7883 Atypical Presentation of Macroprolactinoma: Discordantly Low Serum Prolactin and Therapeutic Challenges

Abstract Disclosure: S. Hossain: None. M.S. Hossain: None. B. Gautam: None. S.C. Kumar: None. H. Liao: None. D.S. Rosenthal: None. Introduction: Pituitary tumors are categorized as microadenomas (1 cm) and macroadenomas (1 cm) with prolactinomas being one of the most common causes. The mean prevalence of prolactinomas is 10 per 100,000 in men and 30 per 100,000 in women. Serum prolactin levels corresponds to the size of the prolactinoma and any discrepancy should be treated with caution as that may indicate co-existence of pluripotent cells. We present a patient who presented with visual field defect and headaches, found to have a pituitary macroadenoma with discordantly low serum prolactin level compared to the size of adenoma, prompting a change in the treatment course. Case Presentation: A 42 year old male with no past medical history was seen as an endocrinology consult requested by neurosurgery for frequent headaches and visual problems for 1 year. Vital signs were normal, with negative orthostatic drop of blood pressure. He had prominent bitemporal hemianopsia confirmed by formal visual field testing by Ophthalmology. No galactorrhea or features suggestive of acromegaly or hyperthyroidism. He complained of erectile dysfunction but appeared to have normal male secondary sexual characteristics. MRI of brain showed a suprasellar mass measuring 3.8 x 2.6 x 2.2 cm with mass effect on the optic chiasm, hypothalamus with extension and infiltration into the right cavernous sinus. Lab results showed elevated prolactin of 1343.5 ng/ml confirmed by serial dilution, low serum testosterone level of 134 ng/dl and inappropriately normal FSH and LH. Complete blood counts, comprehensive metabolic panel, TFT, ACTH, cortisol and IGF-1 were normal. Patient was started on bromocriptine and he noticed improvement in his field of vision and acuity within one week of starting therapy. He continued to show sustained improvement in vision and resolution of his headaches at the one-month mark. MRI 2 months from starting bromocriptine showed modest reduction in size of the tumor to 2.1 x 2.3 x 2.4 cm with continued mass effect on the optic chiasm. Prolactin level decreased to 69.3 ng/ml. Therefore, although there was some improvement in visual fields, surgical resection was recommended to alleviate the mass effect on the optic chiasm. Conclusion: Our case highlights the significance of identifying discrepancies between the size of a prolactinoma and serum prolactin levels. Expected prolactin level from a prolactinoma similar in size to our case would be 3000 ng/ml when ours was 1343.5ng/ml. Despite undergoing treatment and lowering of prolactin levels our patient's pituitary adenoma did not significantly reduce in size after two months of treatment with bromocriptine and the mass continued to exert pressure on the optic chiasm. This suggests possible presence of pluripotent cells in addition to prolactin-producing cells within the tumor. The recommended course of action was surgical resection of the mass. Presentation: 6/1/2024