A 52-year-old man with known pituitary macroadenoma on cabergoline presents with a 3-month history of fatigue, increased thirst, and visual field defect. MRI shows tumor enlargement. Serum prolactin is now 82 µg/L (previously 2800 µg/L on cabergoline). What is the most likely explanation?
- A Cabergoline resistance requiring dose increase
- B Transformation to non-functioning adenoma
- C Hook effect causing spuriously low prolactin masking giant prolactinoma ✓
- D Concurrent GH-secreting adenoma
Explanation
The 'hook effect' (prozone phenomenon) occurs when a markedly elevated antigen (prolactin >100,000 µg/L in giant prolactinomas) saturates both capture and detection antibodies in immunoassay, leading to falsely low readings. The solution is serial dilution of the sample, which reveals the true extremely elevated prolactin. Enlarging tumor with apparent prolactin drop on therapy strongly suggests hook effect rather than true response or resistance. This is a critical diagnostic trap — the clinical scenario of tumor growth with seemingly controlled prolactin should always prompt dilution testing.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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Written and medically reviewed by the StethoPrep medical team.