All three types of renal tubular acidosis (RTA) are associated with hyperchloraemic metabolic acidosis (normal anion gap).

Type 1 RTA (distal)

• inability to generate acid urine (secrete H+) in distal tubule
• A renal loss of bicarbonate can happen in two ways:
  • Impaired bicarbonate reabsorption
  • Increased bicarbonate excretion
• the loss of bicarbonate is compensated by an equivalent increase in chloride reabsorption in the distal tubules → hyperchloraemia
• causes hypokalaemia
• complications include nephrocalcinosis and renal stones
• causes include idiopathic, rheumatoid arthritis, SLE, Sjogren's, amphotericin B toxicity, analgesic nephropathy

Abdominal x-ray showing nephrocalcinosis - a classical finding in type 1 RTA

Type 2 RTA (proximal)

• decreased HCO3- reabsorption in proximal tubule
• causes hypokalaemia
• complications include osteomalacia
• causes include idiopathic, as part of Fanconi syndrome, Wilson's disease, cystinosis, outdated tetracyclines, carbonic anhydrase inhibitors (acetazolamide, topiramate)

Type 3 RTA (mixed)

• extremely rare
• caused by carbonic anhydrase II deficiency
• results in hypokalaemia

Type 4 RTA (hyperkalaemic)

• reduction in aldosterone leads in turn to a reduction in proximal tubular ammonium excretion