a putative causal effect of NAFLD on TSH levels.

The Clinical Relevance of Hypothyroidism in Patients with Solid Non-Thyroid Cancer: A Tantalizing Conundrum

Hypothyroidism in patients with solid non-thyroid cancer is a tantalizing entity, integrating an intriguing thyroid hormones (THs)-cancer association with the complexity of hypothyroidism itself. The present narrative review provides a comprehensive overview of the clinical relevance of hypothyroidism in solid non-thyroid cancer. Hypothyroidism in patients with solid non-thyroid cancer is reminiscent of hypothyroidism in the general population, yet also poses distinct challenges due to the dual role of THs in cancer: promoting versus inhibitory. Close collaboration between oncologists and endocrinologists will enable the prompt and personalized diagnosis and treatment of hypothyroidism in patients with solid non-thyroid cancer. Clinical data indicate that hypothyroidism is a predictor of a decreased or increased risk of solid non-thyroid cancer and is a prognostic factor of favorable or unfavorable prognosis in solid non-thyroid cancer. However, the impact of hypothyroidism with respect to the risk and/or prognosis of solid non-thyroid cancer is not a consistent finding. To harness hypothyroidism, or THs replacement, as a personalized anticancer strategy for solid non-thyroid cancer, four prerequisites need to be fulfilled, namely: (i) deciphering the dual THs actions in cancer; (ii) identifying interventions in THs status and developing agents that block tumor-promoting THs actions and/or mimic anticancer THs actions; (iii) appropriate patient selection; and (iv) counteracting current methodological limitations.

Non-Alcoholic Fatty Liver Disease and Hypothyroidism: Review of Clinical and Experimental Studies

Hypothyroidism is a widespread condition affecting people of different socio-economic background and geographical location. A lot of studies highlight the effect of hypothyroidism on the metabolic processes in various organs, including the liver. On the other hand, liver damage often results in the development of non-alcoholic fatty liver disease; however, the data on the impact of hypothyroidism on liver morphology, which can serve as a direct indicator and marker of liver condition and function, are limited and controversial. In this report, we reviewed the relationship between non-alcoholic fatty liver disease and hypothyroidism with an accent on morphological alteration of the liver discovered in clinical and experimental studies.

Incidences of hypothyroidism and autoimmune thyroiditis are increased in patients with nonalcoholic fatty liver disease

BACKGROUND AIMS

Nonalcoholic fatty liver disease (NAFLD) has emerged as the most prevalent chronic liver disease, with about one in four people being affected in most areas of the world. Due to its strong association with metabolic syndrome, NAFLD is associated with high morbidity and mortality. Recent data have suggested a pathophysiological association between NAFLD and thyroid disease, but most studies are explorative and results remained conflicting. Here, we aimed at evaluating a potential association between NAFLD and hypothyroidism as well as autoimmune thyroiditis primary care patients in Germany.

METHODS

Incidence rates of hypothyroidism and autoimmune thyroiditis were compared between a cohort of n = 40,583 patients with NAFLD and a cohort of equal size without NAFLD that was matched by sex, age, index year, obesity and diabetes within 10 years from the index date.

RESULTS

Within the 10 years observation period, incidence of hypothyroidism was significantly higher among patients with NAFLD compared to patients without NAFLD [hazard ratio: 1.53 (95% confidence interval, 1.43-1.64), P < 0.001]. Importantly, regression analysis revealed that the association was significant in both men and women patients as well as in patients from all age groups. Moreover, NAFLD patients showed a significantly higher incidence rate of autoimmune thyroiditis compared to patients without NAFLD [hazard ratio: 1.55 (95% confidence interval, 1.34-1.79), P < 0.001]. This association was most pronounced in patients >70 years.

CONCLUSION

Our study provides strong evidence for a significant association between NAFLD and hypothyroidism as well as autoimmune thyroiditis in a large population-based cohort in Germany. This finding should trigger a particular awareness of thyroid diseases in this increasingly important patient group.

Hypothyroidism-Associated Dyslipidemia: Potential Molecular Mechanisms Leading to NAFLD

Thyroid hormones control lipid metabolism by exhibiting specific effects on the liver and adipose tissue in a coordinated manner. Different diseases of the thyroid gland can result in hypothyroidism. Hypothyroidism is frequently associated with dyslipidemia. Hypothyroidism-associated dyslipidemia subsequently results in intrahepatic accumulation of fat, leading to nonalcoholic fatty liver disease (NAFLD), which leads to the development of hepatic insulin resistance. The prevalence of NAFLD in the western world is increasing, and evidence of its association with hypothyroidism is accumulating. Since hypothyroidism has been identified as a modifiable risk factor of NAFLD and recent data provides evidence that selective thyroid hormone receptor β (THR-β) agonists are effective in the treatment of dyslipidemia and NAFLD, interest in potential therapeutic options for NAFLD targeting these receptors is growing. In this review, we summarize current knowledge regarding clinical and molecular data exploring the association of hypothyroidism, dyslipidemia and NAFLD.

Obesity and Thyroid Axis

Development of obesity is primarily the result of imbalance between energy intake and energy expenditure. Thyroid hormones influence energy expenditure by regulating cellular respiration and thermogenesis and by determining resting metabolic rate. Triiodothyronine influences lipid turnover in adipocytes and impacts appetite regulation through the central nervous system, mainly the hypothalamus. Thyroid-stimulating hormone may also influence thermogenesis, suppress appetite and regulate lipid storage through lipolysis and lipogenesis control. Subclinical hypothyroidism may induce changes in basal metabolic rate with subsequent increase in BMI, but obesity can also affect thyroid function via several mechanisms such as lipotoxicity and changes in adipokines and inflammatory cytokine secretion. The present study investigated the complex and mutual relationships between the thyroid axis and adiposity.