Why am I still having low serum testosterone levels despite being on testosterone replacement therapy?

Why Testosterone Levels Remain Low Despite Replacement Therapy

If your serum testosterone remains low despite being on testosterone replacement therapy, the most likely causes are inadequate dosing, poor timing of blood draws, incorrect formulation choice, non-adherence, or an undiagnosed underlying condition affecting testosterone metabolism or absorption. 1

Diagnostic Approach to Persistently Low Levels

1. Verify Proper Testing Methodology

Timing is critical. Serum testosterone must be measured in the morning (before 10 AM) after an overnight fast, as testosterone exhibits diurnal variation with peak values in the morning 1. Glucose and food intake suppress testosterone concentrations 1. If you're testing in the afternoon or after eating, levels will appear falsely low even with adequate replacement.

For injectable testosterone: Peak serum levels occur 2-5 days after injection, with return to baseline by 10-14 days 2. If you're testing at the trough (just before your next injection), levels will naturally be low—this creates the "roller coaster" effect. Testing should occur mid-cycle (around day 3-5 post-injection) to assess adequacy.

2. Assess Formulation and Dosing

Different testosterone formulations have vastly different pharmacokinetics:

• Intramuscular injections produce the highest peak levels but significant fluctuations 2
• Transdermal preparations provide more stable levels but may have absorption issues
• Testosterone cypionate cannot be used interchangeably with testosterone propionate due to differences in duration of action 3

If your dose is inadequate, it simply needs to be increased. The goal is to achieve physiologic testosterone levels (typically >300-350 ng/dL, though CDC-harmonized standards suggest >264 ng/dL) 1, 4.

3. Rule Out Conditions Affecting SHBG and Metabolism

Certain conditions alter sex hormone-binding globulin (SHBG) levels, which affects total testosterone measurements 1:

• Obesity (lowers SHBG, may show low total testosterone but normal free testosterone)
• Diabetes and metabolic syndrome (associated with lower testosterone levels) 5
• Thyroid dysfunction (affects testosterone metabolism) 6
• Liver disease (affects SHBG production)
• Medications: Opioids and high-dose glucocorticoids suppress testosterone production 1

In these cases, measure free testosterone by equilibrium dialysis or calculate it from total testosterone, SHBG, and albumin 1. You may have adequate free (bioavailable) testosterone despite low total testosterone.

4. Evaluate for Secondary Causes

Check pituitary function if not already done 6:

• Measure LH, FSH, and prolactin to distinguish primary from secondary hypogonadism
• Elevated prolactin can suppress testosterone production
• Pituitary tumors or other pituitary hormone deficiencies may require specific treatment
• Correcting other pituitary axes may reduce or eliminate the need for testosterone replacement 6

5. Consider Adherence and Absorption Issues

For topical formulations: Poor skin absorption, application technique errors, or washing off the medication too soon can result in inadequate absorption. Transdermal administration has lower rates of achieving target levels compared to injections.

For all formulations: Non-adherence is common. Confirm you're actually taking the medication as prescribed.

Common Pitfalls to Avoid

• Testing at the wrong time of day or in relation to injection timing is the most common reason for apparent treatment failure
• Failing to confirm low testosterone with repeat morning measurements before adjusting therapy—30% of men with initially low levels have normal levels on repeat testing 1
• Not measuring free testosterone in obese or diabetic patients where SHBG alterations are common
• Overlooking medication interactions, particularly opioids which are notorious for suppressing testosterone 1
• Assuming the formulation is working without verifying absorption—some patients simply don't absorb topical preparations well

Management Algorithm

1. Confirm timing: Repeat testosterone measurement in early morning (before 10 AM), fasting, at appropriate time relative to injection schedule
2. If still low: Check free testosterone, SHBG, albumin, LH, FSH, prolactin, and thyroid function 1, 6
3. If dosing inadequate: Increase dose or switch formulation (injections generally achieve higher levels than transdermal) 2
4. If absorption suspected: Switch from topical to injectable formulation
5. If secondary cause identified: Treat underlying condition (thyroid, prolactinoma, discontinue offending medications)
6. If metabolic syndrome present: Address obesity, diabetes, and sleep apnea—these conditions bidirectionally affect testosterone levels 5

The target is achieving physiologic testosterone levels with symptom improvement, not just treating a number. However, persistently low levels despite adequate dosing warrant investigation for the causes outlined above 1, 7.