Abstract #1398813: A Rare Case of Acromegaly with Negative Immunohistochemical Staining for Growth Hormone

Abstract

We are presenting a case of mixed prolactin and growth hormone (GH)-pituitary tumor with negative immunostaining for GH. 81-year-old male with history of type 2 diabetes mellitus, hypertension, obstructive sleep apnea, and colon polyps was found to have a pituitary macroadenoma during evaluation of his visual disturbances. Years before the presentation, he noticed his shoe size had increased and his wife observed his jaw becoming more prominent. Physical exam was notable for enlarged nose, mandibular prognathism, and enlarged hands. Pre-operative laboratory results were remarkable for significantly elevated levels of insulin-like growth factor-1 (IGF-1), GH, and prolactin. He underwent transsphenoidal resection (TSPR) of the mass and pathology demonstrated a lactotroph adenoma, negative for GH immunostaining. After surgery he was started on cabergoline 0.5mg PO twice weekly. Three months afterwards his IGF-1 level remained elevated at 415 ng/mL. Acromegaly was confirmed by an elevated GH level of 13.8 ng/mL 2-hours after oral glucose challenge, then a long-acting somatostatin analogue depot was initiated for treatment. In the first year of therapy, he had weight loss and hypoglycemia that required discontinuation of insulin therapy. His IGF-1 and GH levels decreased to within age-appropriate ranges. His prolactin level returned to normal and cabergoline was stopped after a successful discontinuation trial. Clinical suspicion for acromegaly is confirmed by elevated IGF-1 and GH levels. When a GH-secreting pituitary adenoma is resected, immunocytochemistry with GH staining typically supports the diagnosis. It is not common for these tumors to stain negative for GH in the presence of clinical and biochemical acromegaly. The opposite—termed silent somatotroph— is more common and may represent an early stage in the development of a somatotroph adenoma. Failure to provide adequate diagnostic tissue, histopathology misread, or loss of antigenicity during fixation or embedding may be reasons for false negatives. However, it has been suggested that sparse cytoplasmic stores of GH do not parallel the hypersecretion of GH and may reduce staining capacity. Thus, a diagnosis of acromegaly should be made in patients with clinical and biochemical findings, even in the absence of GH immunostaining.