Laboratory Workup for Low Free Testosterone
Measure serum luteinizing hormone (LH) and prolactin levels immediately—these are the two most critical tests to determine whether the testosterone deficiency is primary (testicular) or secondary (pituitary/hypothalamic) and to identify treatable causes. 1
Essential Initial Laboratory Tests
Mandatory Tests for All Patients
LH measurement is the single most important test to guide further workup, as it distinguishes between primary hypogonadism (elevated LH indicating testicular failure) and secondary hypogonadism (low or low-normal LH indicating pituitary/hypothalamic dysfunction). 1
Serum prolactin must be measured in all patients with low testosterone and low or low-normal LH levels, as hyperprolactinemia is a common and treatable cause of secondary hypogonadism that suppresses LH/FSH secretion. 1, 2
Confirm the testosterone deficiency with repeat morning measurements (two separate fasting samples obtained between 7 AM and 12 PM) using an accurate method such as liquid chromatography-tandem mass spectrometry in a CDC-certified laboratory. 3, 1
Additional Essential Tests
Measure follicle-stimulating hormone (FSH) in men interested in fertility to assess spermatogenesis status—elevated FSH with low testosterone indicates impaired sperm production and warrants semen analysis. 1
Check hemoglobin/hematocrit before initiating any testosterone therapy, as this is required for safety monitoring. 1
Assess thyroid function (TSH and free T4) to exclude central hypothyroidism, which commonly coexists with central hypogonadism and can present with similar symptoms. 2
Conditional Testing Based on Initial Results
If LH is Low or Low-Normal
Obtain pituitary MRI with sellar cuts if total testosterone is <150 ng/dL combined with low or low-normal LH, regardless of prolactin levels, to rule out pituitary adenomas or structural lesions. 1, 2
Order pituitary MRI if prolactin is elevated or if multiple pituitary hormone deficiencies are suspected (check for symptoms of other pituitary dysfunction). 1, 2
Cardiovascular Risk Assessment
- Screen for atherosclerotic cardiovascular disease risk factors including lipid panel, blood pressure, hemoglobin A1c (or fasting glucose), as these must be assessed before initiating testosterone therapy. 1
Critical Pitfalls to Avoid
Never skip LH measurement—ordering pituitary imaging without first checking LH and prolactin is inappropriate unless testosterone is <150 ng/dL with documented low/low-normal LH. 1
Do not rely on total testosterone alone when levels are between 280-400 ng/dL, as total testosterone in this range has poor sensitivity (only 91% at <280 ng/dL) and specificity (only 73.7%) for predicting low free testosterone—you need the actual free testosterone measurement by equilibrium dialysis in these borderline cases. 4
Avoid testing at incorrect times—only 9% of practitioners obtain morning samples despite guidelines requiring 7 AM to 12 PM collection, which leads to falsely low results and overdiagnosis. 5
Do not routinely measure estradiol—this is only indicated for patients with breast symptoms (gynecomastia) before treatment, not as part of the initial workup. 1
Recognize that screening questionnaires cannot substitute for laboratory testing, as they have variable specificity and sensitivity and should not guide clinical decisions. 1
Special Considerations
In obese patients or those on chronic opioids, recognize that functional hypogonadism from increased aromatization (obesity) or opioid-induced suppression may be reversible with weight loss or medication adjustment before committing to lifelong testosterone therapy. 6
For men wishing to preserve fertility, consider selective estrogen receptor modulators (SERMs) like clomiphene citrate rather than testosterone replacement, as exogenous testosterone suppresses spermatogenesis. 6