Vitamin A (retinol) is a fat-soluble vitamin involved in vision, immune function, cell growth, and skin integrity. It is obtained from animal sources (as preformed retinol) and from plant sources (as beta-carotene, which the body converts to retinol). Because vitamin A is fat-soluble, it is stored in the liver and can accumulate over time.
This test measures serum retinol, the active circulating form of vitamin A. It is useful for identifying both deficiency and excess. Deficiency can occur in people with fat malabsorption conditions (such as coeliac disease, Crohn's disease, or cystic fibrosis), those with very restricted diets, or people with liver disease that impairs vitamin A storage and metabolism. Symptoms of deficiency include impaired night vision, dry eyes, dry skin, and increased susceptibility to infections.
Vitamin A toxicity (hypervitaminosis A) is less common but worth understanding, particularly for people taking high-dose supplements or consuming large amounts of liver or cod liver oil. Chronic excess can cause headaches, nausea, dizziness, and liver damage. Because the body stores vitamin A rather than excreting it readily, toxicity can develop gradually without obvious early warning signs.
It is worth noting that serum retinol has some limitations as a marker. Retinol levels in blood are tightly regulated by the liver and may not drop below the normal range until liver stores are substantially depleted. Conversely, levels may not appear elevated until storage capacity is exceeded. For this reason, clinical context matters when interpreting results.
This test does not measure beta-carotene, the plant-derived precursor to vitamin A. If beta-carotene levels are of interest, a separate test is required.
To measure circulating retinol and identify levels that may be too low (deficiency) or too high (toxicity from excess supplementation or dietary intake).
