From the FDA Drug Label
The liver is the major site of degradation for both T4 and T3, with T4 deiodination also occurring at a number of additional sites, including the kidney and other tissues. Approximately 80% of circulating T3 is derived from peripheral T4 by monodeiodination
Liver disease can affect T3 levels because the liver is the major site of degradation for both T4 and T3, and it plays a crucial role in the conversion of T4 to T3 through monodeiodination. Therefore, any liver disease that impairs this process may impact T3 levels. 1
From the Research
Liver disease can significantly affect triiodothyronine (T3) levels in the body, with patients often experiencing decreased T3 levels, a condition known as "low T3 syndrome" or "euthyroid sick syndrome". This occurs because the liver plays a crucial role in converting the inactive thyroid hormone thyroxine (T4) to the active form T3 through enzymes called deiodinases. When liver function is compromised due to conditions like cirrhosis, hepatitis, or fatty liver disease, this conversion process becomes impaired, resulting in lower circulating T3 levels despite normal thyroid-stimulating hormone (TSH) levels.
Key Findings
- The severity of T3 reduction often correlates with the extent of liver dysfunction, as seen in a study published in the Indian journal of endocrinology and metabolism 2.
- Liver disease can affect thyroid hormone transport proteins like thyroxine-binding globulin (TBG), further disrupting normal thyroid hormone metabolism.
- Clinicians should be aware that these alterations in thyroid function tests in liver disease patients don't necessarily indicate primary thyroid disease and typically don't require thyroid hormone replacement unless true hypothyroidism is confirmed.
Clinical Implications
- A study published in the Journal of the Pakistan Medical Association found that serum T3 and FT3 levels were significantly decreased in patients with liver cirrhosis, and these levels correlated with the severity of the disease 3.
- Another study published in the American journal of gastroenterology found that T3 and reverse T3 levels normalized on improvement in clinical status and liver function tests in patients with hepatic cirrhosis 4.
- The most recent and highest quality study, published in 2018, found that the level of FT3 can be used as a prognostic marker for liver cirrhosis patients 2.