Rationale: Abnormally low and high levels of insulin-like growth factor-I (IGF-I) are both associated with increased metabolic risk. Since (U-shaped) associations of IGF-I, within the normal range, have also been found with cardiovascular risk factors and disease in the general population, it would be interesting to investigate if this association can also be found in growth hormone deficient (GHD) adults treated with Growth Hormone (GH). This could be of interest for endocrinologists prescribing GH in clinical practice because strict dosing may become even more important. Next to that, scientific evidence for clinical practice is wanted.

Objective: Next to cardiovascular risk factors (main objectives: body composition and lipid profile; secondary objectives: remainder) we investigate the effect on glucose metabolism, physical performance, and neuropsychological functioning of different levels of IGF-I in GH treated GHD men and women.

Study design: Open-label randomized trial.

Study population: At least 32 subjects, both childhood as adult onset GHD men and women, receiving GH treatment for at least one year, with an age between 20 and 65 years.

Intervention: At entry subjects are already receiving GH treatment according to general clinical practice, and are expected to demonstrate an IGF-I concentration of 0 - 1 SD score (SDS) (normal dose). The group of men and group of women will be randomized to receive either a decrease of their regular dose of GH treatment (IGF-I target level of -2 - -1 SDS) (low dose), or an increase of their regular dose, (IGF-I target level of 1 - 2 SDS) (high dose) for at least 24 weeks.

Inclusion Criteria:

• Ongoing surveillance at our centre (VUmc)
• Stable substitution therapy for other pituitary hormone deficiencies

Exclusion Criteria:

• Subjects with a craniopharyngioma as cause of their GHD or pituitary deficiencies
• Contraindications for the use of GH treatment
• (Receiving treatment for) malignant disease (in the past)
• Cardiovascular event less than one year prior to inclusion
• Participation in other studies
• Subjects, who in the opinion of the investigator, are unsuitable in any other way to participate in this study