Insulin is a peptide hormone with a molecular weight
of approximately 6000 daltons. Serum insulin determinations may be
performed on patients with symptoms of hypoglycemia4,5 where
insulinoma is a possible etiology. Patterns of secretion by insulinomas
may be sporadic or constant. Insulinoma is a rare functional tumor of
pancreatic origin that secretes insulin inappropriately, causing
hypoglycemia. Whipple originally described the tumor and classic
diagnostic criteria called "Whipple's Triad". The criteria included:
1) neurologic symptoms of hypoglycemia
2) low glucose (<50 mg/dL) which are corrected immediately by giving glucose or carbohydrate.
Today,
as described above, we would add inappropriately elevated insulin
levels during an episode of hypoglycemia. Usually hypoglycemia is
induced with a fast or exercise, and the diagnostic criteria may include
a 48 to 72 hour fast with insulin levels greater than 5 µU/mL during a
hypoglycemic episode.4
Hypoglycemia can, however, also be facilitated intentionally or unintentionally (factitious hypoglycemia),4,5
C-peptide levels along with insulin levels may be helpful in ruling out
factitious insulin use, however note the limited reactivity of various
insulin analogs. Accordingly, one use of the insulin test is to screen
patients with low glucose for insulinoma if other factors such as
glucose-lowering drugs or reactive hypoglycemia are excluded.
Insulin levels can be also useful in predicting susceptibility to the development of type II diabetes.4
However, the American Diabetes Association recommendations for the
diagnosis of diabetes do not include the measurement of insulin levels.6
Insulin
levels may also be measured to estimate the patient's capacity for
insulin secretion (eg, glucagon test) or in the evaluation of insulin
sensitivity (eg, oral glucose tolerance test). Although the adequacy of
pancreatic insulin synthesis is frequently assessed via the
determination of C-peptide, fasting insulin is also measured to assess
insulin resistance.
In treated patients therapeutic administration
of insulin can lead to the formation of insulin binding antibodies.
Insulin binding antibodies invariably interfere with insulin measurement
methods and results are unreliable. For patients with known
autoantibodies to insulin should measure free and total insulin to
assess biologically active fraction of insulin (free insulin) and total
insulin. Measurements of free and total insulin, C-peptide, insulin
antibodies and hemoglobin A1C are often tested in conjunction to clarify
the contribution of endogenous and exogenous insulin to overall
diabetic management. See Test 501561 for Free and Total Insulin.
