ARE THERE ANY CAVEATS TO THE INTERPRETATION OF LAB VALUES?
It is important to note that in postmenopausal women who are not taking hormone replacement therapy, LH and FSH will be elevated and estradiol may provide an additional clue in the detection of abnormal function. Conversely, low LH and FSH in postmenopausal women should raise a flag for hypopituitarism.
Another caveat is that GH secretion is pulsatile and serum levels are undetectable between pulses. Therefore, low/undetectable GH does not necessarily suggest deficiency. The GH measurement would only be helpful if it is significantly elevated (suggestive of hypersecretion—gigantism/acromegaly). Otherwise, GH has little value as a screening test.
Instead, IGF-1, which is secreted from the liver in response to GH secretion, has a longer half-life and serves as a better screening tool. IGF-1 has age- and sex-adjusted reference ranges, which are often reported by the lab or given as a Z score.
WHAT IS THE PREFERRED IMAGING STUDY FOR THE PITUITARY GLAND?
The best choice is MRI of the pituitary gland (not the whole brain) with gadolinium. If the incidentaloma was initially diagnosed by a CT, additional testing with MRI should be performed, unless contraindicated.2
Brian is referred for MRI with gadolinium. The radiologist’s report describes a 5 x 4 x 4–mm pituitary microadenoma without sellar extension or involvement of the optic chiasm.
AT WHAT POINT SHOULD OPTIC CHIASM BE A CONCERN?
Since the pituitary gland is located directly beneath the optic chiasm, any compressive effect of growth against the optic nerve(s) can cause visual impairment. This includes bitemporal hemianopsia (loss of peripheral vision) or ophthalmoplegia (abnormal movement of the ocular muscle). Since clinical signs and symptoms can be subtle or absent, all patients with evidence of a pituitary lesion abutting or compressing the optic chiasm should have a formal visual field exam.2
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