Prealbumin, formally known as Transthyretin, is a small protein produced primarily by the liver that circulates throughout the blood and cerebrospinal fluid. Although its name suggests a close relationship to the more abundant protein albumin, prealbumin is a distinct molecule with unique functions. Measuring its concentration in the bloodstream is a common practice in many clinical settings.
What is Transthyretin and What Does It Do?
The scientific community officially renamed prealbumin to Transthyretin (TTR) to more accurately reflect its biological role. The name Transthyretin literally means “transports thyroxine and retinol,” which are its two primary functions. The older term, prealbumin, was assigned because the protein migrates faster than albumin during laboratory electrophoresis.
TTR binds and carries the thyroid hormone thyroxine (T4) through the bloodstream. It also forms a complex with retinol-binding protein (RBP) to transport Vitamin A (retinol) throughout the body. TTR is mainly synthesized and released into the blood by the liver, but it is also produced in the choroid plexus in the brain and the retina of the eye.
Why Doctors Test for Prealbumin
Doctors test for prealbumin because its rapid turnover rate offers a distinct advantage over other protein markers, such as albumin (which has a half-life of around 20 days). Prealbumin has a short half-life of approximately two to three days. This short lifespan means its blood concentration changes quickly in response to protein synthesis or catabolism, making it a sensitive marker for acute changes.
It is frequently used to monitor the effectiveness of nutritional support, such as total parenteral nutrition (TPN) or enteral feeding. Levels can rise noticeably within a few days of starting adequate nutritional support, giving clinicians an early indication of utilization.
However, its utility is complicated by its role as a “negative acute phase reactant.” During inflammation, infection, or trauma, the liver temporarily reduces TTR production as part of the stress response. Consequently, a drop in prealbumin may indicate the presence of an underlying inflammatory process rather than solely a lack of nutrition.
Understanding High and Low Levels
Interpreting a prealbumin test result requires careful consideration of the patient’s overall clinical picture. The most common concern is a low prealbumin level, typically falling below the reference range of 16 to 30 milligrams per deciliter. The three primary causes for a low result are inadequate protein intake, inflammation, and liver dysfunction.
A low value can signal protein-energy malnutrition, reflecting insufficient dietary protein and calories. Low levels also occur during acute inflammatory states, such as major infection or recent surgery. To distinguish between these causes, doctors often order a C-reactive protein (CRP) test; a high CRP alongside a low prealbumin suggests inflammation is responsible.
Liver disease, such as cirrhosis or hepatitis, can also lead to low prealbumin because the liver is the primary site of synthesis. Less commonly, low levels result from excessive protein loss, such as in certain kidney diseases or extensive burns.
High prealbumin levels are less frequent. The most common cause of an elevated level is chronic kidney disease or kidney failure. In healthy individuals, the kidneys clear prealbumin from the body, and impairment allows the protein to accumulate. High levels can also be associated with high-dose corticosteroid therapy, which stimulates the liver to increase protein synthesis. Physicians must integrate the prealbumin value with other laboratory tests and the patient’s physical examination to form an accurate diagnosis.