Biological response modifiers of cancer-related neuroendocrine disorders: efficacy of the long-term dopaminergic agonist cabergoline in the treatment of breast cancer-induced hyperprolactinemia

The evaluation of the biological status of cancer patients should not be limited only to investigation of immune reactivity, but should also include analysis of the endocrine condition, namely concerning those hormones which have appeared to be tumor growth factors, such as prolactin (PRL) for breast and prostate carcinomas. This statement is justified by the fact that the evidence of abnormally high serum concentrations of PRL has been proven to be associated with poor prognosis in breast and prostate cancer patients. Moreover, since hyperprolactinemia negatively influences the efficacy of anticancer therapies in breast cancer, it could be fundamental to achieve a normalization of PRL levels by long-acting dopaminergic agents, such as cabergoline. On this basis, a study was planned to evaluate the effect of cabergoline on PRL levels in hyperprolactinemic metastatic breast cancer subjects. The study included 20 hyperprolactinemic metastatic breast cancer subjects, who were randomized to receive no therapy or cabergoline at 0.5 mg/week orally for 4 consecutive weeks. Cabergoline therapy induced a normalization in all patients, whereas no spontaneous normalization of PRL levels occured in the control group. These results show that a weekly oral administration of the long-acting dopaminergic agent cabergoline is a well tolerated and effective treatment of metastatic breast cancer-related hyperprolactinemia. The possible prognostic impact of PRL normalization needs to be established by successive studies.     

Hyperprolactinémie et fonction sexuelle chez l’homme

Erectile dysfunction (ED), generally associated with reduced sexual desire and sometimes with orgasmic or ejaculatory dysfunction, is the major presenting symptom of hyperprolactinemia (HPRL) in men, a condition which should not be missed since many cases are due to pituitary tumors, likely to result in serious complications. It is generally believed that the mechanism of prolactin (PRL)-induced sexual dysfunction is a decrease in testosterone secretion. In fact, serum testosterone is normal in many hyperprolactinemic males and testosterone-independent mechanisms are also involved, probably mainly involving cerebral neurotransmitter systems. Systematic determinations of serum PRL have found very low prevalences of marked HPRL (>35 ng/ml) in ED patients (0.76% in a compilation of more than 3,200 patients) and pituitary adenoma (0.4%). In addition, the association of HPRL with ED may have been coincidental in some of these cases, since 10% of HPRLs diagnosed by the usual immunological assays are due to macroprolactins, which are biologically inactive or minimally active variants of PRL. Specific identification of PRL requires PRL chromatography which is only available in some specialized laboratories. No consensus has yet been reached concerning screening for HPRL in ED. Systematic determination of serum PRL may be justified, as HPRL is a serious but reversible disease, while there is presently no reliable clinical, psychometric or hormonal criteria (including serum testosterone level) allowing to restrict its determination to certain categories of ED patients without a risk of missing certain cases of HPRL. In the case of consistent HPRL, looking for hypothalamic or pituitary tumor is mandatory. Dopamine-agonist therapy is the first-line treatment for PRL-induced sexual dysfunction. Sexual counselling may be necessary for some patients.     

Surgical treatment of prolactinomas. Short- and long-term results, prognostic factors

All 347 patients surgically treated for a prolactinoma from January 1, 1976 to December 31, 1982, in the neurosurgical ward of Foch Hospital, were retrospectively studied. The frequency of postoperative normalisation of plasma prolactin (PRL) depends on prolactinoma size, preoperative PRL level, duration of first clinical symptom, previous oestroprogestative contraception, and adenoma necrosis. Postoperative PRL values were normalized in 75% of small prolactinomas (grade 0, 1 or 2) with preoperative PRL values less than 200 ng/ml, and clinical duration less than 5 years (n = 102). There was no operative death and minor morbidity (2.7%). Among the 96 patients with postoperative PRL normalisation, operated between 1976 and 1979, 70 were followed up for an average time of 4.4 +/- 0.2 years. 17% of patients had hyperprolactinemia recurrence with a delay of 1.5 +/- 0.4 years. Postoperative PRL levels near the upper normal limit, and weak PRL response to TRH tests were found to be unfavourable prognostic factors for hyperprolactinemia recurrence. Pregnancy did not increase the risk of recurrence, but could reflect genuine long-lasting remission. Selective adenomectomy remains an interesting treatment for prolactinoma, particularly if the adenoma is small, recent and with PRL moderately increased. The frequency of postoperative PRL normalisation after surgery is less than with bromocriptine, but surgery is the only treatment able to achieve a definitive cure with a low iatrogenic risk.     

Paradoxical prolactin response to growth hormone-releasing hormone in a patient with hyperprolactinemia and empty sella.

In a 30-year-old woman with amenorrhea due to hyperprolactinemia, serum PRL increased to twice the basal amount in response to growth hormone-releasing hormone (GHRH). Roentgenological studies revealed no pituitary adenoma but empty sella. Bromocriptine therapy normalized serum PRL and made the paradoxical response to GHRH disappear. The paradoxical response did not occur in any of eight other patients with hyperprolactinemia due to prolactinoma. Although this case is rare, GHRH stimulates PRL as well as GH release remarkably in some cases with hyperprolactinemia without a GH-producing tumor.     

Macroprolactinemia in childhood and adolescence: a cause of asymptomatic hyperprolactinemia

Asymptomatic hyperprolactinemias associated with altered proportions of molecular forms of circulating prolactin (PRL) have been reported in adults. The scarce references available in children and adolescents prompted us to report our experience in the evaluation and follow-up of patients with macroprolactinemia. We studied 5 patients (1 male and 4 females) aged 11.6-18 years with incidentally discovered asymptomatic hyperprolactinemia. Patients underwent repeated evaluations for a period of 3 months to 8 years, and their PRL levels remained elevated (34.4-516 ng/ml). Structural variants of PRL >/=45 kD ranged between 58.9 and 78.6%. Chromatographic profiles showed increases in Big Big PRL in the 5 cases, ranging between 40 and 72% (normal: 9-21%), and in Big PRL in 3 cases, ranging between 30.0 and 32.6% (normal: 5-25%). Little PRL was decreased in all cases, ranging between 20.6 and 41.1% (normal: 50-90%). In conclusion, upon detection of hyperprolactinemia with no clinical manifestations and no alteration of the remaining endocrine functions, macroprolactinemia should be considered as a possible diagnosis. The confirmed absence of functional alterations during the follow-up would favor a no-treatment approach and at the same time avoid repeating imaging studies.     

American Association of Clinical Endocrinologists,American College of Endocrinology Disease State Clinical Review: Clinical Relevance of Macroprolactin in the Absence or Presence of True Hyperprolactinemia

Objective: To review the current literature regarding the prevalence of macroprolactin (macroPRL) in hyperprolactinemic patients and determine recommendations for testing.Methods: An electronic United States National Library of Medicine PubMed search was conducted for search term “macroprolactin.” Only English-language articles were considered.Results: MacroPRL is an under-recognized cause of elevated prolactin (PRL) and is present in approximately 4% to 40% of hyperprolactinemic patients depending on the referral population. Clinical findings which could be due to hyperprolactinemia are the impetus for testing for PRL. Because of this there is significant overlap in the clinical presentation of patients with true hyperprolactinemia and those with macroPRL, differentiation cannot always be made on the basis of symptoms. A lack of recognition of the presence of macroPRL can lead to unnecessary laboratory investigations, imaging, and pharmacologic or surgical treatment.Conclusion: Until there is a commercially available PRL assay that is not subject to interference by macroPRL, clinicians should consider the possibility of macroPRL, especially if the clinical presentation, imaging findings, and/or response to therapy reveal inconsistenciesAbbreviations:DA = dopamine agonistGFC = gel filtration chromatographyIgG = immunoglobulin GmacroPRL = macroprolactinMMP3 = matrix metalloproteinase-3NS = nonsignificantPEG = polyethylene glycolPRL = prolactinRA = rheumatoid arthritisSLE = systemic lupus erythematosus     

Disturbed prolactin responses to dopamine-related substances in patients with acromegaly and hyperprolactinemia

We undertook this study, because conflicting data were reported about the dopaminergic regulation of prolactin (PRL) secretion in patients with acromegaly and hyperprolactinemia. In order to clarify the dopaminergic regulation of PRL secretion in patients with acromegaly and hyperprolactinemia, the effects of nomifensine, a central dopamine agonist, FK 33-824, a centrally antidopaminergically acting agent, and domperidone, a peripheral dopamine antagonist, on plasma PRL in these patients were studied. The results were compared with those observed in normal subjects and hyperprolactinemic patients, with or without a pituitary tumor. Nomifensine did not lower the PRL levels and FK 33-824 did not raise the PRL levels in acromegalic patients. In hyperprolactinemic patients, nomifensine did not lower the PRL levels and FK 33-824 failed to raise the PRL levels. Domperidone did not increase PRL in about a third of acromegalic patients, while TRH increased PRL in the all normoprolactinemic acromegalic patients. These results suggest that in acromegalic patients there may be a disturbance in dopamine related neurotransmission and that such disorders also seem to be present in patients with hyperprolactinemia, with or without a pituitary tumor.     

Catecholamines and pituitary function. VII: Effects of acute and chronic dopamine-receptor blockade on pituitary response to TRH-GNRH in normal women and in patients with hyperprolactinemic amenorrhea

To investigate whether an enhanced dopamine (DA) inhibition on pituitary thyrotrophs and gonadotrophs may account for the abnormal TSH and LH dynamics in pathological hyperprolactinemia, we examined the effect of an acute lysis of the putative DA overinhibition, as obtained with continuous domperidone (DOM) infusion, on both basal and TRH-GnRH stimulated PRL, TSH and LH release in both normal cycling women and patients with pathological hyperprolactinemia. The effect of TRH-GnRH administration was also examined in women with DA-antagonist induced hyperprolactinemia, in order to evaluate the effect of a chronic lack of the physiological DA inhibition on pituitary hormone dynamics. Patients with both pathological and DA-antagonist induced hyperprolactinemia displayed an evident TSH and LH hyper-responsiveness to TRH-GnRH. The PRL response was reduced in the former but enhanced in the latter group. Domperidone infusion resulted in a marked increase in serum PRL levels in normal cycling women, but not in patients with pathological hyperprolactinemia. The abolition of the putative DA-overinhibition at the pituitary level with DOM infusion in patients with pathological hyperprolactinemia was followed by a slight increase in basal TSH output but did not modify the TSH and LH hyperresponsiveness to TRH-GnRH. The similarities in TSH and LH dynamics between patients with pathological and DA-antagonist induced hyperprolactinemia and the ineffectiveness of DOM infusion in modifying the TSH and LH hyper-responses to TRH-GnRH in the former group, seem to exclude the widely accepted idea that endogenous DA overactivity is responsible for the abnormal thyrotroph and lactotroph dynamics in women with hyperprolactinemic amenorrhea.     

Prolactin release in polycystic ovarian syndrome

To evaluate the prevalence of hyperprolactinemia in patients with polycystic ovarian syndrome (PCO), 72 patients with oligo- or anovulation were studied. All of the patients had persisting elevated LH (greater than 25 mIU/ml), normal FSH, high LH/FSH ratio (greater than 2.5), and exaggerated LH responses to LHRH. Mean testosterone and androstenedione concentrations were appreciably increased in these patients. Out of 171 samples for prolactin (PRL) determination from these 72 patients, only 5 patients had a PRL value above 30 ng/ml during the first sampling. The next sampling from these same 5 women disclosed that they were transiently hyperprolactinemic because the next samples showed a normal PRL value. To further investigate the PRL secretory capacity 500 micrograms of TRH and 10 mg of metoclopramide (MCP) were administered to these 72 and 44 patients, respectively. The PRL response to MCP was significantly blunted in these patients compared to normal women while the PRL response to TRH in these patients was not indistinguishable from that in normal women. These results indicate that the true prevalence rate of hyperprolactinemia in PCO may be low rather than high and the association of hyperprolactinemia with PCO may be coincidental rather than a pathogenically related phenomenon.     

Involvement of hypothalamic dopamine in the regulation of prolactin secretion

The neuroendocrine control of prolactin (PRL) secretion is known to be a multifactorial process, but dopamine (DA) secreted by the tuberoinfundibular dopaminergic (TIDA) neurons of the hypothalamus is believed to exert a predominant inhibitory control on the secretion of PRL. The secretory activity of the TIDA neurons, including the rate of biosynthesis of DA and the rate of release of the neurohormone into hypophysial portal blood, can be readily evaluated in the rat. In most conditions in which an altered secretion of PRL has been documented, an altered secretory activity of the TIDA neurons has been found. When an acute reduction in the secretion of DA is observed, an increased secretion of PRL is associated, with an inverse relationship between DA and PRL concentrations in hypophysial portal and systemic blood, respectively. However, the secretion of PRL can be regulated by PRL itself through stimulation of the secretory activity of the TIDA neurons, and consequently hyperprolactinemia can be observed concomitantly with a sustained high secretion of DA, as seen after treatment with estrogen. The short loop feedback of PRL secretion seems to be impaired in the aging rat, since a sustained reduced hypothalamic secretion of DA is observed in spite of long-term hyperprolactinemia.     

Decrease in serum prolactin during bromocriptine-induced pregnancy and subsequent restoration of reproductive function in some hyperprolactinemic women

Twenty-two women with hyperprolactinemia and amenorrhea were followed up for at least 18 months after 22 bromocriptine-induced full term pregnancies. Return of menstruation occurred in 9 of the 22 patients after pregnancy. Five of them conceived subsequently without any treatment. The prolactin (PRL) concentration before or during bromocriptine treatment was not significantly different between the patients who conceived spontaneously and those who did not. However, the PRL concentration after weaning was significantly lower in the patients who conceived spontaneously than in those who did not. Serum PRL levels were normal or only slightly elevated in the patients who conceived spontaneously. In 11 of the 22 patients, PRL levels were serially determined during bromocriptine-induced pregnancies. In 7 of the 11 patients, serum PRL levels fell during the second or third trimester. Spontaneous pregnancy occurred in 3 of the 7 patients who showed a decrease in PRL values during pregnancy, but in none of the 4 patients who did not. Furthermore, PRL levels after weaning correlated with PRL levels during the third trimester in these 11 women. Consequently, a decrease in PRL values during pregnancy may result in a fall in PRL levels after pregnancy, which in turn leads to the restoration of reproductive function in some subjects.     

Possible involvement of prolactin in endocrine-resistant metastatic prostate cancer

The hormone resistance of prostate cancer has been proved to depend at least in part on enhanced neuroendocrine activity and the resultant increase in blood concentrations of chromogranin A. Other experimental observations have suggested the involvement of prolactin (PRL), which appears to be a potential growth factor for prostate cancer. Abnormally high levels of PRL have been detected in metastatic prostate cancer, but the clinical significance of this finding has still to be clarified. In an attempt to explain the prognostic significance of serum PRL levels in prostate cancer, in this preliminary study we have analyzed the PRL levels in a group of metastatic prostate cancer patients with hormone-dependent or hormone-resistant cancer. The study included 50 patients with metastatic prostate cancer, 15 of whom had hormone-resistant tumors. The serum levels of PRL were measured by the RIA method. Abnormally high concentrations of PRL were found in 11/50 (22%) patients. Moreover, the percent of patients with cancer-related hyperprolactinemia was significantly higher in the hormone-resistant group than in the hormone-dependent group (8/15 vs 3/35, p < 0.01). This study confirms the possible existence of a hyperprolactinemic state in metastatic prostate cancer, as previously reported by other authors. Moreover, it appears to demonstrate that the occurrence of hyperprolactinemia is more frequent in hormone-resistant neoplasms, suggesting the possible involvement of PRL in hormone independence. Further studies concomitantly evaluating PRL and chromogranin A blood concentrations will be necessary to establish whether the hyperprolactinemia precedes and promotes the onset of hormone resistance in prostate cancer, or whether it is simply a consequence of the hormone independence.     

Relation between surgery-induced prolactin increase and the menstrual cycle phase at time of surgery in premenopausal breast cancer

It has been suggested that both the menstrual cycle phase and postoperative changes in prolactin (PRL) secretion at the time of surgery may influence the prognosis of breast cancer. The present study was carried out to evaluate the relation between menstrual cycle period and surgery-induced PRL variations. We evaluated 32 premenopausal women with operable breast carcinoma; 17 were in perimenstrual phase (days 1-6 and 21-28) and 15 were in the mid-cycle (days 7-20) period at the time of surgery. To investigate serum levels of PRL, venous blood samples were collected before and 7 days after surgery. Postoperative hyperprolactinemia occurred in 17/32 patients and it was statistically more frequent in patients surgically treated during the perimenstrual phase than in the mid-cycle phase (12/17 vs 5/15; p less than 0.05), while no other parameter (including axillary node and estrogen receptor status) showed a significant influence on hyperprolactinemia rate. The results suggest that in premenopausal breast cancer patients surgery-induced hyperprolactinemia may be influenced by the menstrual cycle phase at the time of surgery.     

Absence of suppressive effect of somatostatin on prolactin levels in patients with hyperprolactinemia

The effect of somatostatin (SRIF: 10 micrograms/min during 120 min) on serum prolactin (PRL) levels was studied in eleven patients with hyperprolactinemia of varying causes: 2 patients with acromegaly; 2 with primary hypothyroidism; 4 with prolactinoma and 3 with drug (sulpiride) induced hyperprolactinemia. During SRIF infusion, no significant change in PRL levels was observed in any of the 4 groups studied except in one female patient with a prolactinoma. The biological activity of SRIF was demonstrated by the significant inhibition (P less than 0.05) of insulin levels seen in all 11 patients (52% fall in relation to basal) without simultaneous modification of glycemia. These data suggest that SRIF does not decrease PRL secretion in most patients with hyperprolactinemia.     

Effects of treatment of normal and hyperprolactinemic rats with cyclosporine in vivo on the release of gonadotropins and prolactin from their pituitaries in vitro.

Cyclosporine (CyA) is extremely useful as an immunosuppressant and it is believed that at least some of its actions are due to antagonizing PRL effects. To determine whether the reported ability of CyA to inhibit gonadotropin release can be modified by PRL, we have examined the effects of treatment of normal and hyperprolactinemic rats with CyA in vivo on the release of LH, FSH and PRL from their pituitaries in vitro. Hyperprolactinemia was induced by implantation of capsules containing diethylstilbestrol (DES) and the animals were examined while the capsules were still in place (DES-IN) or after they had been removed (DES-OUT). Treatment with CyA significantly reduced plasma LH levels in control DES-IN rats without reducing basal LH release from the pituitaries of these animals in vitro. In the DES-IN rats, CyA exposure in vivo did not modify plasma PRL levels, but reduced PRL release in vitro, and interfered with the inhibitory action of dopamine (DA) on PRL release. The effect of DA on gonadotropin release in vitro was modified by CyA treatment. Administration of CyA failed to antagonize the suppressive effects of hyperprolactinemia on plasma LH and FSH levels or on the basal rates of gonadotropin release by incubated pituitaries. We conclude that CyA can reduce PRL release but does not interfere with the actions of PRL on anterior pituitary function.     

Catecholamines and pituitary function. III. Restoration of the prolactin response to thyrotropin-releasing hormone by low-dose dopamine infusion in women with pathological hyperprolactinemia

Previous studies in Rhesus monkeys have demonstrated that a dopamine (DA) infusion rate of 0.1 microgram/kg X min induces peripheral DA levels similar to those measured in hypophysial stalk blood and normalizes serum prolactin (PRL) levels in stalk-transected animals. We therefore examined the effect of such DA infusion rate on basal and thyrotropin-releasing hormone (TRH)-stimulated PRL secretion in both normal cycling women and women with pathological hyperprolactinemia. 0.1 microgram/kg X min DA infusion fully normalized PRL serum levels in 8 normal cycling women whose endogenous catecholamine synthesis had been inhibited by alpha-methyl-p-tyrosine (AMPT) pretreatment. Furthermore, DA significantly reduced, but did not abolish, the rise in serum PRL concentrations induced by both acute 500 mg AMPT administration and 200 micrograms intravenous TRH injection in normal women. A significant reduction in serum PRL levels in response to 0.1 microgram/kg X min DA, similar to that observed in normal cycling women when expressed as a percentage of baseline PRL, was documented in 13 amenorrheic patients with TRH-unresponsive pathological hyperprolactinemia. However, a marked rise was observed in the serum PRL of the same patients when TRH was administered during the course of a 0.1-microgram/kg X min DA infusion. The PRL response to TRH was significantly higher during DA than in basal conditions in hyperprolactinemic patients, irrespective of whether this was expressed as an absolute increase (delta PRL 94.4 +/- 14.2 vs. 17.8 +/- 14.1 ng/ml, p less than 0.002) or a percent increase (delta% PRL 155.4 +/- 18.9 vs. 17.9 +/- 7.1, p less than 0.0005), and there was a significant linear correlation between the PRL decrements induced by DA and the subsequent PRL responses to TRH. These data would seem to show that the 0.1-microgram/kg X min DA infusion rate reduces basal PRL secretion and blunts, but does not abolish, the PRL response to both TRH and acute AMPT administration. The strong reduction in PRL secretion and the restoration of the PRL response to TRH by 0.1 microgram/kg X min DA infusion in high majority of hyperprolactinemic patients, seem to indicate that both PRL hypersecretion and abnormal PRL response to TRH in women with pathological hyperprolactinemia are due to a relative DA deficiency at the DA receptor site of the pituitary lactotrophs.     

Effects of hyperprolactinemia on prolactin and LH pulsatile pattern in female rats.

Prolactin (PRL) and luteinizing hormone (LH) secretions are very closely-related. To further understand these mechanisms, the pulsatile secretion pattern of both hormones in experimentally-induced hyperprolactinemia has been studied in adult female rats. Hyperprolactinemia was induced by the transplanting of two pituitary glands. Nine days after the transplant operation, rats were bled (75 or 100 microliters/7 min for 3 h). Serum samples were analyzed for prolactin and LH values by RIA. Hyperprolactinemia modifies pulsatile PRL secretion by increasing the absolute amplitude and duration of the peaks together with a decrease in their frequency. Also, the mean values of the hormone during the whole studied period were increased. Hyperprolactinemia was followed by an increase in the mean values of LH and in the absolute amplitude of the peaks. All these results suggest that hyperprolactinemia induced by pituitary grafting in adult female rats, is followed by a significant change in prolactin and LH pulsatility, which may explain, to some extent, the effects of hyperprolactinemia on reproduction.     

Delayed puberty and hypoplastic uterus associated with hyperprolactinemia: successful treatment with bromocriptine

A 15-year-old girl referred because of primary amenorrhea was found to have a hypoplastic uterus and persistent hyperprolactinemia (72-110 ng/ml). The gonadotrophin-dependent pubertal signs, i.e. breast and vulvar development, were significantly retarded (Tanner stage 2-3) while sexual hair was well developed; bone age was 13 years. The endocrinological evaluation revealed gonadotrophin secretion (LH-basal: 0.85-1.25; peak after LH-RH: 10.4 mIU/ml; FSH-basal: 1.63-2.5; peak: 8.2 mIU/ml) and E2 levels (26-68 pg/ml) which were appropriate for Tanner stage 3. The high basal levels of PRL were nonresponsive to either stimulatory (TRH) or inhibitory (nomifensine) agents. CT scan of the brain suggested the presence of a pituitary microadenoma. Following therapy with bromocriptine (2.5 mg/day) plasma PRL levels dropped to normal (5-6.8 ng/ml) with an accompanying catch-up of pubertal development and linear growth and a marked increase in size of the uterus as documented by repeated ultrasonographic examinations. Menarche occurred 5 months after initiation of therapy, followed by regular menses thereafter. Repeated CT scan of the brain showed a decrease in the density and size of the still persisting lesion. This patient demonstrates that hyperprolactinemia can cause delayed puberty with a particular inhibitory effect on uterine growth and development.     

Endocrine,paracrine and autocrine actions of prolactin on immune cells

The immune response is regulated by locally released factors, collectively referred to as cytokines. Data on the human immune system have convincingly demonstrated that the hormone prolactin (PRL), in addition to exerting its endocrine control on the immune system, acts as a cytokine in that it is released within the immune system and regulates the lymphocyte response by paracrine and autocrine mechanisms. Both lymphocyte and pituitary PRLs are under the control of immune factors. Synthesis of human PRL by lymphocytes is induced by T-cell stimuli, while increased release of PRL by the pituitary, observed in vivo after immune challenge, may be mediated by cytokines produced by monocyte-macrophages. Since hyperprolactinemia and hypoprolactinemia are both immunosuppressive, physiological levels of circulating PRL must be necessary to maintain basal immunocompetence. The effects of Cyclosporin (CsA) on IL-2 and PRL gene activation and the analysis of the intracellular signaling events downstream IL-2 and PRL receptors suggest coordinate actions of these two cytokines during T cell activation.     

Increase of circulating levels of thymulin in hyperprolactinemia and acromegaly

The production of thymulin by the thymic epithelium is under complex control involving the endocrine system. Experimental models have suggested that prolactin (PRL) and growth hormone (GH) participate in this regulation but this has not been documented in humans. Using a bioassay we measured circulating thymulin levels in patients with hyperprolactinemia (n = 21), acromegaly (n = 15), or both (n = 6). Thymulin was elevated in these three groups of patients compared with normal subjects or with patients with pituitary disease but no excess in PRL or GH. Contrasting with observations in control groups, thymulin did not decrease as a function of age in patients. No correlation between thymulin and PRL or GH levels was observed while thymulin and insulin-like growth factor 1 levels were correlated. A new radioimmunoassay used in some patients for thymulin determination yielded similar results. Overall these data demonstrate that PRL and GH are involved in the hormonal control of thymulin production by the thymic epithelium in the human.