Endocrine Abstracts

Background: Prolactin secreting pituitary adenomas (prolactinomas) is the most common pituitary adenoma. The magnitude of prolactin secretion in prolactinomas is usually proportionate to the tumor’s size. Invasive giant prolactinoma is a large prolactinoma (>4 cm in dimension) presenting with serum prolactin levels of >1000 ng/ml and mass related clinical symptoms. Giant prolactinoma is rare and usually presents in men.

Case report: A 52-year-old man was admitted to our hospital with headache, decreased libido, blurred vision and diplopia existing for 6 months. MRI showed a giant pituitary adenoma (47×35×33 mm) with extension to sylvian, suprasellar cisterns, invading right cavernous sinus and extension from right of third ventricle to the superior, wrapping the right internal carotid and middle cerebral arteries. The hormon levels at admission showed FSH, 4.48 μIU/ml (NR, 1.5–12.4); LH, 3.75 μIU/ml (NR,1.7–8.6); testosterone 0.736 ng/ml (NR 2.84–8); TSH,0.959 μIU/ml (NR,0.27–4.2); free thyroxin index 1.28 ng/dl (NR, 0.9–1.7) and prolactin, 470 ng/ml (NR, 0–15). Because of apoplexy, patient was promptly operated. Postoperative prolactin level was 470 ng/ml as before. Then prolactin assay was repeated in 1 in 100 dilution of the serum, which was reported as 2060 ng/ml.

Conclusion: The intensity of an antigen-antibody interaction depends primarily on the relative proportion of the antigen and the antibody. A relative excess of either will impair adequate immune complex formation. This is called the ‘high dose hook effect’. The high dose hook effect often interferes with the assay result. To overcome the hook effect, the serum sample is diluted and prolactin assayed. In our patient, a 1:100 dilution gets accurate values. We suggest that in order to accurately estimate PRL in patients with large pituitary tumors, PRL should be assayed in 1:100 or even higher dilutions of serum in order to gate an accurate estimate of serum PRL.