Yes, thyroid disorders can raise liver enzymes, and the connection is more common than most people realize. Between 15% and 76% of people with untreated hyperthyroidism show abnormal liver test results, and hypothyroidism independently increases the risk of fatty liver disease. Both an overactive and an underactive thyroid can disrupt liver function through different mechanisms.
How an Overactive Thyroid Affects the Liver
Hyperthyroidism, where the thyroid produces too much hormone, frequently causes liver enzyme elevations. The most commonly affected marker is alkaline phosphatase (ALP), which is abnormal in roughly 44% to 64% of people with untreated hyperthyroidism. ALT, the enzyme most specific to liver cell damage, is elevated in about 33% of cases, while AST runs high in about 23%. GGT, another liver marker, can be elevated in up to 62% of cases.
The elevations are usually mild, typically less than five times the upper limit of normal. The liver is one of the main sites where thyroid hormones are processed and converted into their active form, so when those hormone levels surge, the liver works harder. Excess thyroid hormone increases oxygen demand in liver cells, which can outpace blood supply and cause minor cellular stress. It also ramps up fat metabolism and bile acid production in ways that strain normal liver function.
One important nuance: ALP can come from either the liver or the bones. Hyperthyroidism speeds up bone turnover, so a high ALP reading in someone with an overactive thyroid doesn’t always mean liver damage. If GGT is also elevated alongside ALP, that points toward a liver source. If GGT is normal, the ALP increase is more likely coming from bone.
How an Underactive Thyroid Affects the Liver
Hypothyroidism affects liver enzymes through a slower, more indirect route. The liver depends on thyroid hormones to carry out its core metabolic work, particularly processing fats and cholesterol. When thyroid hormone levels drop, the liver’s ability to break down fatty acids slows while fat delivery continues. Cholesterol and triglyceride levels climb, and fat accumulates in liver cells. This is the pathway to non-alcoholic fatty liver disease (NAFLD), which is one of the most common causes of chronically elevated liver enzymes in the general population.
A large meta-analysis found that overt hypothyroidism raises the odds of developing NAFLD by about 70%, with adjusted figures closer to 80% higher risk. Even subclinical hypothyroidism, where thyroid levels are only slightly off, increases NAFLD risk by 40% to 63%. Among people already diagnosed with fatty liver disease, 15% to 36% also have hypothyroidism, suggesting the two conditions frequently overlap.
Hypothyroidism also increases oxidative stress in the liver, promoting inflammation and insulin resistance that can worsen fat buildup over time. It reduces the activity of enzymes responsible for processing bilirubin, which can lead to higher bilirubin levels and, in some cases, gallstone formation. Sluggish gallbladder emptying, driven by the general slowing of muscle activity in hypothyroidism, contributes to gallstone risk as well.
The Thyroid-Liver Relationship at a Biological Level
The thyroid and liver are deeply interdependent. The liver converts roughly 80% of the inactive thyroid hormone (T4) into its active form (T3), so the liver is essential for thyroid function. In the other direction, thyroid hormones regulate many of the liver’s core jobs: building and burning fat, producing cholesterol, making bile acids, and managing blood sugar.
When thyroid hormone levels are normal, they keep fat production and fat burning in balance inside liver cells. They stimulate the enzyme that converts cholesterol into bile acids, which is the body’s primary way of eliminating excess cholesterol. They also promote mitochondrial health in liver cells, maintaining the energy supply needed for these processes. Disrupting thyroid hormone levels in either direction throws this balance off, and liver enzymes rise as a downstream signal of that disruption.
Medications Used for Thyroid Disease Can Also Raise Enzymes
Sorting out whether elevated liver enzymes come from the thyroid disorder itself or from the drugs used to treat it can be tricky. The two most common medications for hyperthyroidism, methimazole and propylthiouracil (PTU), both carry some risk of liver injury, but in different patterns.
Methimazole causes hepatitis (liver inflammation) at roughly 3.2 per 1,000 person-years, compared to 1.2 per 1,000 for PTU. That risk is dose-dependent, with high-dose methimazole carrying about a fivefold greater hepatitis risk than PTU. However, PTU is associated with a higher rate of acute liver failure, the more dangerous outcome, at 0.68 per 1,000 person-years compared to 0.32 for methimazole. In practical terms, both risks are low, but they mean that new or worsening liver enzyme elevations during treatment shouldn’t automatically be blamed on the underlying thyroid condition.
The good news is that for most people whose liver enzymes were elevated before starting treatment, those levels normalize as thyroid function is brought under control. In studies tracking patients over 10 months of treatment, ALT and AST levels returned to normal in the majority of cases as the hyperthyroidism resolved.
What Elevated Enzymes Mean for You
If you have a thyroid disorder and your blood work shows elevated liver enzymes, the thyroid condition itself is a plausible explanation. Mild elevations, particularly in someone with untreated or poorly controlled thyroid disease, often don’t signal a separate liver problem. They reflect the metabolic strain that abnormal thyroid hormone levels place on the liver.
The pattern of which enzymes are elevated can offer clues. In hyperthyroidism, ALP and GGT tend to be the most commonly affected. In hypothyroidism, the picture often looks more like fatty liver disease, with modest ALT elevations. If liver enzymes are very high, rising rapidly, or don’t improve as thyroid levels normalize, that warrants further investigation for other causes.
For people with hypothyroidism, getting thyroid levels into the normal range with hormone replacement often improves liver enzyme levels over time, partly by reversing the fat accumulation and metabolic slowdown that caused them. For those with hyperthyroidism, enzyme levels typically normalize within months of achieving stable thyroid function, though your doctor may monitor them more closely during the early phase of treatment to distinguish disease-related changes from medication effects.