Blood Tests for Low Libido Evaluation
For patients presenting with low libido, the recommended blood tests include total testosterone, free testosterone, luteinizing hormone (LH), prolactin, and thyroid function tests. 1, 2
Primary Laboratory Workup
- Total testosterone: Morning measurement is essential as the primary hormonal assessment 2
- Free testosterone: Provides better clinical utility than total testosterone alone, as it represents the metabolically active fraction 1, 3
- Luteinizing hormone (LH): Helps distinguish between primary and secondary hypogonadism 1, 2
- Prolactin: Indicated for patients with low testosterone or loss of libido 1
- Thyroid function tests (TSH, free T4): To rule out thyroid disorders affecting libido 2
Additional Tests Based on Initial Findings
If initial testosterone levels are low:
- Follicle-stimulating hormone (FSH): To further characterize hypogonadism 2
- Estradiol: Target range 20-40 pg/mL; abnormal levels can affect libido 2
- Complete blood count: To assess for anemia or polycythemia 4
- Comprehensive metabolic panel: To evaluate liver and kidney function 1
Interpretation Guidelines
Testosterone Assessment
- Normal total testosterone ranges vary by age:
- 20-24 years: <409 ng/dL indicates low levels
- 25-29 years: <413 ng/dL indicates low levels
- 30-34 years: <359 ng/dL indicates low levels
- 35-39 years: <352 ng/dL indicates low levels
- 40-44 years: <350 ng/dL indicates low levels 5
Clinical Pearls
- Free testosterone measurement can prevent unnecessary endocrine evaluations in approximately 50% of patients with suspected hypogonadism 6
- Testosterone thresholds for symptoms vary significantly between individuals, with mean thresholds corresponding to the lower end of the normal range 7
- Morning testosterone measurement is critical as levels fluctuate throughout the day 2
- Both chronic illness and low testosterone can contribute to libido issues, so consider comorbidity burden when interpreting results 8
Important Considerations
- A normal free testosterone level may eliminate the need for further endocrine evaluation even when total testosterone appears low 1, 6
- Hypogonadism is found in approximately 15.6% of men presenting with sexual dysfunction 6
- Clinical symptoms alone (including decreased libido or testicular atrophy) cannot reliably predict hypogonadism, making laboratory testing essential 6
- Hyperprolactinemia occurs in approximately 1.8% of patients with sexual dysfunction 6
Follow-up Testing
- If initial tests suggest hypogonadism, consider MRI of the brain with pituitary cuts, especially with multiple endocrine abnormalities 1
- For patients with low testosterone and normal LH/FSH, consider evaluating for other causes including sleep apnea, obesity, and medication effects 2
- In women with low libido, consider testing DHEA levels in addition to the standard hormone panel 1
Remember that laboratory results should be interpreted in the context of clinical symptoms, as individual testosterone thresholds for libido issues can vary significantly.