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Prolactin hook effect
A 42-year-old man has reduced libido and bitemporal visual field loss. MRI shows a 3.4 cm sellar mass with suprasellar extension. Serum prolactin is reported as 48 ng/mL (normal up to 20). The endocrinologist is not satisfied with this result. What should be done next?
- correctA. Repeat the prolactin on a serially diluted sample
- B. Accept the result and refer for urgent transsphenoidal surgery
- C. Start cabergoline and repeat the MRI in six months
- D. Measure serum growth hormone and IGF-1 instead
Explanation
Key fact
A huge prolactinoma with only a modestly raised prolactin is the hook effect - the assay is saturated and reports a falsely low value. Dilute the sample.
Repeat the prolactin on a serially diluted sample. A 3.4 cm prolactinoma should produce a prolactin in the thousands, not 48.
How it happens
- Tumour size and prolactin level normally correlate closely.
- A macroadenoma over 3 cm secreting prolactin should give a level well over 1,000 ng/mL.
- A value of 48 is far too low for a tumour that size.
- So either the tumour is not a prolactinoma, or the assay is wrong.
- An enormous excess of antigen saturates a two-site immunoassay - the hook effect.
- Diluting the sample brings it back into range and reveals the true level.
At very high antigen the assay reads low: dilute the sample and the true value appears
Why not the others
- B. Accept the result and refer for urgent transsphenoidal surgery — risks operating on a tumour that would shrink on a tablet. Prolactinoma is the one pituitary tumour treated medically first.
- C. Start cabergoline and repeat the MRI in six months — may well be right, but not before the diagnosis is secured. A modest prolactin can also reflect stalk compression by a non-functioning adenoma, which will not respond.
- D. Measure serum growth hormone and IGF-1 instead — is reasonable in a full pituitary assessment, but it does not resolve the discrepancy in front of you.
Remember
Big tumour, small prolactin: dilute the sample. The other explanation is stalk compression, and the dilution tells you which.
Go deeper
The mechanism: excess antigen occupies the capture and detection antibodies separately, so no sandwich forms and the reported value falls.
Stalk effect: any sellar mass interrupting dopamine delivery raises prolactin, but usually only to 30 to 100 ng/mL. Above about 200 ng/mL a prolactinoma is near certain.
Cabergoline shrinks prolactinomas dramatically, and visual fields often recover within weeks - surgery is reserved for failure, intolerance or apoplexy.
Before calling a mild elevation pathological, exclude the common causes: pregnancy, hypothyroidism, chest wall stimulation, renal failure, and dopamine-blocking drugs such as antipsychotics and metoclopramide.
Harrison's Principles of Internal Medicine, 22nd ed. (McGraw Hill, 2025).