What is a normal prolactin level in a 66-year-old male with low free testosterone levels and inappropriately low Luteinizing Hormone (LH) levels?

Normal Prolactin Levels in Males with Secondary Hypogonadism

In a 66-year-old male with low free testosterone and inappropriately low LH levels, prolactin should be measured to screen for hyperprolactinemia, with normal levels typically below 15 ng/mL in men. 1

Understanding the Clinical Context

This patient's presentation—low free testosterone combined with inappropriately low (or low-normal) LH—defines secondary (hypogonadotropic) hypogonadism, indicating a hypothalamic-pituitary axis problem rather than testicular failure. 2 In this setting, measuring prolactin is mandatory because:

• Hyperprolactinemia is a reversible cause of secondary hypogonadism that must be excluded before initiating testosterone therapy. 1
• Elevated prolactin suppresses gonadotropin-releasing hormone (GnRH) secretion, which in turn suppresses LH and FSH, leading to low testosterone. 1
• Prolactinomas and other pituitary lesions can present with this exact hormonal pattern. 1

Defining Normal vs. Elevated Prolactin

Normal Prolactin Range

• Normal prolactin levels in men are generally <15 ng/mL, though reference ranges vary slightly by laboratory. 3
• Basal prolactin levels of at least 5 ng/mL for males are considered sufficient to exclude hypoprolactinemia. 3

Mild Hyperprolactinemia (15.1-50 ng/mL)

• This range warrants further investigation, particularly when combined with low testosterone and low LH. 4
• Prolactin levels >25 ng/mL in the setting of low testosterone significantly increase the likelihood of pituitary abnormalities. 4
• The prolactin-to-testosterone ratio is the best independent predictor of finding a pituitary abnormality on MRI, with a cutoff of 0.10 achieving 90% sensitivity. 4

Marked Hyperprolactinemia (>50 ng/mL)

• Prolactin levels more than twice the upper limit of normal (typically >30-40 ng/mL) strongly suggest pituitary adenoma, with 75% of microadenomas presenting with this degree of elevation. 5
• Prolactinomas characteristically present with prolactin levels >100 times above normal in men with low testosterone. 6

Diagnostic Algorithm for This Patient

Step 1: Measure Prolactin

• Obtain serum prolactin level as part of the initial workup for secondary hypogonadism. 1
• If prolactin is elevated, repeat the measurement to ensure the elevation is not spurious (stress, recent sexual activity, or macroprolactin). 1

Step 2: Interpret Results Based on Prolactin Level

If Prolactin is Normal (<15 ng/mL):

• Proceed with evaluation for other causes of secondary hypogonadism (obesity, medications, sleep apnea, chronic illness). 2
• Consider pituitary MRI if testosterone is <150 ng/dL with low/normal LH, as non-secreting adenomas may be present even with normal prolactin. 1

If Prolactin is Mildly Elevated (15.1-50 ng/mL):

• Calculate the prolactin-to-testosterone ratio. 4
• If the ratio is ≥0.10 OR prolactin is ≥25 ng/mL, order pituitary MRI to evaluate for microadenoma. 4
• This approach achieves 90% sensitivity for detecting pituitary abnormalities while reducing unnecessary imaging by 28%. 4

If Prolactin is Markedly Elevated (>50 ng/mL or >2× upper limit of normal):

• Refer to endocrinology immediately and order pituitary MRI. 1
• The likelihood of identifying pituitary adenoma is high with this degree of elevation. 5
• Initiate cabergoline therapy if prolactinoma is confirmed, which will normalize prolactin, restore testosterone levels, and improve hypogonadal symptoms in 83% of men. 6

Critical Clinical Pearls

Normal Testosterone Does Not Exclude Prolactinoma

• Approximately 11-20% of men with prolactinomas present with testosterone levels within the normal range (≥2.6 ng/mL). 6
• These men still experience hypogonadal symptoms in 55% of cases despite "normal" testosterone. 6
• With cabergoline treatment, testosterone levels increase further within the normal range (from 3.91 to 6.42 ng/mL on average), with clinical improvement in 83% of symptomatic men. 6

When to Order Pituitary MRI

The decision to obtain MRI should be based on:

• Prolactin >2× upper limit of normal (typically >30-40 ng/mL): High likelihood of adenoma. 5
• Prolactin-to-testosterone ratio ≥0.10: 90% sensitivity for pituitary abnormalities. 4
• Testosterone <150 ng/dL with low/normal LH: Consider MRI regardless of prolactin level, as non-secreting adenomas may be present. 1
• Markedly low testosterone (<200 ng/dL): Only 1 in 17 men will have an adenoma, so MRI should be based on individual circumstances. 5

Common Pitfalls to Avoid

• Do not assume normal prolactin excludes pituitary pathology if testosterone is <150 ng/dL with low LH—non-secreting adenomas can present this way. 1
• Do not start testosterone therapy without measuring prolactin in men with secondary hypogonadism, as hyperprolactinemia is a reversible cause that requires different treatment. 1
• Do not order pituitary MRI reflexively in all men with low testosterone and low LH—only 17.6% will have abnormalities, and most will be incidental findings like partially empty sella. 5

Treatment Implications

If Prolactin is Normal:

• Proceed with testosterone replacement therapy if hypogonadism is confirmed and the patient has appropriate symptoms (diminished libido, erectile dysfunction). 2
• Testosterone therapy is absolutely contraindicated if the patient desires fertility—use gonadotropin therapy (hCG plus FSH) instead. 2

If Hyperprolactinemia is Confirmed:

• First-line treatment is cabergoline, not testosterone therapy. 6
• Cabergoline normalizes prolactin, restores testosterone production, and improves hypogonadal symptoms in the majority of men. 6
• Testosterone levels increase by an average of 2.34-3.19 ng/mL with cabergoline treatment. 6