A 56-year-old man comes to the physician with a 7-month history of headache, loss of libido, erectile dysfunction, and visual disturbances. His past medical history is unremarkable. He is currently not taking any medications. The patient has a 15-pack-year smoking history and does not use alcohol. On visual field examination, there is a small temporal field defect seen in both eyes. Genitourinary examination shows atrophic testes. An MRI scan reveals a 1.5-cm pituitary tumor. Which of the following biochemical findings will be most consistent with the diagnosis of a prolactinoma in this patient?

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11ec25c8_d3b7_39e3_bc81_bbea959d2ed5_MD0003_00 Prolactin-producing lactotroph adenomas (prolactinomas) account for approximately 50% of primary pituitary tumors, making them the most common type. Prolactinomas are usually detected early (<10-mm microadenomas) in premenopausal women due to associated endocrine symptoms (oligo/amenorrhea, infertility, galactorrhea). However, in men and post-menopausal women, prolactinomas produce only minimal or nonspecific symptoms at early stages, and many are not diagnosed until they are much larger and produce compressive symptoms such as headaches or visual field defects. Men may also develop symptoms of hypogonadotropic hypogonadism due to suppression of gonadotropin-releasing hormone secretion from the hypothalamus by prolactin. Gynecomastia is present in about a third of men with prolactinomas; galactorrhea may occur but is uncommon. suppression of gonadotropin-releasing hormone secretion The magnitude of prolactin elevation correlates with the size of the prolactinoma, and a level >200 ng/mL is virtually diagnostic of prolactinoma. As seen in this patient, hyperprolactinemia at this level will usually cause hypogonadism with low testosterone levels and low or inappropriately-normal LH. TSH may also be normal or low; this depends on the compressive effect on thyrotroph cells. (Choice B) Very large pituitary masses will cause low TSH and LH levels due to compression of thyrotrophs and gonadotrophs, along with mildly elevated prolactin due to compression of the dopaminergic pathways in the pituitary stalk. This is the characteristic pattern of "nonfunctioning" pituitary macroadenomas (which usually secrete the biologically inert gonadotropin alpha subunit). However, a mildly elevated prolactin level of 45 ng/mL is less consistent with 1.5-cm prolactinoma, which would likely be associated with a prolactin level >200 ng/mL (prolactin levels can be as high as 1000 ng/mL in ~2-cm prolactinomas). (Choice C) Mild elevations of prolactin in the context of otherwise normal pituitary function are typically due to medications (eg, certain antipsychotic drugs). Other causes include hypothyroidism and renal failure. (Choice D) Primary hypothyroidism may cause mild-to-moderate hyperprolactinemia, which is thought to be due to thyrotropin releasing hormone-induced stimulation of lactotrophs. LH in such patients may be normal or mildly low. Educational objective: Prolactin-producing lactotroph adenoma (prolactinoma) is the most common primary pituitary tumor. Serum prolactin levels correlate with the size of prolactinoma, and levels >200 ng/mL are essentially diagnostic. References: Clinical Practice: Prolactinomas (http://www.ncbi.nlm.nih.gov/pubmed/20357284)

A 56-Year-Old Man Comes to the Physician with a 7-Month