['Binding sites specific for growth hormone have been identified in the brain, but the action of GH on the central nervous system is still poorly understood.', 'In a double-blind, placebo-controlled 21-month trial with a cross-over design, with each treatment period lasting for 9 months, we investigated the long-term effect of GH on the cerebrospinal fluid (CSF) concentrations of some brain neurotransmitters and thyroid hormones of importance for mood and cognition.', 'Twenty-four patients with documented GH deficiency acquired in adult life took part.', 'Analysis of CSF collected at the end of the two treatment periods showed that the GH concentration was related to the administered dose of rhGH (r = 0.56, P = 0.0044). After rhGH treatment the concentration of the dopamine metabolite homovanillic acid (HVA) had decreased from 218 +/- 80 to 193 +/- 82 nmol/l (P = 0.002) and that of the excitatory acid aspartate had increased from 233 +/- 81 to 313 +/- 116 nmol/l (P = 0.032). No effects were observed on the concentrations of 5-hydroxyindoleacetic acid (the serotonin metabolite) and of 3-methoxy-4-hydroxyphenyl glycol (the noradrenaline metabolite), or on those of glutamate, glycine and beta-endorphin. However, both CSF and serum levels of free T4 decreased, from 19.8 +/- 6.1 to 16.6 +/- 5.7 nmol/l (P = 0.0002) and 17.0 +/- 5.0 to 13.7 +/- 4.3 nmol/l (P = 0.0001), respectively. The concentration of total T3 was not measurable in CSF but increased in serum from 1.41 to 1.53 nmol/l (P = 0.01).'] Does growth hormone treatment affect brain neurotransmitters and thyroxine [ see comment ]?