Prolactin in the blood

Prolactin is synthesized in specialized lactogenic cells in the anterior lobe of the pituitary gland; its synthesis and release is under the stimulatory-inhibitory influence of the hypothalamus. The hormone is secreted sporadically. In addition to the pituitary gland, prolactin is synthesized by the decidual membrane (which explains the presence of prolactin in the amniotic fluid) and endometrium. Unlike gonadotropins, prolactin consists of a single peptide chain comprising 198 amino acid residues, and has a molecular weight of approximately 22,000-23,000. The target organ for prolactin is the mammary gland, the development and differentiation of which is stimulated by this hormone. During pregnancy, the concentration of prolactin rises due to the enhanced formation of estrogen and progesterone. The stimulating effect of prolactin on the mammary gland leads to postpartum lactation.

High concentrations of prolactin have an inhibitory effect on ovarian steroidogenesis, the formation and secretion of gonadotropins by the pituitary gland. In men, its function is not known.

Prolactin appears in serum in three different forms. Biologically and immunologically active monomeric (small) form predominates (approximately 80%), 5-20% is present in the form of a biologically inactive dimer ("large") form and 0.5-5% in the form of a tetrameric ("very large") form , which has a low biological activity.

The production and secretion of prolactin by lactotrophic a-cells of the anterior lobe of the pituitary gland is under the control of a number of regulatory centers of the hypothalamus. Dopamine has a pronounced oppressive effect on the secretion of prolactin. The release of dopamine by the hypothalamus is controlled by the nucleus dorsomedialis. In addition to dopamine, norepinephrine, acetylcholine and y-aminobutyric acid have an inhibitory effect on the secretion of prolactin. Derivatives of TRH and tryptophan, such as serotonin and melatonin, perform the function of the PRG and have a stimulating effect on the secretion of prolactin. Concentration of prolactin in the blood increases during sleep, exercise, hypoglycemia, lactation, pregnancy, under stress (surgery).

Reference values (norm) of prolactin concentration in blood serum

Age	Prolactin, mIU / l
Children under 10 years old	91-526
Women	61-512
Pregnancy 12 weeks	500-2000
Pregnancy 12-28 weeks	2000-6000
Pregnancy of 29-40 weeks	4000-10000
Men's	58-475

Hyperprolactinemia (in men and women) is one of the main causes of impaired fertility. Prolactin is used in clinical practice in anovulatory cycles, hyperprolactinemic amenorrhea and galactorrhea, gynecomastia and azoospermia. Prolactin is also determined for suspected breast cancer and pituitary tumors.

When determining prolactin, remember that the concentration depends on the time taken to take the blood, since prolactin secretion occurs sporadically and is subject to a 24-hour cycle. Prolactin release is stimulated by breastfeeding and stress. In addition, an increase in serum prolactin concentrations causes a number of drugs (eg, benzodiazepines, phenothiazines), TRH and estrogens. The secretion of prolactin is inhibited by derivatives of dopamine (levodopa) and ergotamine.

Recently, many authors have reported the presence of macroprolactin in the blood of women with various endocrine diseases or during pregnancy. It is also described that there is a different ratio of serum macro-prolactin ("very large" - a molecular weight of more than 160,000) and monomeric prolactin when analyzed by different test systems. A number of test systems collectively determine all variants of the prolactin molecule in a wide range. This circumstance can lead to different results depending on the test system used.

Blood samples with elevated prolactin levels may contain macro-prolactin (prolactin-IgG complex) and oligomeric forms of the hormone. Patients with prolactin levels above the reference values should differentiate different forms of the hormone. Macro-prolactin or prolactin oligomers are determined by pretreating a serum sample with a 25% polyethylene glycol solution (PEG-6000) and then analyzing the supernatant for prolactin. The discrepancy between the prolactin values in the treated and native samples indicates the presence of macroliprolactin and / or oligomers of prolactin.

The amount of macroprolactin and its oligomers is determined by calculating the ratio of prolactin concentration in the initial sample and after PEG precipitation - [(prolactin concentration after PEG precipitation × dilution) / prolactin concentration in the initial sample (before PEG precipitation)] × 100%. The result of the study is evaluated as follows.

• If the ratio exceeds 60% - the sample mainly contains monomeric prolactin.
• Values of 40-60% ("gray zone") - the sample contains both monomeric prolactin, and macro-prolactin and / or oligomers of prolactin. You should inform the clinician that you need to re-examine the patient's blood (for example, by filtration chromatography in a gel or another test system).
• A ratio of less than 40% indicates that the sample contains macroprolactin and / or prolactin oligomers. The result should be compared with clinical data.

To date, the clinical significance of various forms of prolactin remains unclear.

[1], [2], [3], [4], [5], [6]