Causes and Countermeasures for Hypouricemia.
Cause of Hypouricemia
Although hypouricemia had been rarely known, low uric acid levels are becoming recognized in medical checkups or comprehensive medical examinations. There are two main causes of hypouricemia.
Conditions Causing Hypouricemia
Hypouricemia is a condition in which the blood uric acid levels are low. In normal conditions, the levels of uric acid generated in the body through exercise or food intake and the levels of uric acid excreted to the outside the body through urine, feces, or sweat are balanced, with 3.7 – 7.8 mg/dL and 2.6 – 5.5 mg/dL* of uric acid in males and in females, respectively. There are two possible causes which can induce a decrease of uric acid levels:
- Even if uric acid is generated in the body, it is rapidly excreted to the outside of the body.
- The amount of uric acid generated in the body is low.
Rapid Excretion of Uric Acid Causes Hypouricemia
The Kidney produces urine containing uric acid by a process of filtration, reabsorption, and tubular secretion from the bloodstream. A large portion of the filtrated uric acid is resorbed through the renal tubules. Therefore, less than 10% of the uric acid which is initially contained in the urine is excreted. However, it has been confirmed that, in some people, uric acid is not sufficiently resorbed through the renal tubules and a large amount of uric acid is excreted to the outside of the body. These symptoms constitute hypouricemia, and most of the cases of hypouricemia are renal hypouricemia.
Renal hypouricemia is caused by mutations in the genes that absorb uric acid from the renal tubules, which indicates the involvement of genetic factors with the disease.
Inability to Degenerate Purines to Uric Acid Causes Hypouricemia
Purine is a generic term for substances which has a purine structure. It is contained in various substances including nucleic acids (such as adenine or guanine) and ATP, and it is an essential substance for organisms. When these purines are degraded, they eventually become uric acid and are excreted to the outside of the body.
There is an enzyme called xanthine oxidoreductase (XOR) which acts on the process of purine degradation into uric acid. If a person does not have this enzyme in the body, uric acid cannot be generated. Inability to generate uric acid due to deficiency of XOR is called xanthinuria, and it is exceptionally rare.
Both renal hypouricemia and xanthinuria only present low uric acid levels and do not cause any symptoms which require treatment nor subjective symptoms.
Secondary Hypouricemia Caused by Drugs or Other Factors
Besides the above, there are cases that uric acid levels decrease due to administration of drugs to treat preexisting diseases which may have the effects, for example, to suppress uric acid synthesis or to accelerate excretion. In addition, some diseases may lead to a decrease in uric acid levels along with other symptoms. These cases may require treatments unlike renal hypouricemia and xanthinuria, and consulting a clinician is highly recommended.
* Cited from common reference interval of Japanese Society of Laboratory Medicine, Japan Society of Clinical Chemistry, Japanese Association of Medical Technologists and Japanese Society for Laboratory Hematology.
