SAT-090 An Elusive Diagnosis: Deciphering A Longstanding Pituitary Macroadenoma

Abstract Disclosure: L. Lee: None. R.C. Rajasoorya: None. Background: Prolactinomas are the most common functioning pituitary adenomas, with macroprolactinomas more common in elderly males. Non-functioning pituitary adenomas (NFPAs) are the most common pituitary macroadenomas. Dopamine agonists (DA) are first-line treatment for macroprolactinomas and also used for NFPAs. Diagnosis between the two can be difficult due to atypical and overlapping features. Clinical Case: A 74-year-old male, presenting after a fall, was incidentally found to have a parasellar mass on CT brain imaging. MRI Pituitary Gland confirmed a 2.2 x 2.0 x 2.2cm enhancing lesion encasing the cavernous sinus, right cranial nerves III, IV, V and VI, sphenoid sinus and suprachiasmatic region with displacement of the pituitary gland and stalk to the left side. Posterior T1 bright spot was preserved. He had no prior brain imaging.History revealed longstanding headaches, bilateral mastectomy for gynecomastia at age 32, and progressive loss of libido, which he attributed to aging. He fathered three children prior to onset of symptoms. He worked as a taxi driver with no visual symptoms.Examination showed non-eunuchoid proportions, Tanner Stage 4 with normal testicular size, preserved body hair except for sparse leg hair, and bilateral mastectomy scars. Visual field testing was normal. He was clinically euthyroid with no cushingnoid or acromegalic features.Hormonal workup showed persistently elevated prolactin (276 µg/L n < 16.2 ug/L) after serial dilution. Macroprolactinemia was excluded with PEG precipitation. Medications, renal and liver impairment were excluded. He had biochemical hypergonadotrophic hypogonadism (FSH 57.3 U/L n < 12.4 u/L, LH 47.4 U/L n < 8.6 u/L, total testosterone 3.8nmol/L n > 7.3 nmol/L). Other pituitary axes were normal. He declined bone mineral density scan but had no history of fragility fractures.He declined surgery and opted for Cabergoline therapy. It normalized prolactin (6.5 µg/L) in 6 months, restoring libido without hypersexual behaviour. Follow-up MRI showed minor reduction in the sphenoid sinus hypo-enhancing component but stable overall tumour size. Conclusion: This case highlights diagnostic complexity between NFPAs and macroprolactinomas, given the discordance between tumor size and serum prolactin, absence of hypopituitarism, and minimal adenoma shrinkage with DA therapy. A serum prolactin > 250 ug/L is rarely seen in NFPAs, though a macroadenoma greater than 2cm would typically yield higher prolactin levels. Definitive subtype confirmation requires histology, which was declined. Non-invasive methods like ¹²³I-methoxybenzamide pituitary scintigraphy could aid diagnosis but are not routinely available. The insidious onset, cultural perceptions of aging and sexual inactivity also contributed to diagnostic delay. This case underscores the need for a nuanced approach, particularly in atypical presentations. Presentation: Saturday, July 12, 2025