ABSTRACT
Type IV renal tubular acidosis (RTA) is due to renal tubular bicarbonate wasting associated with mineralocorticoid deficiency. In its five subtypes, IV-4 is due to pseudohypoaldosteronism (PHA) evidenced by increased plasma renin and aldosterone. PHA is believed to result from distal tubular unresponsiveness to circulating aldosterone and has normal renal and adrenal fuction. Hypoaldosteronism can easily be suspected when the patient shows typical electrolyte imbalance (hyponatremia coupled with hyperkalemia) and the diagnosis of PHA is confirmed by elevated serum aldosterone level. But some patients of PHA show negligible electrolyte imbalance, thus metabolic acidosis is a sole abnormal finding in routine laboratory examination. We experienced a case of IV-4 RTA in a 2-month-old male infant who presented with normal anion gap-metabolic acidosis as a sole abnormal finding in routine laboratory examination. RTA was suspected and the test of urine pH during systemic acidosis and fractional excretion of bicarbonate (FEHCO3-) during the condition of normal plasma bicarbonate concentration revealed the disease to be type IV RTA. With elevated plasma renin activity and aldosterone level, the diagnosis of type IV-4 RTA (pseudohypoaldosteronism) was made. Type IV RTA is the most common form of RTA, therefore it is recommended that young infants with suspected RTA should be checked for serum aldosterone level first.