How the workup of low renin hypertension lead to the diagnosis of SIADH
Chara Kolentini1, Nils Morgenthaler2, Gerhard Mertes3, Angela Cupisti4, Matthias Schott1, Werner A Scherbaum1 & Holger S Willenberg1
A patient was referred to us because he had hypokalemic hypertension and no detectable blood concentrations of renin and aldosterone. In addition, this patient had an adrenal mass. Since he was hyponatremic, he was put on fludrocortisone.
At presentation, we found that the patient was normonatremic and hypokalemic and had a high sodium to urinary sodium to potassium to urinary potassium (SUSPUP) ratio. But review of older laboratory results showed an inadequate high urinary concentration of sodium and a very low SUSPUP ratio. Hormonal studies revealed that the patient had undetectable low renin and aldosterone levels. However, pro-brain natriuretic peptide (proBNP) and copeptin as a measure of vasopressin were elevated to an astonishing degree. The diagnosis of a syndrome of inadequate secretion of antidiuretic hormone (SIADH) was made.
A computed tomography scan of the thorax showed a tumor in segment 7 and 8. The histology gave the diagnosis of a mall cell lung cancer and staining of the biopsy samples to copeptin were positive.
We conclude that SIADH may present as a form of low renin hypertension and that the use of modern markers such as proBNP and copeptin are useful in establishing the diagnosis of SIADH. We also conlcude that the SUSPUP ratio may help to distinguish between mineralocorticoid excess and other form of hypertension.