Elevated liver enzymes found during a routine blood test can be concerning, especially for individuals with hypothyroidism. Hypothyroidism, characterized by an underactive thyroid gland, causes insufficient production of thyroid hormones needed to regulate metabolism. A strong relationship exists between the thyroid and the liver, meaning this hormonal imbalance can directly affect liver health. Elevated liver enzymes, specifically Alanine Transaminase (ALT) and Aspartate Transaminase (AST), are commonly observed in people with untreated or poorly managed thyroid deficiency.
Understanding ALT and AST Enzymes
Alanine Transaminase (ALT) and Aspartate Transaminase (AST) are enzymes routinely measured as part of a liver function panel. These molecules, known as transaminases, play a crucial role in amino acid metabolism within cells. When cells are injured or damaged, these enzymes leak into the bloodstream, allowing their levels to be detected in a blood sample.
ALT is primarily concentrated in the liver, making it the more specific indicator of liver cell damage (hepatocellular injury). AST is also found in the liver, but it is less specific because it is present in significant quantities in other tissues, including the heart, skeletal muscles, and kidneys. Therefore, an elevation in AST alone may indicate muscle injury rather than a problem solely within the liver. Measuring both ALT and AST helps physicians determine the likely origin of the cellular damage.
The Mechanism Linking Hypothyroidism to Elevated Enzymes
The link between an underactive thyroid and elevated liver enzymes stems from the systemic metabolic slowdown caused by hormone deficiency. Thyroid hormones, particularly triiodothyronine (T3), are essential for maintaining the metabolic rate of nearly all organs, including the liver. Low levels of T3 and thyroxine (T4) compromise liver function, leading to subtle injury of the main liver cells, the hepatocytes.
One proposed mechanism involves decreased blood flow to the liver, which is insufficient to meet the organ’s metabolic needs. This mismatch creates a state of relative oxygen deprivation, or hypoxia, particularly around the central veins of the liver lobules. This lack of oxygen causes stress and damage to the liver cells, resulting in the leakage of ALT and AST into the circulation.
Hypothyroidism also disrupts normal lipid metabolism, causing fat accumulation within the liver cells, known as hepatic steatosis or fatty liver. This fat buildup can cause inflammation and injury, prompting enzyme release. The resulting elevation of ALT and AST is typically mild to moderate, rarely exceeding five times the upper limit of the normal range. In some cases, elevated AST may result from myopathy (muscle damage), a known complication of severe or long-standing thyroid deficiency.
Alternative Causes of Elevated Liver Enzymes
Elevated ALT and AST levels are not unique to hypothyroidism; they represent a non-specific sign of cellular injury. When a physician encounters these abnormal results, they must consider several other potential causes in a process called differential diagnosis. The single most common cause of mildly elevated liver enzymes in the general population is Non-Alcoholic Fatty Liver Disease (NAFLD), which is now often referred to as Metabolic Dysfunction-Associated Steatotic Liver Disease (MASLD).
Other frequent causes include chronic viral infections, such as Hepatitis B and C, which directly target and damage liver cells. Medications are another major factor, as many common prescription and over-the-counter drugs, including certain cholesterol-lowering statins and high doses of acetaminophen, can cause liver stress. Excessive alcohol consumption is also a well-known cause of liver inflammation and enzyme elevation. Since AST is present in muscle tissue, conditions like intense exercise or muscle trauma must also be ruled out, especially if the AST level is disproportionately higher than the ALT.
Normalization of Enzymes Through Thyroid Treatment
When medical evaluation confirms that hypothyroidism is the primary reason for increased liver enzymes, the prognosis is generally favorable. The elevated enzyme levels are expected to return to the normal range once the underlying thyroid condition is successfully treated with hormone replacement therapy. Treatment typically involves a synthetic thyroid hormone, such as levothyroxine, which restores the body’s metabolic balance.
As the medication corrects the thyroid hormone deficiency, the metabolic stress on the liver is relieved, allowing damaged hepatocytes to recover. While the thyroid-stimulating hormone (TSH) level may take several weeks to stabilize, a corresponding drop in ALT and AST levels usually follows within weeks to a few months. Follow-up blood work monitoring both TSH and liver enzymes is necessary to confirm the resolution of the enzyme elevation and that the patient has achieved a stable euthyroid state.