What labs should be ordered for a patient with a draining breast and concerns about prolactin levels?

Laboratory Evaluation for Draining Breast and Prolactin Concerns

For patients with a draining breast and concerns about prolactin levels, serum prolactin measurement should be the primary initial laboratory test, with additional testing guided by the prolactin result and clinical presentation. 1, 2

Initial Laboratory Evaluation

• Measure serum prolactin in a single blood sample collected at any time of day as the first-line test 1, 2
• Consider serial measurements (2-3 samples at 20-60 minute intervals using an indwelling cannula) if initial prolactin is modestly elevated to distinguish stress-related elevation from true hyperprolactinemia 1, 2
• Ensure proper sample collection - fasting morning samples are preferred to minimize pulsatile variations 2
• Assess for macroprolactin if prolactin is mildly or incidentally elevated, especially if the patient is asymptomatic 1, 2

Additional Laboratory Testing Based on Prolactin Results

If Prolactin is Elevated:

• Measure serum luteinizing hormone (LH) levels to help establish the etiology of hyperprolactinemia 1, 2
• Screen for secondary causes with the following tests:
  • Thyroid function tests (TSH, free T4) to rule out hypothyroidism 1, 2
  • Renal function tests (BUN, creatinine) as renal insufficiency can cause hyperprolactinemia 1, 2
  • Liver function tests to exclude hepatic impairment 1, 2
  • Pregnancy test in women of childbearing age 1, 2
• Consider serum dilutions for prolactin measurement if a large pituitary lesion is suspected but prolactin levels are only modestly elevated (to rule out the "hook effect") 1, 2
• Measure IGF-1 to rule out mixed prolactin and growth hormone hypersecretion 1, 2

For Men:

• Measure serum estradiol if breast symptoms or gynecomastia are present 1, 2
• Measure total testosterone levels - men with total testosterone <150 ng/dL combined with low/low-normal LH should undergo further evaluation regardless of prolactin level 1, 2

Imaging Studies

• Pituitary MRI should be obtained when prolactin levels are significantly elevated or when clinical suspicion for a prolactinoma exists 2
• Men with total testosterone <150 ng/dL and low/low-normal LH should undergo pituitary MRI regardless of prolactin levels 1

Clinical Pitfalls to Avoid

• Do not overlook medication effects - review all medications as many can cause hyperprolactinemia, particularly antipsychotics and other dopamine antagonists 2, 3
• Be aware of the "hook effect" - falsely low prolactin levels in large tumors due to assay saturation 1, 2
• Do not miss macroprolactinemia - present in 10-40% of patients with hyperprolactinemia and may not require treatment 1, 2
• Do not overlook primary hypothyroidism as a cause of hyperprolactinemia and pituitary enlargement that can mimic a prolactinoma 4

Referral Considerations

• Refer patients with persistently elevated prolactin levels of unknown etiology to an endocrinologist 1, 2
• Consider referral if prolactin levels remain elevated after excluding common causes 2
• Men with total testosterone <150 ng/dL and low/low-normal LH should be referred regardless of prolactin level 1

Treatment Implications

• If a prolactinoma is confirmed, dopamine agonists (cabergoline preferred over bromocriptine) are the first-line treatment 2, 5, 6
• Initial cabergoline dosing is 0.25 mg twice weekly, with gradual increases based on prolactin response 5
• Monitor prolactin levels to assess treatment response 2, 5