Pendred syndrome is a recessive autosomal disorder characterized by thyroid 
goiter and sensorineural hearing loss. The Pendred syndrome gene (SLC26A4) 
encodes a new anion exchanger named pendrin which mediates iodide transport by 
thyrocytes and regulates ion and fluid transport by the endolymphatic sac 
epithelium. Pendrin defects result in inner ear malformations, with enlargement 
of the endolymphatic sac and duct in association with a large vestibular 
aqueduct. Furthermore, patients may develop endolymphatic hydrops requiring 
diuretic therapy, mainly in the form of thiazides. Pendrin could also account 
for apical Cl(-)/ HCO3(-) exchange at level of intercalated cells of the 
cortical collecting duct in the kidneys, however, humans with Pendred syndrome 
have no symptoms attributable to renal pendrin abnormalities in basal 
conditions. We report the case of a child with Pendred syndrome and intercurrent 
endolymphatic hydrops, who developed profound hypokalemia and severe 
hypochloremic metabolic alkalosis (potassium 1.7, chloride 70, sodium 129, HCO3 
43.8, base excess +17.8 mmol/l, pH 7.52) following thiazide therapy. In subjects 
with Pendred syndrome thiazide therapy seems to provoke more severe Cl(-) and 
extracellular volume depletion. A possible explanation could be the defective 
action of the disrupted pendrin, which exacerbates the effects of the inhibition 
of C1(-) reabsorption mediated by the thiazide-sensitive NaCl cotransporter 
(SLC12A3).