Summary
Uric acid is the final breakdown product of purines — nitrogen-containing compounds found in cell nuclei and in high-purine foods such as red meat, shellfish, and beer. When uric acid levels are too high (hyperuricaemia), crystals form in joints (causing gout) and kidneys (causing stones). Uric acid is now also recognised as an independent risk factor for cardiovascular disease, hypertension, and metabolic syndrome.
Humans lack the enzyme uricase (present in most other mammals) that converts uric acid to the more soluble allantoin — making us uniquely susceptible to uric acid accumulation. Uric acid is renally excreted, and both over-production (high cell turnover or purine-rich diet) and under-excretion (chronic kidney disease, medications, dehydration) can elevate serum levels.
Hyperuricaemia is present in around 20% of UK adults, but gout — the symptomatic manifestation — affects approximately 2–3%, making it the most common inflammatory arthritis in men.
Modern research has linked elevated uric acid beyond gout to insulin resistance, hypertension, non-alcoholic fatty liver disease, and chronic kidney disease — suggesting it has broader biological effects than purely crystal formation.
What It Is
Purines (adenine and guanine) are broken down through a metabolic cascade ending at xanthine, which is oxidised to uric acid by xanthine oxidase. In humans, uric acid is the end product because we lack active uricase.
Uric acid in plasma exists mostly in the ionised form (urate) at physiological pH. Its solubility limit is approximately 420 μmol/L (6.8 mg/dL) at body temperature — concentrations above this lead to supersaturation and crystal formation in cooler, lower-pH environments such as peripheral joints.
Reference ranges: < 360 μmol/L in women, < 420 μmol/L in men (corresponding to approximately 6.0 and 7.0 mg/dL). Gout treatment targets serum urate < 360 μmol/L (< 300 μmol/L in patients with tophi or frequent attacks) to dissolve existing crystals.
Functions
Gout risk indicator
Elevated uric acid is the primary risk factor for gout — chronic hyperuricaemia leads to monosodium urate crystal deposition in joints.
Kidney stone risk marker
Uric acid stones account for 10% of all kidney stones — hyperuricaemia and acidic urine both promote urate stone formation.
Metabolic syndrome marker
Elevated uric acid is increasingly recognised as a marker of — and possibly contributor to — insulin resistance, hypertension, and cardiovascular risk.
Antioxidant reservoir
Uric acid is a potent antioxidant in plasma — it may have evolved as a compensatory antioxidant mechanism. This partly explains why moderate elevations are associated with reduced neurological disease risk.
Reference Ranges
Serum Uric Acid (Urate)
Measured in μmol/L| Status | Range (μmol/L) | Range (mg/dL) | What it means |
|---|---|---|---|
| Low | < 140 | < 2.4 | Low uric acid — rarely clinically significant. May be seen in some medications or rare genetic conditions. |
| Optimal | 140–360 | 2.4–6.0 | Normal uric acid levels — below the supersaturation threshold for crystal formation. |
| Borderline | 360–420 | 6.0–7.0 | Approaching the solubility limit — gout risk increases. Lifestyle modification recommended. |
| High | > 420 | > 7.0 | Hyperuricaemia — above the solubility threshold. Gout risk is significantly elevated. |
Reference ranges are sex-specific. The crystal solubility threshold is approximately 360 μmol/L for women and 420 μmol/L for men. Gout can occur below these thresholds; some people tolerate higher levels without symptoms. Treatment targets for established gout: < 360 μmol/L.
Symptoms of Imbalance
Elevated uric acid is often asymptomatic; gout presents as sudden, intensely painful joint attacks, typically starting in the big toe.
- Very low uric acid is generally not clinically significant
- Rare: some neurological conditions (multiple sclerosis, Parkinson's) associated with lower uric acid
- Acute gout attacks: sudden, severe joint pain (often big toe, ankle, or knee), redness, warmth, and swelling
- Chronic tophaceous gout: chalky deposits under the skin around joints
- Kidney stones: loin-to-groin pain, haematuria
- Recurrent joint attacks
- Joint damage from chronic crystal deposition
Causes of Imbalance
- Very low purine diet
- Some medications (allopurinol, rasburicase, high-dose vitamin C)
- Rare: xanthinuria (genetic XO deficiency), Wilson's disease
- High-purine diet (red meat, organ meats, shellfish, oily fish)
- Excess alcohol (particularly beer — contains purines and inhibits urate excretion)
- Fructose excess (high-fructose corn syrup — accelerates ATP catabolism to uric acid)
- Dehydration — reduces urate excretion
- Obesity and metabolic syndrome
- Diuretics (thiazides and loop diuretics — reduce renal urate excretion)
- Chronic kidney disease
- Psoriasis and high cell turnover states
FAQs
The first metatarsophalangeal joint (big toe) is the classic — and most common — site, affected in about 50% of first gout attacks (podagra). This is because the big toe is cooler and more acidic than central joints, favouring crystal formation. However, gout can affect any joint — ankle, knee, wrist, elbow, and finger joints are all commonly involved, particularly in recurrent disease.
Dietary modifications can meaningfully reduce uric acid by 10–20%: avoid red meat, organ meats, shellfish, and high-fructose corn syrup; reduce alcohol (especially beer); stay well hydrated; and eat more low-fat dairy, cherries, and vegetables (which promote urate excretion). However, diet alone is rarely enough to normalise significantly elevated uric acid — urate-lowering medication (allopurinol) is the most effective treatment for established gout.
Yes — thiazide and loop diuretics (e.g. bendroflumethiazide, furosemide) reduce renal urate excretion, raising serum uric acid by 10–20%. This is one of the most common drug-induced causes of gout. If gout develops in a patient on diuretics, their continued use should be reviewed and an alternative antihypertensive considered if possible.
Allopurinol is the first-line urate-lowering therapy for established gout. It works by inhibiting xanthine oxidase — the enzyme that converts xanthine to uric acid — thereby reducing uric acid production. Started 2–4 weeks after an acute attack resolves (not during, as it can prolong attacks), and titrated to achieve a target urate < 360 μmol/L. It is taken indefinitely and is generally very well tolerated.
Epidemiological data consistently shows that hyperuricaemia is an independent risk factor for cardiovascular disease, hypertension, and all-cause mortality. Proposed mechanisms include uric acid-induced oxidative stress, endothelial dysfunction, and renal salt retention. Whether treating hyperuricaemia reduces cardiovascular events (beyond treating gout) remains under investigation — but it is another reason to address elevated uric acid through lifestyle changes.
References
- Richette P, Doherty M, et al. 2016 updated EULAR evidence-based recommendations for the management of gout. Ann Rheum Dis. 2017;76(1):29–42. View source
- Dalbeth N, et al. Gout. Lancet. 2016;388(10055):2039–2052. View source
- Feig DI, et al. Uric acid and cardiovascular risk. N Engl J Med. 2008;359(17):1811–1821. View source