THU098 Gonadotropin Secreting Pituitary Tumor Presenting As Primary Hypogonadism

Disclosure: J.N. Shapiro: None. T.X. Lu: None. J. Eapen: None. J.G. Karam: None. Gonadotroph adenomas are the most common subtype of nonfunctional pituitary adenomas, with 35 percent secreting enough LH or FSH to raise serum gonadotropin levels, but clinical syndromes due to hypersecretion of gonadotropins are rare. A low testosterone level with high FSH or LH can mislead to a diagnosis of concomitant primary hypogonadism rather than a secreting pituitary adenoma. We present a case of a patient with an FSH-secreting pituitary macroadenoma presenting with low testosterone levels. The patient is a 67-year-old man incidentally found to have a pituitary macroadenoma measuring 12 x 13 x 12 millimeters, impinging upon the optic chiasm on MRI of the brain obtained for headaches. Visual field tests revealed abnormalities compatible with glaucoma, but not chiasm compression. He has no symptoms of hormonal excess or deficit, although his wife reported decreased erectile function. Physical examination was unremarkable except for an atrophic left testicle. Pituitary hormone function tests (ACTH, AM cortisol, IGF-1, prolactin, thyroid function tests, and 24-hour urine free cortisol) were within normal range, except for an elevated FSH 14.8-18.1 mIU/mL and decreased testosterone in range of 185-256 ng/dL, with normal LH at 7.0 mIU/mL, for which he was referred for urology for evaluation of primary hypogonadism. The patient underwent transsphenoidal resection of the pituitary adenoma. Postoperative pathology revealed a pituitary adenoma, gonadotroph cell type, with immunohistochemical staining scattered positive FSH, and rare positive LH. Postoperative MRI showed post-surgical changes with no residual nodular enhancement. Following surgery, the patient suffered from severe clinical hypogonadism with episodes of profuse diaphoresis and extreme fatigue. Postoperative laboratory values revealed panhypopituitarism with undetectable testosterone level, and low LH and FSH levels. The patient was successfully treated with hydrocortisone, levothyroxine, and testosterone, with resolution of his symptoms. Our patient presented with a pituitary macroadenoma, low testosterone, and high FSH suggestive initially of primary hypogonadism, but found ultimately to have an FSH-secreting adenoma. He likely had underlying hypogonadism, primary or secondary, and although his adenoma secreted FSH, it was not enough to maintain testosterone level in normal range, possibly in part due to biologically inactive FSH molecules or to primary testicular failure. The extremely low testosterone levels postoperatively suggest that his testicular function was mostly stimulated by the adenoma secreted FSH and was therefore drastically lost after surgery. This case increases awareness to consider a gonadotropin-secreting tumor in the differential diagnosis of a pituitary adenoma with high FSH or LH, even when testosterone level is unexpectedly low. Presentation: Thursday, June 15, 2023