When should serum testosterone levels be checked in an adult male receiving intramuscular testosterone replacement therapy for hypogonadism?

Timing of Testosterone Level Monitoring with IM Injections

For men receiving intramuscular testosterone cypionate or enanthate, serum testosterone levels should be measured midway between injections (days 5-7 after injection), targeting mid-normal values of 500-600 ng/dL. 1

Pharmacokinetic Rationale

The timing of testosterone measurement is critical because IM testosterone exhibits predictable pharmacokinetics:

• Peak serum levels occur 2-5 days after injection, often rising transiently above the upper limit of normal with standard dosing 2, 1
• Testosterone levels return to baseline by days 10-14 after injection 2, 1
• Measuring at the midpoint (days 5-7) captures the average testosterone exposure rather than the extremes 1

Critical Pitfalls to Avoid

Do not draw testosterone levels at the peak (days 2-5), as this will show supraphysiologic levels that do not reflect average exposure and may lead to inappropriate dose reduction 1. Conversely, do not draw levels at the trough (days 13-14), as testosterone may have returned to baseline or below, potentially leading to unnecessary dose escalation 1.

Initial Monitoring Timeline

First follow-up visit should occur at 1-2 months after initiating therapy to assess clinical response and consider dose escalation if symptoms persist with suboptimal levels 2, 1. This early assessment is crucial because nearly half of men on testosterone therapy never have their levels checked—a dangerous practice pattern 1.

After the initial assessment:

• Monitor every 3-6 months for the first year 2, 1
• Then yearly thereafter once stable levels are confirmed 2, 1

Dose Adjustment Algorithm

The decision to adjust dosing should be based on both testosterone levels and clinical response:

• If clinical response is adequate, no dose adjustment is needed even if levels are in the low-normal range 2, 1
• If clinical response is suboptimal AND testosterone levels are low-normal or below, increase the dose 2, 1
• If maximal recommended transdermal doses fail to achieve adequate levels, consider switching to IM injection therapy 2

Additional Monitoring Requirements

Beyond testosterone measurement, comprehensive monitoring at each visit must include:

• Hematocrit monitoring—withhold treatment if >54% and consider phlebotomy in high-risk cases 2, 1
• PSA levels in men over 40 years—refer for urologic evaluation if PSA increases >1.0 ng/mL in first 6 months or >0.4 ng/mL per year thereafter 2, 1
• Digital rectal examination to assess for prostate abnormalities 2, 1
• Assessment of symptomatic response, particularly sexual function and libido 2, 1

Special Considerations for Long-Acting Formulations

For testosterone undecanoate (long-acting injectable), the pharmacokinetics differ substantially:

• Peak levels occur at the end of the first week after injection 3
• Terminal elimination half-life is 18-23 days 3
• Steady-state is achieved after 2-3 injections 4
• Trough levels should be measured before the next scheduled injection (at 10-12 weeks) 5, 4

The standard regimen is 750 mg initially, repeat at 4 weeks, then every 10 weeks, though some patients may benefit from 8-week intervals 5.

Clinical Context

The primary indication for testosterone therapy is diminished libido and erectile dysfunction in men with confirmed biochemical hypogonadism 1, 6. Testosterone therapy produces small but significant improvements in sexual function (standardized mean difference 0.35) but little to no effect on physical functioning, energy, vitality, depressive symptoms, or cognition 1. This context is important when interpreting testosterone levels—if the patient reports adequate clinical response to supplementation, there is no need for dosage adjustment even if levels are in the low-normal range 2.