Pitfalls in pituitary diagnosis: peculiarities of three cases

Interference by macroprolactin in assays for prolactin: will the In Vitro Diagnostics Regulation lead to a solution at last?

Cross reactivity with high molecular weight complexes of prolactin known as macroprolactin is a common cause of positive interference in assays for serum prolactin. All prolactin assays currently available are affected with 5-25% of results indicating hyperprolactinaemia falsely elevated due to macroprolactinaemia - hyperprolactinaemia due to macroprolactin with normal concentrations of bioactive monomeric prolactin. Macroprolactinaemia has no pathological significance but, if it is not recognised as the cause, the apparent hyperprolactinaemia can lead to clinical confusion, unnecessary further investigations, inappropriate treatment and waste of healthcare resources. Macroprolactinaemia cannot be distinguished from true hyperprolactinaemia on clinical grounds alone but can be detected by a simple laboratory test based on the precipitation of macroprolactin with polyethylene glycol. Laboratory screening of all cases of hyperprolactinaemia to exclude macroprolactinaemia has been advised as best practice but has not been implemented universally and reports of clinical confusion caused by macroprolactinaemia continue to appear in the literature. Information provided by manufacturers to users of assays for prolactin regarding interference by macroprolactin is absent or inadequate and does not comply with the European Union Regulation covering in vitro diagnostic medical devices (IVDR). As the IVDR is implemented notified bodies should insist that manufacturers of assays for serum prolactin comply with the regulations by informing users that macroprolactin is a source of interference which may have untoward clinical consequences and by providing an estimate of the magnitude of the interference and a means of detecting macroprolactinaemia. Laboratories should institute a policy for excluding macroprolactinaemia in all cases of hyperprolactinaemia.

Challenges and pitfalls in the diagnosis of hyperprolactinemia

The definition of the etiology of hyperprolactinemia often represents a great challenge and an accurate diagnosis is paramount before treatment. Although prolactin levels > 200-250 ng/mL are highly suggestive of prolactinomas, they can occasionally be found in other conditions. Moreover, as much as 25% of patients with microprolactinomas may present prolactin levels < 100 ng/mL, which are found in most patients with pseudoprolactinomas, drug-induced hyperprolactinemia, or systemic diseases. On the other hand, some conditions may lead to falsely low PRL levels, particularly the so-called hook effect, that is an assay artifact caused by an extremely high level of PRL, and can be confirmed by repeating assay after a 1:100 serum sample dilution. The hook effect must be considered in all patients with large pituitary adenomas and PRL levels within the normal range or only modestly elevated (e.g., < 200 ng/mL). An overlooked hook effect may lead to incorrect diagnosis and unnecessary surgical intervention in patients with prolactinomas. Another important challenge is macroprolactinemia, a common finding that needs to be identified, as it usually requires no treatment. Although most macroprolactinemic patients are asymptomatic, many of them may present galactorrhea or menstrual disorders, as well as neuroradiological abnormalities, due to the concomitance of other diseases. Finally, physicians should be aware that pituitary incidentalomas are found in at least 10% of adult population.

Coexistence of macroprolactinaemia and hyperprolactinaemia in women with oligo-/amenorrhoea is associated with high risk of pituitary adenomas

Macroprolactin may cause elevation of prolactin (PRL) concentrations measured by standard assays. In our study, we assessed the prevalence of pituitary lesions in women with macroprolactinaemia and either oligomenorrhoea or secondary amenorrhoea. Pituitary MRI scans were performed in 61 women aged 31.0  ±  6.7 years (mean  ±  SD), with raised PRL concentrations due to macroprolactinaemia, detected by 25% polyethylene glycol (PEG) precipitation method (PRL recovery <40%). After PEG precipitation of macroprolactin, free PRL concentrations were still raised in 36 (59%) women. Microadenomas were detected in 10 patients out of 61 (16.4%), with raised free PRL in 9 of these cases, while macroadenomas were detected in 4 out of 61 (6.6%) women, all of whom also had raised free PRL. In case of coexistence of macroprolactinaemia and raised free PRL after PEG precipitation of macroprolactin, the chance of finding of either a micro- or a macroadenoma was 36% (13 cases out of 36). We conclude that hyperprolactinaemia and macroprolactinaemia may coexist in the same patient. Furthermore, if free PRL is raised after PEG precipitation of macroprolactin, then the chance of finding either a pituitary micro- or macroadenoma in women with oligo-/amenorrhoea is over 30%. Therefore pituitary magnetic resonance imaging is mandatory in all such cases.

Atypical thyrotropin-secreting pituitary microadenoma revealed by severe osteoporosis in a young man

For 10 years, a young man was followed for a severe osteoporosis associated with a considerable reduction in height and a massive weight loss. The constant increase of signs of tissue impregnation with thyroid hormones and the molar ratios of alpha-TSH suggested an inappropriate secretion of thyrotropin. Magnetic resonance imaging finally revealed a thyrotropic microadenoma of the pituitary gland. This case gives some new additional information on thyrotropin-induced osteoporosis. To our knowledge such a case has never been reported in the literature.

The impact on clinical practice of routine screening for macroprolactin

BACKGROUND

Macroprolactin has reduced bioactivity in vivo and accumulates in the sera of some subjects, resulting in pseudo-hyperprolactinemia and consequent misdiagnosis.

METHODS

We have audited our experience of routine screening for macroprolactin using polyethylene glycol (PEG) precipitation over a 5-yr period in a single center.

RESULTS

Application of a reference range for monomeric prolactin (the residual prolactin present in macroprolactin-depleted serum) for normal individuals revealed that 453 of 2089 hyperprolactinemic samples (22%) identified by Delfia immunoassay were explained entirely by macroprolactin. The percentage of hyperprolactinemic samples explained by macroprolactinemia was similar across all levels of total prolactin (18, 21, 19, and 17% of samples from 700-1000, 1000-2000, 2000-3000, and greater than 3000 mU/liter, respectively). Application of an absolute prolactin threshold after polyethylene glycol treatment of sera, rather than the traditional method, i.e. less than 40% recovery, minimizes the opportunity for misclassification of patients in whom macroprolactin accounted for more than 60% of prolactin and the residual bioactive prolactin was present in excess. Macroprolactinemic patients could not be differentiated from true hyperprolactinemic patients on the basis of clinical features alone. Although oligomenorrhea/amenorrhea and galactorrhea were more common in patients with true hyperprolactinemia (P < 0.05), they were also frequently present in macroprolactinemic patients. Plasma levels of estradiol and LH and the LH/FSH ratio were significantly greater in macroprolactinemic compared with true hyperprolactinemic subjects (P < 0.05). Reduced use of imaging and dopamine agonist treatment resulted in a net cost savings, offsetting the additional cost associated with the introduction of screening.

CONCLUSION

Routine screening of all hyperprolactinemic sera for macroprolactin is recommended.

Common disorders of the pituitary gland: hyposecretion versus hypersecretion

An understanding of pituitary disorders requires much more than a knowledge of the definitions associated with them. To grasp fully their complexity, one must recognize not only the powerful role the pituitary gland plays in the endocrine system, but also its effects on homeostasis throughout the entire human body, its association with the hypothalamus, and ultimately its associated end organs. Because of the many hormones stored or produced by the pituitary, associated disorders often are confusing and challenging. However, by simplifying the classification of these disorders into the categories of hyposecretion and hypersecretion, the understanding of pituitary disorders is made simple, and the associated interventions become apparent.