Medication for excessive urine production in patients treated with tolvaptan

INTERVENTION: Subjects will be treated with hydrochlorothiazide, metformin and placebo for two weeks each, followed by one wash‐out week, in random order. Hydrochlorothiazide will be initiated at 12,5mg QD, after one week the dose will be increased to 25mg QD if well tolerated. Metformin will be initiated at 500mg BID, after one week the dose will be increased to 1000mg BID, if well tolerated. CONDITION: Polycystic kidney disease (ADPKD), Polyuria, Tolvaptan ; ; PRIMARY OUTCOME: The primary outcome variable will be change in 24‐hour urine volume SECONDARY OUTCOME: ‐Change in glomerular filtration rate (as measured with the iohexol plasma clearance technique) ; ; ‐Change in plasma copeptin ; ; ‐Tolerability of the study medication INCLUSION CRITERIA: 1. Diagnosis of ADPKD, based upon modified Ravine criteria 2. Using tolvaptan 120mg daily 3. Age between 18 and 50 years 4. >45 eGFR (CKD‐EPI) 5. Providing informed consent