Prolactin is a protein hormone (closely related to human growth hormone) that is secreted by specialized cells in the anterior part of the pituitary gland. In addition, the hormone is also produced and secreted by a broad range of other cells in the body, most prominently various immune cells, the brain and the lining of the uterus. Most cells respond to prolactin. In fact, it is hard to identify any tissue that does not have prolactin receptors.
Although prolactin’s major target organ is the breast where it stimulates development and milk production, the hormone has many other functions. Several hundred different actions have been reported for prolactin and
Immune cells are rich in prolactin receptors and certain types of lymphocytes in fact synthesize and secrete prolactin. These observations suggest that prolactin may to some extent act as a regulator of the body’s immune activity.
In an area in the brain known as the hypothalamus, a chemical called dopamine is released. Dopamine suppresses prolactin synthesis and release by the pituitary gland. As such it acts as a “hypothalamic brake set” causing prolactin only to be secreted when the “brake” is released. Treatment with dopamine agonists such as bromocriptine (Parlodel) and cabergoline (Dostinex) , by enhancing serotonin production lowers prolactin
Several other hypothalamic hormones, including thyroid releasing hormone (TRH) and gonadotropin releasing hormone (GnRH) cause an increase in prolactin secretion Stimulation of the nipples (including but not limited to nursing) leads to hypothalamic activation and prolactin release Estrogens also exerts a positive control over prolactin synthesis and secretion
Even modestly raised prolactin levels (20ng/ml-40ng/ml) can interfere with estrogen-induced endometrial proliferation as well as egg/ ovarian follicle growth and development. Accordingly treatment with dopamine agonists (bromocriptine/ Cabergoline) might be of benefit in such cases.
Increased PRL secretion reduces the pulsatility of GnRH impairing the pituitary production of FSH and LH and may directly impair the endocrine activity of ovarian follicles as well as endometrial response to estrogen. This can lead to dysfunctional or failed ovulation, a defective luteal phase, and a poorly developed endometrial response to estrogen (a thin endometrial lining). About n 5% of unselected, asymptomatic infertile women have hyperprolactinemia. In such cases long-term use of dopaminergic drugs such as bromocryptine and can normalized prolactin levels leading to reestablishment of functional ovulation and improved endometrial development. About half of the pregnancies occurring during dopaminergic therapy start after the first 6 months of this drug therapy. Treatment should continue for at least 1 year.
Common manifestations of significantly increased prolactin secretion (hyperprolactinemia):
- In women:
- Oligo/amenorrhea (reduction or absence of menstrual flow) and galactorrhea (excessive or spontaneous breast secretion of milk).
- A modest elevation in blood prolactin can also point to an underlying state of hypothyroidism
- Markedly elevated prolactin levels (i.e. >60ng/ml) might point to a prolactin producing pituitary macroadenoma or microadenoma as well as other intracranial lesions such as craniopharyngiomas, meningiomas etc.
- In men: Such men rarely have galactorrhea
- Breast enlargement (gynecomastia)
- Erectile dysfunction
- Decreased Libido
- Sperm dysfunction resulting in infertility and with impotence.
Causes of hyperprolactinemia: Main causes of pathologic hyperprolactinemia (40).
- Idiopathic (commonest variety) ….cause unknown Acromegaly
- Empty Sella Turcica
- Renal Failure
- Polycystic Ovarian Syndrome (PCOS)
- Certain drugs:
- Antipsychotic drugs ( phenothiazines, haloperidol, monoamine oxidases (MAO) risperidone, fluoxetine, butyrophenones,
- Anti-emetics: metoclopramide, domperidone,
- Tricyclic antidepressants
- Antihypertensives and Ganglion blockers
Drug-induced hyperprolactinemia can be reversed by modifying or withdrawing the causative medication. In cases where this cannot safely be done, bromocryptine derivatives can be used.
- Pituitary adenomas (prolactinomas)
- Some pituitary adenomas are treated by surgical removal but in most case prolonged treatment with bromocryptine or cabergoline will effectively lower blood concentrations and lead to shrinkage/disappearance of the tumor. Such treatment is also safe during pregnancy.
- Intracranial lesions such as craniopharyngiomas, meningioma causing hyperprolactinemia are usually treated by surgical removal.
Hyperprolactinemia and Reproductive Dysfunction:
- Hypothyroidism in women is often caused by an autoimmune process where antithyroid antibodies progressively replace thyroid glandular tissue with functionless connective tissue. In roughly 50% of such cases there will be increased uterine natural killer cell activity (NKa) which may profoundly impair implantation leading to “perceived infertility” or recurrent pregnancy loss. Thus, ATA with NKa can be present prior to the development of clinically overt autoimmune hypothyroidism (Hashimoto’s disease). Since women with NKa are often infertile, or experience recurrent pregnancy loss, it is important that any unexplained hyperprolactinemia associated with reproductive failure or infertility be evaluated for an immunologic implantation dysfunction (IID) through testing for the presence of antithyroid antibodies and if the ATA level is elevated, that an NKa test (K-562 target cell test) be done. What is not often commonly recognized is that even in cases where autoimmune hypothyroidism is clinically overt, treatment with thyroid hormone replacement will usually not solve the reproductive dysfunction which will usually require selective immunotherapy with Intralipid (IL) infusions plus steroid therapy. IL is administered intravenously about 4-7 days prior to ovulation or egg retrieval and then repeated one more time upon biochemical confirmation of early pregnancy. The steroids are continued to the 8th week of pregnancy and then tailed off over 2 weeks.
- Ovarian Hyperstimulation Syndrome (OHS): Prolactin facilitates production by ovarian follicle cells of VEGF (a vasoactive substance that increases vascular permeability of blood vessels) In cases of severe ovarian With severe ovarian hyperstimulation syndrome (OHSS) where there are a large number of follicles present (>25) and the blood estradiol level is markedly elevated (4,000pg/ml), even modestly elevated prolactin release can markedly worsen the situation. There is strong evidence to suggest that women with ovarian Hyperstimulation (>20 follicles and blood estradiol levels that peak above 3,000pg/ml) who receive O.5mg of oral administration Cabergoline daily for 7 days, starting on the day of the hCG trigger, experience a significant reduction in the risk and severity of severe ovarian stimulation syndrome (OHSS). This is thought to be due to Cabergoline suppressing the production of vascular vasoactive substances such as VEGF that are produced by luteinized follicular granulosa cells, that increase the vascular permeability of local pelvic blood vessels.