Thiazides and thiazide-like diuretics are one of the five major classes of antihypertensive drugs. This study is to investigate whether urinary PGE2 concentration at baseline (prior to thiazide initiation) is associated with the development of TIH within the first four weeks of treatment.

Thiazides and thiazide-like diuretics are one of the five major classes of antihypertensive drugs. They act by inhibiting the apical Na+-Cl- -cotransporter in the distal convoluted tubules of the kidneys. Thiazides and thiazide-like diuretics often cause adverse effects, importantly a drop in plasma sodium levels that is called thiazide-induced hyponatremia (TIH). Data suggest a crucial role of urinary PGE2 in water reabsorption. Since urinary PGE2 concentrations were higher in patients with TIH, quantification of urinary PGE2 prior and after thiazide initiation might allow identification of patients at risk for TIH, presenting PGE2 as a potential novel predictive marker for the development of TIH.

This study is to investigate whether urinary PGE2 concentration at baseline (prior to thiazide initiation) is associated with the development of TIH within the first four weeks of treatment. Hospitalized and ambulatory patients in whom a thiazide or thiazide-like diuretic will be newly prescribed are screened for inclusion.

The study procedure contains the screening and inclusion, visit 1 before thiazide initiation, visit 2 4 weeks (+/-7days) after thiazide initiation and a 3-months follow-up (visit 3). An additional visit (visit 2.1) will only be added in case of a dose change of the thiazide or thiazide-like diuretic (4 weeks +/- 7 days after the dose change). The 2 hours- challenge is optional if the patient agrees to additional testing.

Inclusion Criteria:

• Newly prescribed thiazide or thiazide-like diuretic
• ≥ 18 years of age
• Informed Consent as documented by signature

Exclusion Criteria:

• Intake of thiazide or thiazide-like diuretic in the preceding month
• Hyponatremia (plasma sodium <135 mmol/L) at baseline
• Acute infectious / inflammatory disease (CRP ≥ 20 mg/L [1, 11])
• Symptomatic urinary tract infection
• Chronic treatment with NSAID and / or NSAID intake 48 hours prior to urine sampling at visit 1 and 2 (intake of acetylsalicylic acid will be no exclusion criteria)
• End of life care, no informed consent or inability to follow the procedures of the study, e.g., due to language barriers, psychological disorders, dementia

Hospitalized and ambulatory patients in whom a thiazide or thiazide-like diuretic will be newly prescribed. Screening will take place at the University Hospital Basel and Kantonsspital Baselland, primarily at the Endocrine Outpatient Clinic and the Hypertension Clinic of the Medical Outpatient Clinic or Nephrology Department and is based on the patients' charts as well as consultation with the treating physician.