Growth hormone and prolactin secretion in the carcinoid syndrome

We evaluated plasma growth hormone (GH) and plasma prolactin (PRL) levels in ten patients with metastatic carcinoid tumors and the carcinoid syndrome ("active tumors") and seven patients with metastatic carcinoid tumors without the carcinoid syndrome ("inactive tumors"). The patients with active tumors had elevated serum serotonin levels and increased urinary 5-hydroxyindoleacetic acid (5-HIAA) while these values were normal in patients with inactive tumors. Forty-five per cent of patients with active tumors had elevated fasting plasma GH levels that were either not suppressed or showed a paradoxical increase in response to I.V. glucose. There was a positive correlation between the plasma GH levels and serotonin production by the tumor. Twenty-eight per cent of patients with inactive tumors had elevated fasting plasma GH levels. GH levels were decreased by the administration of serotonin antagonists in some but not all of the patients. Parachlorophenylalanine (PCPA) an inhibitor of serotonin synthesis caused a paradoxical rise in GH levels. GH release in response to insulin hypoglycemia was normal. Plasma prolactin levels were normal in most of the patients with metastatic carcinoid tumors. PCPA administration did not systematically alter plasma prolactin levels. We conclude that elevated plasma GH levels are frequently present in patients with the carcinoid syndrome. Both serotonin produced by the tumors and the tumor itself may be responsible for the elevated GH levels.