(13.01) Discordance between Level of Prolactin and Prolactinoma Size

Introduction : Prolactinomas are the most common hormone-secreting pituitary tumors. In general, serum prolactin (PRL) levels correlate to the size of the tumor. When the tumor size is smaller than 1 cm, the serum prolactin level is typically below 200 ng/ml. It is uncommon to find a markedly elevated PRL level that is disproportionate to the size of the prolactinoma. We present a case of a 55-year-old female with an incidental 6mm microadenoma with a PRL level of 464 ng/ml. Based on our literature review, only one similar case report exists that describes a 32-year-old man with a macroprolactinoma measuring 9 x 8 x 9 mm, and a markedly elevated PRL level of 1302 mcg/L

Case(s) Description : A 55-year-old female with a history of hypertension, diabetes, morbid obesity, and right malignant otitis externa was referred to our endocrine clinic for evaluation of a 6-mm pituitary microadenoma, which was an incidental finding during magnetic resonance imaging (MRI) of the internal auditory canal.
Upon history taking, the patient disclosed that she had been consistently experiencing milk discharge from her breasts since childbirth 20 years ago, and she was unable to conceive after her first child. Additionally, she endorsed irregular menstrual cycles and eventually reached menopause at age 50. She denied any nausea, vomiting, headaches, or visual disturbance. She had no history of antipsychotic, antidepressant, antiemetic, or oral contraceptive use. She denied tobacco, alcohol, and recreational drug use.
Physical exam was notable for milky discharge from her right breast. She had no peripheral vision loss. The hormonal panel revealed an elevated PRL level of 464 ng/mL (3.4 – 24.1 ng/mL). Other anterior pituitary hormones were normal, including adrenocorticotropic hormone (ACTH) of 42.7 pg/mL (7.2 – 63.3 pg/mL), free thyroxine (FT4) of 1.2 ng/dL (0.9 – 1.8 ng/dL), thyroid stimulating hormone (TSH) of 1.26 uIU/mL (0.27 – 4.20 uIU/mL), luteinizing hormone(LH) of < 0.3 IU/L, follicle-stimulating hormone(FSH) of 0.6 IU/L, insulin-like growth factor 1 (IGF-1) of 146 ng/mL (49 – 234 ng/mL). A pregnancy test was negative.
The patient was started on cabergoline 0.25mg twice weekly due to suspected prolactin-secreting pituitary microadenoma. She experienced an improvement in her milky discharge following the administration of cabergoline.

Discussion : Our case demonstrates that the degree of prolactinemia may not always correspond to prolactinoma size. Further research is warranted to understand the underlying mechanisms and implications of these uncommon scenarios.