Why should I check prolactin and luteinizing hormone in a 32‑year‑old woman with undetectable total and free testosterone and otherwise normal labs?

Why Check Prolactin and LH in a 32-Year-Old Woman with Undetectable Testosterone

You must check prolactin and LH because undetectable testosterone with low or low-normal LH indicates central (hypothalamic-pituitary) hypogonadism, and elevated prolactin is the most common treatable cause that suppresses both LH secretion and testosterone production. 1, 2

Diagnostic Algorithm for Low Testosterone in Women

Step 1: Measure LH to Determine the Etiology

• LH measurement distinguishes between primary ovarian failure (high LH) and central hypogonadism (low or low-normal LH). 1, 2
• In your patient with undetectable testosterone, if LH is low or inappropriately normal (not elevated), this confirms the pituitary is not responding appropriately to the low sex hormone state. 2
• Low LH levels (<3 IU/L) combined with low testosterone indicate hypothalamic amenorrhea or hypogonadotropic hypogonadism. 2

Step 2: Measure Prolactin When LH is Low or Low-Normal

• Hyperprolactinemia is a common and treatable cause of suppressed LH/FSH secretion and subsequent testosterone deficiency. 1, 2
• Elevated prolactin suppresses GnRH pulsatility, which directly blocks LH secretion and prevents normal testosterone production. 2, 3
• Prolactin screening is mandatory because it can indicate pituitary tumors such as prolactinomas, which require specific treatment. 1, 2

Step 3: Interpret Results and Determine Next Steps

If prolactin is elevated:

• Repeat the prolactin measurement to confirm it's not spurious. 1, 4
• Persistently elevated prolactin warrants immediate endocrinology referral for evaluation of prolactinoma or other pituitary pathology. 1, 2
• Order pituitary MRI with sellar cuts to rule out pituitary adenomas or structural lesions. 1, 2

If prolactin is normal but LH remains low:

• This still indicates central hypogonadism from hypothalamic dysfunction. 2
• Consider functional causes: excessive exercise, eating disorders, stress, or significant weight loss. 2
• Endocrinology referral remains warranted for comprehensive evaluation. 2
• Check FSH alongside LH, as both should be inappropriately low relative to the hypoestrogenic state. 2

Clinical Significance in Your Patient

• At age 32, undetectable testosterone with low LH is NOT normal and requires investigation. 2
• This hormonal pattern indicates the pituitary-hypothalamic axis is failing to respond to low sex hormones. 2
• Untreated chronic hypogonadism leads to serious complications including accelerated bone loss, infertility, reduced energy, fatigue, depression, and reduced libido. 1, 2

Additional Workup to Consider

• Check estradiol levels to fully characterize the hypogonadal state. 1, 2
• Measure TSH and free T4 to exclude central hypothyroidism, which commonly coexists with central hypogonadism. 2
• Assess for menstrual irregularities (oligomenorrhea with cycles >35 days or amenorrhea with no bleeding >6 months). 2
• Bone mineral density testing should be performed if chronic hypogonadism is confirmed, as prolonged hypoestrogenism accelerates bone loss. 1, 2

Common Pitfalls to Avoid

• Do not assume low testosterone alone is diagnostic—interpretation must include LH, prolactin, and clinical context. 2
• Confirm with repeat morning measurements, as transient fluctuations can occur. 1, 2
• Do not delay prolactin testing—hyperprolactinemia is highly treatable and missing a prolactinoma has serious consequences. 1, 2
• Consider medication effects: GnRH analogs, corticosteroids, and certain antiepileptic drugs can suppress LH levels. 2