Monitoring FSH and LH in Testosterone Replacement Therapy Patients
FSH and LH levels are significantly suppressed in patients on testosterone replacement therapy (TRT), with the degree of suppression varying by testosterone formulation and dosage. 1, 2
Physiological Effects of TRT on FSH and LH
- TRT causes negative feedback suppression of the hypothalamic-pituitary-gonadal axis, resulting in decreased FSH and LH production 3
- This suppression occurs in a dose-dependent manner, with higher testosterone doses causing greater suppression of gonadotropins 3
- The suppression of FSH and LH leads to decreased testicular function, reduced spermatogenesis, and potential testicular atrophy 1
Patterns of FSH and LH Suppression by TRT Formulation
- Long-acting injectable testosterone (enanthate or undecanoate) causes the most significant suppression, decreasing FSH by approximately 86% and LH by 72% 1
- Intermediate-acting daily gels/patches decrease FSH by about 60% and LH by 59% 1
- Short-acting intranasal testosterone causes the least suppression, decreasing FSH by 38% and LH by 47% 1
- Suppression begins approximately 5 days after administration of injectable testosterone enanthate 4
Monitoring Recommendations
- FSH and LH levels should be measured at baseline before initiating TRT to establish the etiology of testosterone deficiency 5
- Baseline measurements help determine if hypogonadism is primary (testicular) or secondary (pituitary/hypothalamic) 5
- After initiating TRT, testosterone levels should be measured at 2-3 months, with subsequent monitoring at 3-6 month intervals for the first year, then yearly thereafter 6
- While not explicitly required for routine monitoring once on established TRT, FSH and LH can be periodically assessed without HRT to evaluate underlying gonadal function 5
Clinical Implications of FSH/LH Suppression
- Fertility is significantly compromised during TRT due to gonadotropin suppression 5
- Approximately 73% of men on TRT will have at least one LH measurement below 1 IU/ml during treatment, though only 22% maintain this level of suppression throughout treatment 2
- Testicular size and consistency often diminish during TRT due to gonadotropin suppression 5
- Men interested in fertility should have a reproductive health evaluation performed prior to TRT initiation 5
Special Considerations
- Patients with low testosterone and low/normal LH levels may be candidates for selective estrogen receptor modulator therapy instead of TRT, particularly if fertility preservation is desired 5
- For men receiving injectable testosterone, clinicians must interpret blood test results based on the interval since the most recent injection, as peak levels occur 2-5 days post-injection 5, 6
- Exogenous testosterone cannot replace the role of LH in maintaining intratesticular testosterone levels necessary for spermatogenesis 7
Practical Monitoring Protocol
- Measure baseline FSH, LH, and testosterone levels before initiating TRT 5
- For injectable testosterone, measure levels midway between injections (typically day 5-7 for weekly injections) 6
- For transdermal formulations, measure levels 2-4 hours after application 6
- Monitor for signs of testicular atrophy during follow-up visits 5
- Consider periodic "drug holidays" to assess recovery of the hypothalamic-pituitary-gonadal axis if fertility is a future concern 1
TRT monitoring should focus primarily on achieving appropriate testosterone levels and monitoring for adverse effects, with the understanding that FSH and LH suppression is an expected physiological response that varies by formulation and dosage.