Hyperprolactinaemia masked by the hook effect in a patient with a giant prolactinoma with growth hormone co-secretion.

We report a woman in her 50s with a giant co-secreting pituitary macroadenoma producing both prolactin and growth hormone. She presented with headache, proptosis, chemosis and secondary amenorrhoea since her 20s. Workup showed hyperprolactinaemia (251 μg/L; normal ≤23 μg/L). A brain MRI showed a giant mass at the base of the skull. Repeat prolactin assay after serum dilution showed marked hyperprolactinaemia (386 020 μg/L), consistent with a giant prolactin-secreting tumour. Additional tests established the presence of growth hormone excess. She was treated with cabergoline, leading to substantial clinical improvement and progressive decrease in tumour size and prolactin. Additionally, she was initially treated with lanreotide which was ineffective. She was switched to pegvisomant therapy, resulting in insulin-like growth factor I normalisation. This case emphasises the critical importance of recognising the hook effect artefact in patients with large sellar masses. Slow uptitration of dopamine agonist therapy is advisable in patients with large tumours to minimise the risks of cerebrospinal fluid leak, apoplexy and herniation.