This randomized, active-controlled, parallel-arm, single-blind trial is to compare the effects of Dopamine agonists (DA) therapy targeting different established treatment strategies on glucose metabolism assessed by an oral glucose tolerance test.

Prolactinomas are the most common pituitary tumors, leading to hyperprolactinemia, which causes hypogonadism, infertility, and is associated with adverse metabolic effects such as insulin resistance, dyslipidemia, and obesity. Dopamine agonists (DAs), especially cabergoline, are the first-line treatment. They reduce prolactin levels and tumor size effectively. Despite their widespread use, there are no evidence-based guidelines regarding target prolactin levels during DA therapy. Limited evidence suggests that different prolactin levels may have different effects on metabolic health. This trial aims to assess glucose tolerance, insulin sensitivity, and beta-cell function-using OGTT with insulin levels-after 12 months of DA treatment, to target treatment options.

In Switzerland, cabergoline is the preferred DAs for treating hyperprolactinemia.

Cabergoline is available in tablet form, with doses of 0.5 mg per tablet. The standard dosing for hyperprolactinemia typically starts at 0.25 mg to 0.5 mg per week, which can be gradually increased based on the patient's response, with a usual range of 0.25 mg to 2 mg per week.

Inclusion Criteria (treatment-naïve patients):

• Diagnosed adult patients (at least 18 years of age) with prolactinoma-induced hyperprolactinemia, defined as a prolactin level ≥ two times the local laboratory maximum and radiographic criteria, based on current guidelines.

Inclusion Criteria (treatment-naïve patients):

• Diagnosed adult patients (at least 18 years of age) with prolactinoma-induced hyperprolactinaemia based on current guidelines.
• Patients treated with cabergoline as DA therapy and prolactin levels within the normal range

Exclusion Criteria:

• alternative explanation for hyperprolactinaemia

• Active substance use disorder within the last six months

• Current or previous psychotic disorder

• Pregnancy or breastfeeding within the last 8 weeks

• Severe hepatic insufficiency or cholestasis

  • Child Pugh C or
  • AST/ ALT > 3 x the upper limit of normal ULN or
  • Cholestasis (total bilirubin > 2x ULN)

• Severe renal impairment (eGFR < 30 ml/min)

• History of pulmonary, pericardial, and/or retroperitoneal fibrotic disorders

• Concomitant treatment with strong or moderate CYP3A4 inhibitors

• Local complications on morphological imaging, related to signs or clinical symptoms which make surgical intervention necessary or a clear patient's preference for surgical treatment

• Gastrointestinal disease or previous surgery: chronic active inflammatory bowel disease, active gastrointestinal ulcer disease, or surgery on the gastrointestinal tract (e.g. sleeve stomach, gastric band)

• Patient incapable of giving informed consent due to cognitive impairment or other reasons (e.g., legal incapacity)

Outcome assessors (e.g., study nurses and study physicians) will all remain blinded to the treatment allocation.