Blood sugar, serum immunoreactive insulin and free fatty acids were determined in three young male patients with islet cell tumors and in thirty young adult controls before and after intravenous injection of tolbutamide. The serum insulin response was compatible with the diagnosis in two of the patients who had high fasting insulin levels, but was of no value in the one patient with normal fasting insulin concentrations. Evaluation of the fasting blood sugar and free fatty acid concentrations in relation to the fasting insulin levels (FBS/IRI and FFA/IRI) made the diagnosis more apparent in all three insulinoma patients. The FBS/ IRI for the thirty male controls was 5.6 ± 1.2 (mean ± 2 S.E.M.). Only three of these values were below 2.5. In contrast, not one of the thirty-four preoperative FBS/IRI values from the three patients was above 2.5. All seventeen postoperative FBS/IRI values were above 2.5. The FFA/IRI for the thirty male controls was 30.6 ± 9.4. Only four of these values were below 15. In the two patients where data were available, twelve of the fourteen values were below 15.

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