Prolactin

HOSP #		WARD	ENT Clinic
CONSULTANT		DOB/AGE	35 Y Male

Abnormal Result

Prolactin 10 986.0 ug/L (4-15.2) 

Dilutions:

1/10  >4700;

1/100 = 10821;

1/50 = 10 986.

Presenting Complaint

Epistaxis

History

Patient with epistaxis referred to the ENT specialist clinic.  No relevant medication history.

Examination

35 y male with a large left post-nasal space mass, a vascular mass involving the pituitary fossa.

?NBL (non-benign lesion)

?Sinonasal malignancy

?Pituitary Tumour

Laboratory Investigations

TSH 0.91 pmol/L (0.27-4.20)

Free T4 15.7 pmol/L (12-22)

FSH 0.8 IU/L ↓ (1.5-12.4)

LH 0.2 IU/L ↓ (1.7-8.6)

Testosterone 0.2 nmol/L ↓ (8.6-29.0)

PTH 1.7 pmol/L (1.6-6.9)

Prolactin measuring method:

The Elecsys prolactin sandwich immunoassay uses two monoclonal
antibodies directed against human prolactin.

R1 = biotinylated antibody – recognizes the N-terminal end of the
prolactin molecule

R2 – ruthenium complexed antibody probably reacts with a region in the
middle of the prolactin molecule.

1^st incubation: a biotinylated monoclonal prolactin-specific
antibody and a monoclonal prolactin-specific antibody labeled with a ruthenium
complex form a sandwich complex.

2^nd incubation: after addition of streptavidin-coated
microparticles, the complex becomes bound to the solid phase via interaction of
biotin and streptavidin.

Reaction mixture aspirated into the measuring cell where microparticles
are magnetically captured into the surface of the electrode.   Unbound substances are then removed with
ProCell. 

Application of a voltage to the electrode then induces
chemiluminescent emission which is measured by a photomultiplier, results
calculated by a standard curve.

Other Investigations

Monomeric prolactin – 7744 ug/L (70% recovery after PEG precipitation)

Biopsy: confirmed tumour stained strongly positive
with prolactin suggesting a prolactinoma.

Final Diagnosis

Pituitary Macroprolactinoma

Take Home Messages

Sandwich immunoassays are prone to high dose hook-effect. There are
various ways to overcome this effect. (This will later be expanded on – see AFP
/ Beta-HCG).

Prolactin appears in the serum as:

1. Active monomeric
   prolactin (“little”) (80%) 23kDa
2. Inactive dimeric
   prolactin (“big”) (5-20%) 50-60kDa
3. Low activity
   tetrameric prolactin (“big-big”) (0.5-5%) 150-170kDa 

Precipitation by PEG yields the active monomeric
prolactin, expressed as a percentage recovery after precipitation.  Big-big prolactin consists of an
antigen-antibody complex of monomeric prolactin-immunoglobulin G and is defined
as macroprolactin.  This has a long
half-life in blood when compared to normal prolactin and gives false high
readings of prolactin, leading to unnecessary investigations in certain
cases.  A high prolactin should thus be
confirmed by doing a PEG precipitation.