Because diphenylhydantoin (DPH) has been shown to inhibit nontumorous insulin release, we evaluated its effects in patients with insulinoma.

Three patients with surgically demonstrated insulinomas were tested with oral and intravenous glucose before and after DPH. In two of these patients, we compared the effects of DPH and diazoxide.

Ingestion of DPH for several days did not alter fasting glucose or insulin. Poststimulatory insulin was appreciably reduced in only one patient during treatment with the drug. During diazoxide administration, this patient had increased fasting glucose and decreased fasting insulin, in addition to reduction of poststimulatory insulin. Combined DPH-diazoxide therapy in this patient did not produce greater effects than did diazoxide alone. Neither fasting ingulin or glucose nor poststimulatory insulin secretion were influencse by DPH or diazoxide in the second patient, or affected by DPH alone in the third.

The accepted role of DPH in the treatment of insulinoma involves its ability to alter seizure threshold. It did not influence fasting hypoglycemia in our patients. Reduced insulin secretion after stimuli might reflect effects of DPH on responses of nontumorous endocrine pancreas.

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