A 34-year-old man comes to the office due to a 6-month history of progressively worsening headaches. He has also noticed blurring of his peripheral vision while driving. The patient has no history of head injuries and has had no associated dizziness, weight change, diplopia, or polyuria. Medical history is unremarkable, and he takes no medications. He does not use tobacco, alcohol, or illicit drugs. Physical examination shows a bitemporal visual field deficit. Laboratory studies show a serum prolactin level >20 times the upper limit of normal. Cranial imaging reveals a large pituitary mass. Which of the following changes in serum GnRH, LH, and testosterone are most likely to be found in this patient?

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11ec160b_3ac8_c41b_9249_67c249bb47cb_MD0001_00 This patient's bitemporal hemianopsia and headaches suggest mass effect symptoms due to an expanding lesion near the optic chiasm. Cranial imaging (typically MRI scan) can confirm the diagnosis of a pituitary adenoma. Unlike other pituitary hormones, prolactin secretion is under negative regulation by dopamine from the hypothalamus, and any disruption of dopaminergic pathways in the pituitary stalk can cause a moderate increase in prolactin release. However, very high prolactin levels, as seen in this patient, suggest a prolactin-secreting adenoma (prolactinoma). Increased circulating levels of prolactin suppress GnRH secretion, leading to a reduction in release of LH from the pituitary and decreased testicular testosterone production. In premenopausal women, prolactinomas typically present early with galactorrhea and amenorrhea. However, in men and postmenopausal women, early symptoms may be subtle and nonspecific (eg, decreased libido), and patients frequently present later with mass effect symptoms (eg, headache, visual field defects). (Choices B and D) Physiologic GnRH secretion occurs in a pulsatile fashion and initiates gonadotropin (eg, LH, FSH) and subsequently testosterone secretion. However, prolonged, continuous GnRH secretion suppresses gonadotropins, resulting in lower levels of sex steroids. Long-acting GnRH agonists (eg, leuprolide) are used to suppress sex steroid-dependent diseases (eg, prostate cancer). (Choice C) Primary hypogonadism with low testosterone levels results in loss of negative feedback on GnRH release, causing elevated levels of GnRH and LH. (Choice E) Intake of exogenous testosterone (eg, anabolic steroid use) will suppress release of GnRH and subsequently LH. Educational objective: Large prolactin-secreting pituitary tumors can compress the optic chiasm, causing bitemporal hemianopsia. In addition, elevated prolactin levels suppress release of GnRH, leading to decreased LH secretion and subsequently impaired testosterone production in men. __________ References: Pituitary adenomas: an overview. (http://www.ncbi.nlm.nih.gov/pubmed/24010395)

A 34-Year-Old Man Comes to the Office Due to a 6-Month