Endocrine Abstracts

Introduction.: Circulating prolactin is predominantly monomeric, but when there presents more than 60% of polymeric, less bioactive forms, phenomenon of macroprolactinemia is diagnosed.

For prolactinomas, serum prolactin levels generally parallel to tumor size, but tumor mass and prolactin levels may be dissociated due to hook-effect, when falsely low prolactin level observed. It is excluded by serial dilutions of serum samples.

The frequency and clinical significance of that phenomenons are still less understood.

Objective: To assess the prevalence and clinical significance of Hook-effect and macroprolactinaemia phenomenons in patients with prolactinomas.

Materials and methods: 190 patients with prolactinomas (87% females and 13% males) were examined with conventional laboratory and instrumental methods. Pituitary tumors were classified according to size as microadenomas – <10 mm, mesoadenomas – from 10 to 20 mm, macroadenomas – from 20 to 40 mm, giant – >40 mm in diameter.

Results:: Macroprolactin was revealed in 65 cases (34%) of asymptomatic hyperprolactinemia. Phenomenon of macroprolactinaemia was found in 5% of patients with microadenomas, and was negative in patients with other tumor sizes (P<0.01). Average prolactin content in those patients was 1550±1.7 mU/l. Average macroprolactin content was 64.5±4%.

Hook-effect phenomenon was observed in 3 patients with macroadenomas and marked in elevation of prolactin level from 1500 to 22 390 mU/l, from 3180 to 130 785 mU/l, from 4200 to 240 168 mU/l after dilution. In patients with other tumor size this phenomenon was negative (P<0.01).

Conclusion: As phenomenons of macroprolactinaemia and hook-effect cannot be reliably distinguished on clinical criteria alone, we recommend routine screening for macroprolactin in patients with asymptomatic microprolactinomas and hook-effect exclusion by serum dilution in patients with macroadenomas in oder to avoid misdiagnosis and mismanagement.