Hook effect of prolactin in large pituitary adenomas

Hello everyone,

Here's something I learnt today when a case of large pituitary adenoma causing visual field loss was presented today.

But let's talk about my favorite subject first - Immunology!

The intensity of an antigen-antibody interaction depends primarily on the relative proportion of the antigen and the antibody. A relative excess of either will impair adequate immune complex formation. This is called the “high-dose hook effect” or the “prozone phenomenon.”

This is important consideration whe measuring prolactin. Extremely high levels of prolactin can interfere with the assay and produce falsely low readings.

This high-dose hook effect occurs because there is not enough antibody to bind to both ends of all antigenic peptides, in this case, prolactin.

Most prolactin is complexed to a single antibody. Only few remaining prolactin peptides are “sandwiched” and therefore detectable.

This results in a falsely low prolactin value.

Hence, as the antigen concentrations increase, there is a proportional increase in assay titers up to a certain level. Antigen concentrations above this threshold level would “hook” down the assay values resulting in very low measurements.

In order to avoid the high-dose hook effect, the serum prolactin should be estimated in appropriate dilution in all patients with large pituitary tumors.

-IkaN (tired Internal Medicine Resident)

Source:
The 'hook effect' on serum prolactin estimation in a patient with macroprolactinoma. https://www.ncbi.nlm.nih.gov/m/pubmed/11303248/