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Nonalcoholic Fatty Liver Disease: Thyroid Function and Gut Health as Disease Risk Factors

With increasing rates of obesity and diabetes, the associated condition of nonalcoholic fatty liver disease (NAFLD) has become increasingly common, such that it is now the number one cause of liver disease in Western countries. NAFLD affects 10 to 46% of the U.S. population, with a worldwide prevalence of 10 to 40%. NAFLD is subdivided into nonalcoholic fatty liver (NAFL) and nonalcoholic steatohepatitis (NASH). The former presents without significant liver inflammation, while the latter is associated with hepatic inflammation that cannot be distinguishable histologically from its alcoholic counterpart. About 30 and 40% of adults in the U.S. have NAFL, while about 3 to 12% of US adults have the more severe form of NASH.

NAFLD is a condition predominantly characterized by fatty infiltration of the liver. Fat, in the form of triglycerides, accumulates in the cells of the liver. This triglyceride accumulation may be due to increased levels in circulation which eventually deposit in the liver, decreased export out of the liver, or decreased fatty acid breakdown. Increased levels of triglycerides are common in type 2 diabetes, and correspondingly between about 60 to 75% of individuals with type 2 diabetes also have been shown to have NAFLD. Obesity plays a role in both the development of both type 2 diabetes and NAFLD, and NAFLD has been shown to be present in 30 to 90% of individuals who are obese.

NAFLD is considered a “silent” liver disease: with its asymptomatic nature and oftentimes normal labs, it is easy to miss clinically even with proper screening. In addition to this, there are additional reasons why NAFLD may be underdiagnosed, particularly in the population without known risk factors such as diabetes and obesity where screening may be less stringent.

One contributor to underdiagnosis is that the standard laboratory ranges to which the alanine aminotransferase (ALT) and aspartate aminotransferase (AST) values are compared often utilize “normal” upper limits which are too high, usually around 40 IU/L, when in fact, the normal upper limit of ALT ranges from 29 to 33 IU/L in men and 19 to 25 IU/l in women.

Another factor is the poor sensitivity of diagnostic ultrasound, the most common tool used to evaluate for fatty liver changes. NAFLD is diagnosable by having liver fat of more than 5% (in absence of alcohol consumption of more than 15 drinks a week for men and 10 drinks a week for women), however, liver ultrasounds are optimally sensitive to detecting liver changes only when fat percentage is greater than 12.5%. Ultrasound has even worse sensitivity for diagnosing NAFLD in the obese.

One thing that may contribute to a lack of rigorous screening is the lack of a well-studied efficacious medication for the treatment of NAFLD. Beyond providing immunizations for hepatitis A and B viruses to prevent further liver-affecting complications, the only recommendations typically suggested that may impact elevated liver enzymes are alcohol abstinence, weight loss, and the proper medical management of blood sugar and cholesterol.However, the race to develop an FDA-approved medication is on, with a phase 2 study recently completed which shows promise for a medication known as VK2809.This medication, a thyroid receptor β-agonist, selectively affects hepatic thyroid receptors that play a role in cholesterol metabolism. In the phase II studies, it was shown to significantly reduce low density lipoprotein (LDL) cholesterol and reduce hepatic fat content between 56.5 to 59.7% depending on the dose studied. However, similar thyromimetics have made it this far only to fail in phase III, so if this drug makes it to the clinic remains to be seen.

As we seek natural treatment strategies for NAFLD, it is important to consider the underlying conditions that may contribute to fatty liver changes and hepatic inflammation. Not surprisingly, one factor may be thyroid function, and the other, the health of the gut.

Hypothyroidism and NAFLD
Hypothyroidism is very common condition population-wide with up to 2% of the population experiencing overt hypothyroidism, and numbers as high as 10% affected by subclinical hypothyroidism. A 2017 meta-analysis of 13 studies assessing the possible relationship between NAFLD and hypothyroidism found that both overt hypothyroidism and subclinical hypothyroidism were independently correlated with NAFLD. In one study, individuals with hypothyroidism were approximately twice as likely to have NAFLD, and about four times as likely to have NASH. From these and other findings, hypothyroidism has even been suggested as an independent risk factor for NAFLD.

Hypothyroidism is associated with metabolic changes, including insulin resistance, dyslipidemia, and obesity,each being factors that can contribute to the increased risk of NAFLD. In addition to these factors, higher levels of oxidative stress have been seen in patients with hypothyroidism,which can contribute to the development of NAFLD. Both animal and human studies have shown that treatment with thyroid hormone has the ability to improve NAFLD.

Hypothyroidism also is a contributing factor to gut dysbiosis. Hypothyroidism contributes to prolonged gastric and intestinal transit times and diminished bile secretion, both of which play a role in the development of small intestinal bacterial overgrowth (SIBO). In the other direction, digestive diseases associated with malabsorption and even Helicobacter pylori infection can be factors that make hypothyroidism resistant to treatment with typical oral therapies.

The Gut–Liver Connection
Increasing evidence shows that the gut and liver have multiple levels of associated interdependence, and disturbance of the gut–liver axis has been implicated in several conditions linked to obesity, including NAFLD.Liver enzyme elevation and fatty liver changes are commonly seen in gastrointestinal conditions such as SIBO, celiac disease, and inflammatory bowel disease (IBD). A recent meta-analysis also found that patients with gastroesophageal reflux disease were at a significantly increased risk of developing NAFLD. It doesn’t stop there; an association has also been shown with H. pylori infection and NAFLD as well.

One common denominator among these conditions is the integrity, or lack thereof, of the gut mucosal barrier. “Leaky gut,” the common term for increased intestinal permeability, has been demonstrated in each of these conditions, and it has not been a stretch for hepatologists and gastroenterologists alike to connect this common underpinning with NAFLD. With the compromised intestinal barrier that is hallmark to leaky gut, bacterial-derived endotoxin, also known as lipopolysaccharide (LPS), is able to pass into circulation and trigger a defensive inflammatory response.

In addition to alterations in the gut microbiome such as SIBO or H. pylori infection, a high-fat diet (HFD) also contributes to increased intestinal permeability and related endotoxemia. Endotoxemia contributes to the backup of bile flow at the level of the hepatocytes and related cellular inflammation and damage.[43] However, much like the gut-brain axis where there is communication in both directions, the biliary stasis related to endotoxemia can further contribute to an altered balance of gastrointestinal flora and diminished motility.

Finally, an additional mechanism that dysbiotic gut flora may contribute to fatty liver changes is through the production of alcohol or other toxic metabolites. Significantly higher levels of blood alcohol have been observed in obese animals while mice protected from dysbiosis have decreased liver disease despite leaky gut. Increased levels of alcohol-producing gut bacteria as well as elevated blood alcohol levels have been seen in children with NASH, while alcohol production attributable Candida albicans or Saccharomyces cerevisiae overgrowth has also been reported.

Much like many medical conditions, NAFLD is a multifactorial issue, impacted by metabolic function, the endocrine system, and gut health as well. Given the many interweaving pathways connecting each of these conditions, a holistic plan of management should include comprehensive evaluation and appropriately selected treatment strategies which attempt to address the cause, and not just the symptom, of the underlying disease.

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