Nonalcoholic fatty liver disease (NAFLD) is the most prevalent liver disease, affecting about 25% population worldwide (1). From a pathological perspective, NAFLD includes liver injuries of varying severity degrees and the resulting liver fibrosis (2). Of these, hepatic steatosis (fatty liver) alone is defined as Nonalcoholic fatty liver (NAFL), while a more severe liver damage with apparent inflammation in hepatocytes is known as Nonalcoholic steatohepatitis (NASH) (2). NASH is not only known to be the early stage of multiple irreverseble liver diseases, such as cirrhosis, liver failure and hepatocellular carcinoma, but also strongly associated with adverse outcomes in non-liver diseases like cardiovascular disease and malignant disease (3). Therefore, before developing to inrreverseble status, it still warrants the attention of assessing other risk factors relating to NAFLD progression while still awaiting the effective therapies of NAFLD.

In the human body, thyroid hormones (THs), including thyroxine (T4), triiodothyronine (T3) and thyroid-stimulating hormone (TSH), are all essential hormones in mediating the synthesis and metabolism of fatty acids and cholesterol in the liver (4). Their participations in regulating the whole body’s metabolism are critical to maintain specific functions of several tissues and cell types (5). Imbalances of these hormones may result in overactive metabolism due to excess thyroid hormones (hyperthyroidism), or underactive activity due to deficient thyroid hormones (hypothyroidism). Depending on the severity of this imbalance, both types of THs disorder can be divided into clinical or subclinical status, with the latter being more common (6). Subclinical hypothyroidism (SH) has been considered as a risk factor for NAFLD. Furthermore, the possibility of liver steatosis improvement through SH treatment is also raised (7). As the liver and thyroid are closely associated with thyroid hormone, its important role in de novo lipogenesis (DNL), beta-oxidation (FAO), cholesterol metabolism and carbohydrate metabolism has led to a number of animal studies and clinical trials investigating TH analogues and TR agonists as potential therapies for NAFLD and hyperlipidemia (8). However, the relevant evidence above the clinical level is incomplete and still needs to be further supplemented.

So far, four published meta-analyses (9–12) have explored the relationship between thyroid and NAFLD. Interestingly, their conclusions are mutually controversial. Gu (9) suggested that the level of TSH might be a risk factor for the development and progression of NAFLD, which was independent of other THs levels; He et al. (10) found that both subclinical and overt hypothyroid patients were at higher risks of NAFLD than those with normal thyroid function. On the other hand, Mantovani et al (11) suggested there was a significant and independent association between primary hypothyroidism and the incidence and severity of NAFLD, but whether subclinical hypothyroidism independently predicts the progression of NAFLD remain uncertain. However, analysis conducted by Jaruvongvanich et al (12). showed there were no significant associations between NAFLD and subclinical, overt or overall hypothyroidism. They also found that the THs level between participants with and without NAFLD had no significant differences.

Although the relationship between the thyroid and NAFLD is widely discussed, it is still not clear if thyroid hormone levels associate with the development of NAFLD. Therefore, we conducted this dose-response meta-analysis of observational studies (incuding cohort and case-control studies) to further investigate on the dosage-dependent correlationship between THs levels, thyroid diseases and NAFLD.

The purpose of this study was to evaluate the relationship between the thyroid function and NAFLD risk as well as the NAFLD-related fibrosis markers. After finishing this meta-analysis of 10 published observational studies, we found that high levels of FT3 are significantly associated with a high risk of NAFLD. Also, there was a significant nonlinear inverse association between elevated FT4 and incidence of NAFLD. What’s more, we found that indicators of liver fibrosis were significantly higher in individuals with high TSH hormone levels.

Thyroid hormones are known to play significant role in regulating the whole-body energy expenditure as well as hepatic lipid metabolism (4). The inner relationship between NAFLD and thyroid hormones was then questioned and studied nowadays. Interestingly, in some clinical traials, thyroxine supplementation has been found to improve obesity and NAFLD to some extent (33, 34). High dosages of T3 were also previously used to promote weight loss in obese patients and to treat hypercholesterolemia (4). Although severe cardiac problems and loss of lean body mass prevent it from becoming a practical treatment, the finding still reinforces the validation of a potentially close relationship between thyroid hormones and metabolic disorders such as NAFLD. Several published meta-analyses (9, 10, 12) and clinical studies (15, 33, 35, 36) reached the conclusion that hypothyroidism strongly associated with NAFLD. A cross-sectional study from Korea (37), of which subjects were classified into subclinical hypothyroidism and overt hypothyroidism according to their TSH, T4, and FT4 levels, found that the spectrum of hypothyroidism correlated with NAFLD in a dose-dependent manner. This finding was consistent with a study from China (38), which suggested that even within the reference range, the gradual increase in serum FT4 level accompanied with reduced prevalence of NAFLD.

Our study also supported the notion that the risk of NAFLD increased with decreasing FT4 in a dose-dependent way. However, our analysis result does not fit perfectly with the assumption that strong association exists between hypothyroidism and NAFLD risk. Another study from Germany (39) came up with a similar result that even though serum TSH and FT3 concentrations were inconsistently associated with hepatic steatosis, low FT4 concentrations were significantly associated with hepatic steatosis. We did not find a clear dose-dependent relationship between TSH and NAFLD, and high levels of FT3 were associated with higher risk of NAFLD. It is possible that polymorphisms in the TSH receptor affect the plasma TSH to thyroid hormone ratio (40), genetic determinants, and aging can alter the set point of the TSH-FT4 feedback mechanism, resulting in a weaker TSH-FT4 association (15). The overall meta-analysis of TSH on the degree of liver fibrosis in NAFLD conducted by us found that individuals with higher levels of TSH had more significant liver fibrosis, which may confirm the relationship between TSH and FT4, and is also in line with other clinical studies. Martínez-Escudé et al. (41) found that TSH could be an independent risk factor for NAFLD and liver fibrosis in the general population. The study found the FIB-4 index of patients with subclinical hypothyroidism was higher than that of subjects with normal thyroid function (42). Punekar et al. (43) found significantly higher TSH levels in patients with cirrhosis compared to controls. Therefore, it is very likely that there is a correlation between TSH levels and the severity of liver disease. However, we unfortunately did not obtain a significant dose-response relationship between TSH and liver fibrosis after integrating the results of more large observational studies.

Studies have shown that low normal FT4 levels are significantly associated with higher insulin resistance but the relationship between FT3 and various metabolic syndrome features is unclear (40). In addition, clinical pharmacists and endocrinologists generally agree that FT3 levels within the normal range are not strongly related to thyroid function. Therefore, FT3 and FT4 may not be completely synchronized, but this does not mean that FT3 not relate to NAFLD. The investigators (19, 23) found that low FT3 levels appeared to be an independent risk factor for NAFLD and advanced liver fibrosis. Physiologically, FT3 is a biologically active form of thyroid hormone that, by binding to TRβ in hepatocytes (44), reduces intrahepatic triglyceride and cholesterol content (45), as well as inflammation and oxidative stress, and restores mitochondrial function in hepatocytes (46), to prevent the onset and progression of liver fibrosis. Correspondingly, more and more researchers (20, 47, 48) have validated the FT3/FT4 ratio as a noninvasive marker of NAFLD through clinical studies. If this hypothesis is true, the clinical diagnosis and prevention of NAFLD will be further improved.

As there is a correlation between hypothyroidism and NAFLD, it’s worth exploring if there also exists a causal relationship, which could be explained by the dose-response relationship between thyroid hormone/hypothyroidism and NAFLD. This question is also important as to whether thyroid hormone can be used clinically as a serum marker of NAFLD. A recently published mendelian randomization study (49) noted a suggestive association between hypothyroidism and NAFLD. However, after adjusting for genetically predicted BMI, the association was not significant, suggesting that this association might be influenced by BMI. The association between thyroid dysfunction and NAFLD affected by BMI, dyslipidemia and other factors was also observed in the prospective cohort study conducted by Bano et al (15). In our subgroup analysis, the association between TSH and NAFLD was also changed after adjusting for age, gender and other factors. In summary, we may hypothesize that thyroid dysfunction contributes to the development and progression of NAFLD, but it may be influenced by other factors (50), which need to be considered if thyroid hormones are used to model NAFLD non-invasive diagnostic tools. Of course, it still needs to be further investigated through analyzing longer prospective studies with larger samples.

Furthermore, this meta-analysis is the first to include high-quality (NOS>6) observational studies (case-control studies, cohort studies), and to conduct two methods (overall and dose-response meta-analysis) to explore the relationship between thyroid hormones and the risk of NAFLD and liver fibrosis. Be that as it may, this study still suffers from the following shortcomings. Firstly, although we included observational studies, the prospective cohort studies were more convincing in terms of the level of evidence for evidence-based medicine. Secondly, in the included studies the investigators focused more on TSH and FT4, but not on FT3 or other thyroid peroxidase antibody titers for which the amount of data was too small to allow adequate meta-analysis. Thirdly, the small range of thyroid hormones explored in the current article fails to observe the effect of higher levels of thyroid hormones on NAFLD, as well as possible implications for the study results. Forthly, the heterogeneity of the study results cannot be ignored. Although we explored the sources of heterogeneity through subgroup and sensitivity analyses, we were still unable to explain this phenomenon well. Due to the limited number of studies, the possibility of chance (“false positives”) for “positive” results in subgroup analyses cannot be ruled out.

In conclusion, this meta-analysis suggests that high FT4, FT3 levels were associated with increased NAFLD, while high TSH levels positively correlated with liver fibrosis. However, more comprehensive prospective studies with larger samples are still needed to further clarify the relationship between thyroid-related hormones and NAFLD and to achieve more clinically meaningful evidence.

LX: Writing – review & editing, Writing – original draft, Visualization, Methodology, Funding acquisition, Data curation, Conceptualization. YC: Writing – original draft, Methodology, Conceptualization. JS: Writing – review & editing, Project administration, Conceptualization. RL: Writing – original draft, Software, Investigation, Data curation. FQ: Writing – original draft, Project administration, Methodology, Data curation. YZ: Writing – original draft, Investigation, Data curation. WZ: Writing – original draft, Investigation, Data curation. lY: Writing – review & editing, Funding acquisition. XZ: Writing – review & editing, Supervision, Funding acquisition, Conceptualization.