Calcitonin in plasma and cerebrospinal fluid from normal subjects and patients with medullary thyroid carcinoma: possible restriction of calcitonin by the blood-brain barrier.

Calcitonin (CT) was measured by RIA in samples of plasma and cerebrospinal fluid (CSF) obtained simultaneously from 27 normal volunteers and 8 patients with medullary thyroid carcinoma to examine the extent of penetration of CT across the blood-brain barrier. CT concentrations in plasma and CSF were 68.2 +/- 25.1 pg/ml (mean +/- SD; range, less than 25-110) and 38.5 +/- 12.1 pg/ml (range, less than 25-56), respectively, in normal subjects and 5,650 +/- 4,120 pg/ml (range, 980-21,100) and 36.1 +/- 11.1 pg/ml (range, less than 25-45), respectively, in patients with medullary thyroid carcinoma. No significant difference was observed in the mean concentrations of CT in CSF from normal subjects and that from patients with medullary thyroid carcinoma. The concentration of CT in CSF from patients with medullary thyroid carcinoma was only 0.05-2.5% the concentration of CT in plasma. Moreover, there was no significant correlation between the concentrations of CT in plasma and CSF from normal subjects, from patients with medullary thyroid carcinoma, or when normal and patient groups were combined. It is concluded that penetration of CT from blood in CSF is minimal.