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How Your Liver Activates Thyroid Hormone
Your thyroid gland primarily produces T4 (thyroxine), which is relatively inactive. For thyroid hormone to work in your tissues, T4 must be converted to T3 (triiodothyronine) by enzymes called deiodinases.
Your liver contains the highest concentration of deiodinase type 1 (DIO1) and performs most of this conversion. When liver function is impaired:
- T4-to-T3 conversion slows down
- Free T3 levels may drop while T4 and TSH remain normal
- You may experience hypothyroid symptoms despite "normal" standard thyroid tests
This is sometimes called "low T3 syndrome" or "euthyroid sick syndrome" and is common in people with chronic liver conditions.
Fatty Liver and Thyroid Dysfunction
Research consistently shows a strong link between hypothyroidism and fatty liver disease (NAFLD). The relationship is bidirectional:
How low thyroid causes fatty liver:
- Reduced metabolic rate slows fat burning
- Decreased lipid oxidation in the liver
- Increased lipogenesis (fat creation)
- Impaired VLDL secretion, trapping fat in liver cells
How fatty liver impairs thyroid function:
- Reduced T4-to-T3 conversion
- Inflammation affects thyroid hormone receptors
- Altered thyroid hormone binding proteins
Studies show that TSH levels correlate with hepatic steatosis severity. Higher TSH (even within "normal" range) is associated with more liver fat.
Why Standard Thyroid Tests May Miss the Problem
Standard thyroid screening typically includes TSH and sometimes free T4. But these tests can appear normal while T3 levels are suboptimal due to poor liver conversion.
Consider this scenario:
- TSH: 2.5 mIU/L (normal)
- Free T4: 1.0 ng/dL (normal)
- Free T3: 2.2 pg/mL (low end of range)
- ALT: 65 U/L (elevated)
- GGT: 45 U/L (elevated)
This pattern suggests the liver isn't efficiently converting T4 to T3. The person may have hypothyroid symptoms despite "normal" TSH and T4. Without testing liver enzymes and free T3, this connection would be missed.
Key Markers to Test Together
The Free T3-to-Free T4 ratio can indicate conversion efficiency. A low ratio with elevated liver enzymes suggests liver-related conversion problems.
A Note on Thyroid Treatments for Liver Disease
The connection between thyroid hormones and liver health is so strong that thyroid hormone receptor beta like resmetirom have been developed specifically to treat NASH (advanced fatty liver disease). These drugs activate thyroid hormone pathways in the liver to promote fat burning and reduce liver inflammation.
This underscores how important thyroid hormone signaling is for liver health.
What You Can Do
If your thyroid markers are suboptimal and your liver enzymes are elevated, addressing liver health may improve thyroid function naturally. Weight loss, reducing alcohol, and improving metabolic health can restore the liver's ability to convert T4 to T3.
If your liver enzymes are normal but thyroid markers are off, the issue is more likely primary thyroid dysfunction requiring different evaluation.
Testing both systems helps you understand what's actually happening and where to focus. Read our guide on what liver function testing is and it's importance.
Want to check your liver health? Take our liver health quiz below to get your liver score.
