Prolactin (PRL) among other pituitary hormones has been detected in human cerebrospinal fluid (CSF) (2.,7.,8.). The normal CSF prolactin concentration is about 1 ng/ml (2.,3.). Patients with prolactin secreting hvpophyseal tumor have elevated PRL levels both in their blood and cerebrospinal fluid. The suprasellar extension of prolactinoma can brinq about higher prolactin concentration in the CSF than that in the serum ( 5 . , 9 . ) .Under physiological conditions serum and CSF prolactin concentrations are linearly correlated ( 2 . ) . This correlation prevails also at slightly elevated serum prolactin levels. In order to further investigate the interrelationship of the serum and CSF prolactin strictly normoprolactinemic neurological patients were chosen to this study. 107 non-endocrine neurological patients could be included to this study when a serum prolactin normal level criterion of 20 ng/ml (maximum) was used. The diagnostic neurological workuus of the patients were done in the departments of neurology, University of Turku and Tampere, Finland. The mean age of the patients was 50 5 14 (SD) years. Simultaneous serum and lumbar CSF samples were obtained as routine diaqnostic laboratory specimens. Paired serum and CSF samples were kept frozen (-20%) until analyzed by human prolactin radioimmunoassay. The assay reagents were a donation from the National Pituitary Agency, N.I.H., Bethesda, Md. to dr. M.T.Hyy~pa. The sensitivity limit of this assav was 0 , 5 ng/ml. The interassay variation was 12 % and the intra-assay variation 3 %. The mean serum prolactin concentration in this normoprolactinemic patient group was 6,2 2 3,5 (SD) ng/ml. The simultaneous level in cerebrospinal fluid was 1,O 2 0,4 (SD) nq/ml. After intraindividual comparisons the mean CSF-PRL / S-PRL ratio proved to be 0,20 2 0,lO (SD) . This is in accordance with the data of earlier study (2.). In that pioneer study of Assies et al. a highly significant and linear correlation (r = 0,9097) between CSF and Serum concentrations was found. Therefore a suggestion of a bulk transport mechanism for hormone from the blood to the CSF space was presented. We,however, found in this study a tendency for CSF prolactin level to remain relatively constant also at lower serum prolactin concentrations (see Table I.). TABLE I. With low normal serum PRL concentrations the CSF PRL concentration remains at the same level as in the high normal grouu of normoprolactinemic patients. (mean 2 SD)
Patient groupSerum PRL ng/ml CSF PRL ng/ml S-PRL > 5 ng/ml 8,5 f 3,2 1,l 2 0,4 S-PRL < 5 ng/ml 3 , 5 2 0,6 1,0 2 0 , 5 P < 0,001 P > 0,lO (mean 2 SD) n = 4 9 n = 5 8
