Levothyroxine is the most commonly prescribed medication for hypothyroidism in the UK, and many patients wonder whether it could affect their liver health. Concerns about fatty liver disease are understandable, particularly given the complex relationship between thyroid function and metabolism. Current evidence does not establish levothyroxine as a direct cause of non-alcoholic fatty liver disease (NAFLD), now termed metabolic dysfunction-associated steatotic liver disease (MASLD). However, thyroid disorders themselves can influence liver metabolism, and patients with hypothyroidism may face overlapping risk factors for developing fatty liver. This article examines the evidence, explores the thyroid-liver connection, and provides guidance on managing liver health whilst taking levothyroxine.

Understanding Levothyroxine and How It Works

Levothyroxine is a synthetic form of thyroxine (T4), the primary hormone produced by the thyroid gland. It is the most commonly prescribed medication for hypothyroidism—a condition where the thyroid gland does not produce sufficient thyroid hormone. In the UK, levothyroxine is available under various brand names and is typically taken as a once-daily oral tablet.

The mechanism of action of levothyroxine closely mirrors that of natural thyroid hormone. Once absorbed, levothyroxine is converted in peripheral tissues (including liver, kidney, and muscle) to triiodothyronine (T3), the active form of thyroid hormone, through a process called deiodination. T3 binds to thyroid hormone receptors in cells throughout the body, regulating numerous metabolic processes including:

• Energy metabolism and basal metabolic rate

• Protein synthesis and cellular growth

• Cardiovascular function including heart rate and contractility

• Lipid metabolism and cholesterol regulation

Thyroid hormones are essential for maintaining normal metabolic homeostasis. When thyroid hormone levels are optimised through levothyroxine replacement, patients typically experience improvements in symptoms such as fatigue, weight gain, cold intolerance, and cognitive difficulties.

Taking levothyroxine correctly: For optimal absorption, take levothyroxine consistently at the same time each day, at least 30 minutes before breakfast (or at bedtime, at least 3 hours after food). Separate levothyroxine from iron supplements, calcium supplements, antacids, and cholestyramine by at least 4 hours, as these can reduce absorption. The MHRA emphasises the importance of consistent brand use where possible, as bioavailability can vary between different levothyroxine preparations. If you switch products and experience symptoms, contact your GP; a thyroid function test may be needed.

Safety considerations: The medication is generally well-tolerated when dosed appropriately. Most adverse effects arise from either under-replacement (persistent hypothyroid symptoms) or over-replacement. Excessive thyroid hormone can cause hyperthyroid symptoms such as palpitations, tremor, anxiety, and—importantly—may increase the risk of atrial fibrillation and reduced bone mineral density, particularly in older adults. Caution is needed in patients with ischaemic heart disease or the elderly, who may require lower starting doses and gradual titration.

Monitoring: Dosing is individualised based on thyroid function tests, particularly thyroid-stimulating hormone (TSH) levels. Your GP will typically check TSH (and free T4 if indicated) 6–8 weeks after starting treatment or changing dose, then at least annually once stable. Regular monitoring ensures that thyroid hormone levels remain within the therapeutic range, minimising the risk of complications.

Reporting side effects: If you experience any side effects whilst taking levothyroxine, report them to your GP. You can also report suspected side effects directly via the MHRA Yellow Card scheme at yellowcard.mhra.gov.uk or through the Yellow Card app.

References: MHRA/EMC Summary of Product Characteristics (levothyroxine sodium); MHRA Drug Safety Update (levothyroxine product switching, 2019/2021); NICE NG145: Thyroid disease: assessment and management; NHS: Underactive thyroid (hypothyroidism).

Can Levothyroxine Cause Fatty Liver Disease?

There is no established direct causal link between levothyroxine therapy and the development of non-alcoholic fatty liver disease (NAFLD), now increasingly termed metabolic dysfunction-associated steatotic liver disease (MASLD). Current evidence from clinical studies and pharmacovigilance data does not identify levothyroxine as a hepatotoxic medication or a direct cause of hepatic steatosis (fat accumulation in the liver). The MHRA Summary of Product Characteristics for levothyroxine does not list fatty liver disease among its recognised adverse effects.

Levothyroxine undergoes conversion by deiodination in several tissues (including liver and kidney) and is metabolised in the liver through conjugation (glucuronidation and sulphation), with enterohepatic circulation. It is excreted via urine and bile. Levothyroxine is not recognised as causing direct liver injury when used at appropriate replacement doses. Abnormal liver function tests may occasionally occur in the context of thyrotoxicosis or over-replacement, rather than as a direct drug effect. In fact, optimising thyroid function with levothyroxine may improve aspects of liver health in patients with hypothyroidism, as thyroid hormones play important roles in hepatic lipid metabolism—though the evidence for this is largely observational and from small trials, and causality is not proven.

However, it is important to distinguish between direct causation and association. Some patients taking levothyroxine may develop fatty liver disease, but this is typically related to:

• Underlying hypothyroidism itself (which can contribute to metabolic dysfunction)

• Shared risk factors such as obesity, insulin resistance, and metabolic syndrome

• Coexisting conditions that predispose to both thyroid dysfunction and liver disease

If fatty liver disease is detected in a patient taking levothyroxine, the focus should be on identifying other contributing factors rather than attributing causation to the medication itself. Discontinuing appropriately prescribed levothyroxine would be inappropriate and potentially harmful, as untreated hypothyroidism carries its own metabolic risks and may itself worsen metabolic liver disease. Instead, a comprehensive assessment of metabolic health, lifestyle factors, alcohol intake, and other potential causes of hepatic steatosis should be undertaken. Any concerns about liver health whilst taking levothyroxine should be discussed with a GP or endocrinologist for proper evaluation.

References: MHRA/EMC Summary of Product Characteristics (levothyroxine sodium); NICE NG49: Non-alcoholic fatty liver disease: assessment and management; NHS: Non-alcoholic fatty liver disease; systematic reviews on hypothyroidism and NAFLD association (e.g., Mantovani et al.).

The Link Between Thyroid Function and Liver Health

The relationship between thyroid function and liver health is bidirectional and complex. Thyroid hormones play crucial roles in hepatic metabolism, whilst the liver is essential for thyroid hormone activation and clearance. Understanding this interplay is important when considering liver health in patients with thyroid disorders.

Hypothyroidism and liver metabolism: Untreated or inadequately treated hypothyroidism can adversely affect liver function through several mechanisms. Thyroid hormones regulate hepatic lipid metabolism, including cholesterol synthesis, lipoprotein production, and fatty acid oxidation. When thyroid hormone levels are low, these processes become dysregulated, potentially leading to:

• Increased hepatic lipid accumulation and elevated cholesterol levels

• Reduced hepatic lipid clearance and impaired fat oxidation

• Insulin resistance which further promotes hepatic steatosis

• Decreased metabolic rate contributing to weight gain and metabolic syndrome

Observational research has demonstrated that patients with hypothyroidism have a higher prevalence of NAFLD/MASLD compared to euthyroid individuals. Studies suggest that thyroid dysfunction may be associated with increased risk of developing fatty liver disease, with some evidence indicating that even subclinical hypothyroidism (elevated TSH with normal thyroid hormone levels) may be associated with increased liver fat content. However, much of this evidence is observational, and associations do not prove causality; confounding factors such as obesity and metabolic syndrome may contribute.

Thyroid hormone replacement and liver health: Importantly, appropriate treatment of hypothyroidism with levothyroxine can help normalise hepatic metabolism. Evidence—largely from observational studies and small trials—suggests that optimising thyroid function may:

• Improve lipid profiles and may reduce hepatic fat accumulation

• Enhance insulin sensitivity and glucose metabolism

• Support weight management efforts

• Potentially reduce the progression of existing fatty liver disease

Some studies have shown improvements in liver enzyme levels and hepatic steatosis following thyroid hormone replacement in hypothyroid patients, though high-quality randomised controlled trial evidence is limited. This underscores that the thyroid disorder itself, rather than its treatment, poses metabolic risks to liver health. Maintaining optimal thyroid function through appropriate levothyroxine therapy is therefore an important component of metabolic health management.

Hyperthyroidism and the liver: It is also worth noting that thyrotoxicosis (including iatrogenic over-replacement with levothyroxine) can be associated with abnormal liver function tests, including transaminase or cholestatic abnormalities.

References: Systematic reviews/meta-analyses on thyroid dysfunction and NAFLD; NICE NG49: Non-alcoholic fatty liver disease: assessment and management; EASL-EASD-EASO MASLD guidance.

Risk Factors for Fatty Liver in Thyroid Patients

Patients with thyroid disorders, particularly hypothyroidism, may face multiple overlapping risk factors for developing non-alcoholic fatty liver disease (NAFLD) or metabolic dysfunction-associated steatotic liver disease (MASLD). Understanding these risk factors enables better prevention and early detection strategies.

Metabolic and lifestyle factors represent the primary drivers of NAFLD/MASLD in thyroid patients:

• Obesity and increased body mass index (BMI): Excess body weight, particularly central adiposity, is the strongest risk factor for fatty liver disease

• Insulin resistance and type 2 diabetes: These conditions frequently coexist with both hypothyroidism and NAFLD/MASLD, creating a metabolic triad

• Dyslipidaemia: Elevated triglycerides and cholesterol, common in hypothyroidism, contribute to hepatic fat accumulation

• Metabolic syndrome: The clustering of obesity, hypertension, dyslipidaemia, and insulin resistance significantly increases NAFLD/MASLD risk

• Sedentary lifestyle: Physical inactivity compounds metabolic dysfunction

• Dietary factors: High intake of refined carbohydrates, saturated fats, and fructose promotes hepatic steatosis

• Alcohol intake: Excessive alcohol consumption must be excluded before diagnosing NAFLD/MASLD; even moderate intake can contribute to liver disease

Thyroid-specific considerations also influence liver health:

• Inadequate thyroid hormone replacement: Suboptimal levothyroxine dosing that leaves patients biochemically hypothyroid perpetuates metabolic dysfunction

• Duration and severity of hypothyroidism: Longer periods of untreated thyroid dysfunction may increase metabolic complications

• Autoimmune thyroid disease: Conditions like Hashimoto's thyroiditis may be associated with other autoimmune or inflammatory conditions affecting metabolism

Additional risk factors to consider include:

• Age (NAFLD/MASLD prevalence increases with age)

• Male sex (though post-menopausal women have increased risk)

• Polycystic ovary syndrome (PCOS), which frequently coexists with both thyroid dysfunction and insulin resistance

• Certain medications (corticosteroids, some antipsychotics, amiodarone, tamoxifen, methotrexate, valproate)

• Genetic predisposition

Assessment and risk stratification: Before diagnosing NAFLD/MASLD, secondary causes of liver disease should be excluded, including viral hepatitis and, where appropriate, conditions such as Wilson's disease. For thyroid patients with suspected or confirmed fatty liver disease, comprehensive metabolic assessment is important. NICE guidance on NAFLD emphasises identifying and managing cardiovascular risk factors.

In UK primary care, non-invasive fibrosis risk stratification is recommended using the FIB-4 score or NAFLD fibrosis score as first-line tools, with age-adjusted thresholds. If these suggest intermediate or high risk of advanced fibrosis, the Enhanced Liver Fibrosis (ELF) test may be used to further assess fibrosis risk. Patients with high-risk scores or indeterminate results with clinical concern should be referred to hepatology services for specialist assessment.

Regular monitoring of weight, blood pressure, lipid profiles, and glucose metabolism—alongside thyroid function tests—enables early identification of metabolic deterioration. Lifestyle interventions including weight management (7–10% body weight loss can significantly improve liver health), increased physical activity, and dietary modification remain the cornerstone of NAFLD/MASLD prevention and management, complementing optimal thyroid hormone replacement.

References: NICE NG49: Non-alcoholic fatty liver disease: assessment and management; BSG guidance on abnormal liver blood tests and NAFLD/MASLD risk stratification; NHS: Non-alcoholic fatty liver disease.

When to Seek Medical Advice About Liver Concerns

Patients taking levothyroxine should be aware of signs and symptoms that warrant medical evaluation for potential liver problems, though it is important to emphasise that serious liver complications are uncommon with thyroid hormone replacement therapy.

Contact your GP if you experience:

• Persistent upper abdominal discomfort or pain, particularly in the right upper quadrant

• Unexplained fatigue that is disproportionate to your thyroid status or worsens despite stable thyroid function

• Jaundice (yellowing of the skin or whites of the eyes)

• Dark urine or pale-coloured stools

• Unexplained nausea, loss of appetite, or weight loss

• Easy bruising or bleeding, which may indicate impaired liver synthetic function

• Abdominal swelling or distension

Seek urgent medical help (contact 111 or 999) if you experience:

• Severe jaundice with confusion or drowsiness

• Vomiting blood or passing black, tarry stools

• Severe right upper quadrant pain with fever

• Rapidly worsening abdominal swelling

It is worth noting that many of these symptoms overlap with hypothyroidism itself, which is why regular thyroid function monitoring is essential. If your thyroid levels are well-controlled but you develop new symptoms, further investigation may be warranted.

Routine monitoring considerations: There is no UK guidance recommending routine liver function testing solely for patients taking levothyroxine. However, your GP may arrange liver blood tests if:

• You have risk factors for liver disease (obesity, diabetes, metabolic syndrome, significant alcohol intake)

• Abnormalities are detected during routine blood tests

• You develop symptoms suggestive of liver dysfunction

• You have other conditions affecting the liver

If fatty liver disease is suspected or detected, your GP may use non-invasive fibrosis risk scores (FIB-4 or NAFLD fibrosis score) as a first step. If these suggest intermediate or high risk of advanced fibrosis, an Enhanced Liver Fibrosis (ELF) blood test may be arranged to further assess fibrosis risk. Patients with high-risk results or indeterminate results with clinical concern will be referred to hepatology services for specialist assessment, as per NICE NG49 guidance.

Important safety advice: Never discontinue levothyroxine without medical supervision, even if you have concerns about liver health. Untreated hypothyroidism poses significant health risks and may itself worsen metabolic liver disease. If fatty liver disease is diagnosed, management typically focuses on:

• Lifestyle modification: Weight loss (7–10% of body weight can significantly improve liver health), increased physical activity, and dietary changes

• Optimising thyroid function: Ensuring levothyroxine dosing is appropriate

• Managing comorbidities: Controlling diabetes, hypertension, and dyslipidaemia

• Avoiding hepatotoxins: Limiting or stopping alcohol consumption and reviewing other medications

Your GP can arrange appropriate investigations, which may include liver function tests, ultrasound scanning, or referral to hepatology or endocrinology services if needed. Early detection and intervention offer the best outcomes for managing fatty liver disease whilst maintaining optimal thyroid health.

Reporting side effects: If you suspect levothyroxine or any other medicine is causing side effects, you can report them via the MHRA Yellow Card scheme at yellowcard.mhra.gov.uk or through the Yellow Card app.

References: NICE NG145: Thyroid disease: assessment and management; NICE NG49: Non-alcoholic fatty liver disease: assessment and management; BSG guidance on abnormal liver blood tests and referral pathways; NHS: Non-alcoholic fatty liver disease; NHS: Underactive thyroid (hypothyroidism).

Frequently Asked Questions