Which lab tests are performed to assess growth hormone–releasing adenoma in hyperpituitarism?

Updated: Oct 24, 2016
  • Author: Alicia Diaz-Thomas, MD, MPH; Chief Editor: Robert P Hoffman, MD  more...
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Answer

See the list below:

  • Serum insulin-like growth factor-I (IGF-I): Measurement of serum IGF-I concentration is a sensitive screening test for acromegaly. Serum total (and free) IGF-I levels closely correlate to 24-hour mean integrated GH secretion. An elevated IGF-I level in a patient with appropriate clinical suspicion almost always indicates GH excess. Potential confusion may arise when evaluating healthy adolescents because significantly higher IGF-I levels occur during puberty than those during adulthood. For accurate control comparison, the IGF-I level must be compared with that of control subjects who are matched for age, gender, and Tanner stage. Note that a single measurement of GH is inadequate, because GH is secreted in a pulsatile manner during deep sleep (at night). Therefore, the use of a random GH measurement can lead to both false-positive and false-negative results and provides practically no clinically relevant data.

  • Serum insulinlike growth factor-binding protein-3 (IGFBP-3): IGFBP-3 levels may also be useful in the diagnosis of GH excess. In patients with confirmed somatotroph adenomas, increased IGFBP-3 level has been reported as a sensitive marker of GH hypersecretion and may be elevated even when circulating IGF-I levels are within the reference range.

  • Inability to suppress serum GH levels during an oral glucose tolerance test (OGTT): The single best laboratory criterion for diagnosing GH excess is failure to suppress serum GH levels to less than 5 ng/dL within 3 hours after a 1.75-g/kg oral glucose challenge (maximum dose is 75 g). This test essentially indicates the loss of negative feedback by IGF-I on GH secretion. Glucose induces insulin secretion, which suppresses hepatic IGFBP-1 release, thereby increasing circulating free IGF-I, which suppresses pituitary GH secretion. These test findings can be misleading in patients who have diabetes.


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