Hook effect: a pitfall leading to misdiagnosis of hypoaldosteronism in an infant with pseudohypoaldosteronism.

We report herein the case of a premature infant who presented with failure to thrive, hyponatremia, hyperkalemia and metabolic acidosis. Initial serum hormone profiling suggested isolated hypoaldosteronism (aldosterone: 0.01 pg/ml, normal range: 50-900 pg/ml). A gas chromatography-mass spectrometry spot urinary steroid profile showed grossly elevated levels of 18-hydroxy-tetrahydro-11-dehydrocorticosterone (18-hydroxy-THA: 5,893 microg/l; normal upper limit 36 microg/l) and tetrahydroaldosterone (TH-Aldo: 5,749 microg/l; normal upper limit 36 microg/l) which are aldosterone precursor metabolite and aldosterone metabolite, respectively. Thus, aldosterone synthase deficiency was excluded and pseudohypoaldosteronism (PHA) was suggested. A repeated test after dilution of the serum revealed a very high level of aldosterone (6,490 pg/ml), confirming the diagnosis of PHA in this case.