Oxytocin (OT) is a hypothalamic peptide that enters the peripheral circulation via the posterior pituitary gland. OT plays a key role in regulating appetite, psychopathology, prosocial behavior and sexual function. Hypopituitarism is associated with increased obesity, increased psychopathology, sexual and prosocial dysfunction despite appropriate hormone replacement. A few studies suggest the existence of a possible OT deficient state in hypopituitarism. In animal models, melatonin has shown to increase OT release.

This study is designed to evaluate oxytocin values after administration of melatonin in adults (healthy volunteers and patients with hypopituitarism).

The investigators hypothesize that OT response will be blunted following melatonin in patients with hypopituitarism compared to healthy controls.

Inclusion Criteria:

• Patients with hypopituitarism (HYPO) (>1 pituitary hormone deficiency) and stable hormone replacement for the prior three months
• At least one clinical sign of hypothalamic damage

Exclusion Criteria:

• uncorrected hormone deficiency
• creatinine >1.5mg/dL
• alanine aminotransferase (ALT) or aspartate aminotransferase (AST) >2.5x upper limit of normal
• hematocrit less than 30%
• suicidality or active psychosis
• participation in a trial with investigational drugs within 30 days
• using a high glucocorticoid dose
• vigorous physical exercise
• alcohol intake within 24 hours before the study participation
• evidence of any acute illness or any illness that the Investigator determines could interfere with study participation or safety
• pregnancy or breastfeeding for last 8 weeks
• known allergies towards melatonin
• patients refusing or unable to give written informed consent
• patients receiving fluvoxamine and/or impossibility to stop hypnotics 48 hours prior to the study visit.
• Additionally for healthy controls: the presence of brain or pituitary tumor, radiation involving the hypothalamus or pituitary, history of hypopituitarism or receiving testosterone or glucocorticoids esters.