Endocrine Abstracts

Background: Giant macroprolactinomas (>4 cm) are rare pituitary adenomas associated with markedly elevated prolactin levels, often exceeding 5300 mIU/l, and can extensively invade adjacent skull base structures. Unlike most skull base tumours requiring surgery or radiotherapy, prolactinomas usually respond dramatically to dopamine agonists such as cabergoline. Accurate preoperative identification is essential, as misdiagnosis may lead to unnecessary invasive treatment. The Hook effect, an immunoassay artefact, can falsely lower prolactin readings in the presence of very high serum concentrations, masking the true diagnosis.

Case Presentation: A 23-year-old man presented with bilateral proptosis and papilloedema, incidentally detected during an optometrist visit. He had no headache, diplopia, or visual field defect. MRI revealed a large skull base mass replacing the sphenoid bone and extending into the orbits, nasal cavity, and temporal fossae, encasing the optic nerves and internal carotid arteries. The initial serum prolactin level was 3873 mIU/l, suggesting a chordoma with mild stalk-effect hyperprolactinaemia. Trans-sphenoidal biopsy demonstrated a pituitary neuroendocrine tumour (PitNET) of Pit-1 lineage, showing strong prolactin expression with scanty growth hormone staining, consistent with a lactotroph adenoma; however, a mammosomatotroph or mixed somatotroph–lactotroph adenoma could not be excluded. On repeat prolactin testing after serial dilution (1:200), the true concentration exceeded 855,932 mIU/l, confirming a giant macroprolactinoma with a marked Hook effect. MEN1 genetic testing was negative. Treatment with cabergoline and lanreotide achieved normalisation of prolactin within nine months, with follow-up imaging showing marked tumour regression and improved proptosis.

Discussion: - This case illustrates an unusual presentation of a giant macroprolactinoma manifesting solely with proptosis. - Recognising the Hook effect enables timely dopamine agonist therapy, avoiding unnecessary surgery. - The tumour’s size and extensive local invasion poses ongoing challenges in decision making for debulking surgery/radiotherapy, requiring ongoing multidisciplinary guidance and monitoring for CSF leakage while on medical therapy.