Glucagon like Peptide -1 (GLP-1) receptor agonists are well known to stimulate glucose-induced insulin secretion and to reduce energy intake. Recent findings from animal and human studies suggest a role of GLP-1 in regulating water and salt homeostasis. GLP-1 has been shown to reduce fluid intake after an oral salt load or during a meal - pointing to a hypodipsic effect. The aim of this study is to elucidate whether these putative hypodipsic properties of GLP-1 might be of advantage in persons with an exaggerated thirst perception as is the case in patients with primary polydipsia.

GLP-1 analogues are currently used for the treatment of hyperglycaemia associated with type 2 diabetes mellitus and given his properties as a natural satiety hormone, the GLP-1 analogue liraglutide was recently approved by the FDA for weight management.

In studies related to the influence of GLP-1 and -analogues in controlling food intake a concomitant reduction of fluid consumption has been observed.

The investigators hypothesize that GLP-1 analogues not only modulate appetite and provide satiety but also reduce fluid intake and thirst sensation in humans - especially in those with excessive thirst perception (patients with primary polydipsia). In view of future therapeutic options for these patients we aim to investigate the influence of the long-acting GLP-1 analogue dulaglutide on fluid intake, thirst perception and quality of life in patients with primary polydipsia compared to placebo.

Inclusion Criteria:

• Age over 18 years
• Polyuria of > 50 ml/Kg/day
• Polydipsia of > 3 liters/day

Exclusion Criteria:

• Known or probable central or nephrogenic Diabetes insipidus, expected from patient's history
• Polyuria secondary to diabetes mellitus, hypokalemia, hypercalcemia
• Pregnancy
• Previous treatment with GLP-1 agonists within the last 3 month
• History of pancreatitis
• Severe renal insufficiency (eGFR (CKD EPI) <30 ml/min/1,73 m2)
• Cancer