Uric acid is a naturally occurring chemical compound in the blood, formed as a byproduct when the body breaks down purines found in human cells and many foods. Most uric acid is dissolved in the blood and filtered out by the kidneys, passing out of the body in the urine. Maintaining a balanced concentration is necessary for health, as excessively high or low levels can signal underlying medical issues.
The Fundamental Role of Uric Acid in the Body
Uric acid is the final product of purine catabolism, a metabolic pathway that breaks down nucleic acids like DNA and RNA. This process involves the degradation of purine compounds, such as adenine and guanine, which are converted into hypoxanthine and then xanthine. The enzyme xanthine oxidase transforms xanthine into uric acid. Unlike most mammals, humans lack the enzyme uricase, which results in naturally higher circulating uric acid levels. In the bloodstream, uric acid acts as a potent water-soluble antioxidant, accounting for more than half of the total free radical scavenging activity in human plasma.
Defining Standard Reference Ranges
The concentration of uric acid considered typical varies depending on physiological factors, particularly sex and age. For adult males, the standard reference range generally falls between 4.0 and 8.5 milligrams per deciliter (mg/dL). Adult females tend to have lower levels, typically 2.7 to 7.3 mg/dL, due to estrogen promoting urate excretion. After menopause, a woman’s uric acid level often rises to approximate that of an adult male. Children typically have lower levels than adults, ranging from 2.5 to 5.5 mg/dL, with values rising after male puberty.
Measurement and Target Levels
The level is typically measured via a serum urate blood test, though a 24-hour urine collection may also assess kidney excretion. In clinical practice, especially for individuals with gout, a therapeutic target range is used rather than just a normal range. This goal is usually to maintain the serum uric acid level below 6.0 mg/dL to prevent crystal formation.
Mechanisms Behind Elevated Uric Acid
An elevation of uric acid in the blood, known as hyperuricemia, occurs when the body’s production and excretion mechanisms fall out of balance. This imbalance is overwhelmingly caused by underexcretion, where the kidneys fail to remove enough uric acid, accounting for up to 90% of cases. The renal handling of urate involves filtration, reabsorption, and secretion in the kidney tubules. A key player is the urate transporter 1 (URAT1), which reabsorbs approximately 90% of the filtered uric acid back into the bloodstream. Medications like thiazide diuretics, alcohol consumption, and high fructose intake can interfere with URAT1, promoting enhanced reabsorption and leading to urate buildup.
Overproduction Causes
The less common cause of hyperuricemia is the overproduction of uric acid. This results from a diet rich in purines or from the rapid turnover of cells, such as during chemotherapy for cancer. Rare genetic enzyme defects can also accelerate purine synthesis, leading to excessive production.
Specific Health Conditions Resulting from Hyperuricemia
The most well-known consequence of prolonged hyperuricemia is gout, a painful form of inflammatory arthritis. Gout develops when serum uric acid exceeds its solubility limit, approximately 6.8 mg/dL. When this threshold is breached, the urate anion forms monosodium urate (MSU) crystals. These needle-shaped MSU crystals deposit in the joints and tissues, triggering an acute inflammatory response. Immune cells recognize the crystals, releasing inflammatory mediators that cause the sudden, severe pain, swelling, and redness typical of a gout flare.
Uric Acid Kidney Stones
Hyperuricemia can also contribute to the formation of uric acid kidney stones (urolithiasis). Stone formation depends highly on the acidity of the urine. When the urine pH is low, typically below 5.5, the soluble urate ion converts into the less soluble, undissociated uric acid molecule. This chemical change causes the uric acid to precipitate, forming crystals that aggregate into stones within the urinary tract.