LH and FSH on TRT: What Your Lab Numbers Actually Mean

How TRT Shuts Down LH and FSH

Exogenous testosterone suppresses LH and FSH through negative feedback on the hypothalamic-pituitary-gonadal (HPG) axis. The mechanism is direct and predictable. Elevated circulating androgen (and its aromatized product, estradiol) signals the hypothalamus to reduce gonadotropin-releasing hormone (GnRH) pulse frequency, which in turn drops pituitary secretion of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) to levels at or below the assay detection limit, typically <0.5 mIU/mL on standard chemiluminescent platforms [1].

This is not a side effect. Suppression of gonadotropins is the expected pharmacological consequence of adding exogenous androgen. If LH and FSH remain elevated despite TRT, the differential includes non-compliance, poor absorption (for topical formulations), or a primary pituitary process that requires further investigation.

LH suppression precedes FSH suppression by a small margin. LH half-life is roughly 20-30 minutes; FSH half-life is 3-4 hours. Both reach nadir within 48-72 hours after the first therapeutic testosterone cypionate or enanthate injection [2]. Topical gels and creams suppress the axis more gradually because serum testosterone rises more slowly, but steady-state suppression is equally complete after 2-4 weeks of consistent application.

Clinically, knowing your LH and FSH are suppressed on TRT tells you three things. First, the exogenous testosterone is being absorbed. Second, endogenous (testicular) production has stopped. Third, spermatogenesis is compromised, because FSH is the primary driver of Sertoli cell function and sperm maturation.

Total Testosterone Range on TRT: Targets and Timing

A single testosterone number means little without knowing when the blood was drawn relative to the last dose. For men on weekly testosterone cypionate injections, the Endocrine Society's 2018 clinical practice guideline recommends measuring a trough level (drawn just before the next injection) and targeting 400-700 ng/dL, though individual symptom relief may occur at slightly different points within the 300-1 to 000 ng/dL reference range [3].

Trough matters. Peak serum testosterone after a 100 mg testosterone cypionate injection can exceed 1 to 200 ng/dL within 24-48 hours, then fall toward 400-500 ng/dL by day 7. Drawing blood at an unspecified time produces an uninterpretable number.

For men on daily subcutaneous testosterone cypionate (a protocol popularized partly to minimize peak-to-trough swings), or for those using transdermal gels applied every 24 hours, draw time still matters but the peak-to-trough delta is smaller, typically within 150-200 ng/dL. The Endocrine Society guideline states directly: "We suggest that testosterone therapy in men be monitored by measuring serum testosterone levels 3 to 6 months after treatment initiation" [3].

The T Trials (N=788 men, mean age 72, average baseline testosterone 232 ng/dL) demonstrated that raising testosterone to the mid-normal range (500-600 ng/dL) improved sexual function, walking distance, bone density, and mood versus placebo over 12 months [4]. The trials did not endorse supraphysiologic levels, and most guidelines cap the target at 1 to 000 ng/dL or less at any draw time.

Free Testosterone: Why the Calculation Matters

Only 2-3% of testosterone in men circulates as free (unbound) hormone [5]. The rest binds to sex hormone-binding globulin (SHBG, roughly 44%) and albumin (roughly 54%). SHBG-bound testosterone is biologically unavailable; albumin-bound testosterone is loosely held and partially available at the tissue level. "Free" plus "albumin-bound" is sometimes called bioavailable testosterone.

Direct free testosterone immunoassays (labeled "free T" on most commercial panels) are unreliable. The American Association of Clinical Endocrinology (AACE) and the Endocrine Society both note that analog immunoassay methods systematically underestimate free testosterone by up to 30-40% [3]. The preferred methods are equilibrium dialysis (gold standard, but slow and expensive) and the Vermeulen calculated free testosterone using measured total testosterone, SHBG, and albumin (set to 4.3 g/dL if not measured directly).

The Vermeulen equation is freely available at https://www.issam.ch/freetesto.htm. A typical target on TRT is calculated free testosterone of 15-25 pg/mL (150-250 pmol/L), though labs report in different units.

SHBG drives this calculation. High SHBG (common in older men, men with hyperthyroidism, or men taking certain anticonvulsants) means total testosterone may look adequate while free testosterone is low. Conversely, low SHBG (common in obesity, insulin resistance, hypothyroidism) means free testosterone may be adequate even when total testosterone is near the lower end of reference range. Always review SHBG alongside total testosterone; treating total T in isolation misses the clinical picture.

The HealthRX Free-T Interpretation Framework

| SHBG Level | Total T (trough) | Free T (Vermeulen) | Likely Clinical Status | |---|---|---|---| | Low (<20 nmol/L) | 400-500 ng/dL | Often adequate (>15 pg/mL) | Dose may be appropriate | | Normal (20-50 nmol/L) | 400-700 ng/dL | 15-25 pg/mL | Target range | | High (>50 nmol/L) | 400-700 ng/dL | Often low (<12 pg/mL) | May need dose increase or SHBG-lowering strategy |

This framework helps clinicians avoid both over-dosing men with low SHBG and under-treating men with high SHBG, based on total testosterone alone.

Estradiol on TRT: Why the Assay Type Changes Everything

Testosterone aromatizes to estradiol (E2), and E2 on TRT is a two-edged marker. Too low (below 20 pg/mL) causes joint pain, low libido, low bone density, and mood disturbance. Too high (above 40-50 pg/mL in most clinical guidelines) correlates with gynecomastia, water retention, and in some men, mood swings and erectile dysfunction [6].

The assay type is the most misunderstood point in TRT monitoring. Standard immunoassay estradiol tests (often labeled "Estradiol" or "E2" without further qualification) are calibrated for the high estradiol ranges seen in women. They perform poorly at the lower male range, producing results that can be off by 10-20 pg/mL. The correct test is the estradiol sensitive assay, which uses liquid chromatography-tandem mass spectrometry (LC-MS/MS). It may be labeled "Estradiol, Sensitive" or "Estradiol, LC-MS/MS" depending on the laboratory.

A 2013 paper in the Journal of Clinical Endocrinology and Metabolism demonstrated that the standard immunoassay overestimated estradiol by a mean of 12.6 pg/mL in men compared to LC-MS/MS [7]. Ordering the wrong assay could lead a clinician to prescribe anastrozole unnecessarily, driving E2 below 15 pg/mL and producing the very symptoms the patient came to fix.

Aromatase inhibitor (AI) use should not be routine. The Endocrine Society 2018 guideline states: "We suggest against the routine use of aromatase inhibitors during testosterone therapy" [3]. Some men with high aromatase activity (typically men with BMI above 30) may genuinely need AI support, but only when confirmed with a sensitive assay and accompanied by symptoms.

CBC on TRT: Erythrocytosis Is the Most Common Lab Abnormality

Testosterone stimulates erythropoietin production in the kidneys and directly acts on bone marrow erythroid progenitors, raising red blood cell mass. This effect is dose-dependent and appears within 3-6 months of starting therapy [8].

The complete blood count (CBC) with hematocrit is the most clinically important safety lab on TRT. Erythrocytosis (hematocrit above 50-52%) is the most common laboratory abnormality in TRT patients. Guidelines from the Endocrine Society set a clear threshold: hold or reduce the testosterone dose if hematocrit exceeds 54% [3]. Hematocrit above 54% raises blood viscosity and carries theoretical risk of thromboembolic events, though the absolute risk in otherwise healthy men is debated.

Factors that increase erythrocytosis risk on TRT include: injectable formulations (higher peak testosterone than topical), older age, baseline hematocrit above 48%, sleep apnea (which independently raises hematocrit), high doses, and infrequent injection schedules that produce large peak-to-trough swings.

Management options for elevated hematocrit include dose reduction, switching from weekly to twice-weekly injections (or daily subcutaneous), switching from injectable to topical formulation, therapeutic phlebotomy (donating blood), and treating underlying sleep apnea. The choice depends on the degree of elevation and the patient's cardiovascular risk profile.

CBC should be drawn at baseline, at 3 months, at 6 months, and annually once stable [3]. If hematocrit is trending upward at 6 months but is still below 54%, monitoring frequency should increase to every 3 months rather than waiting for the annual draw.

PSA, CMP, and the Rest of the TRT Panel

A complete TRT monitoring panel extends well beyond testosterone and gonadotropins. Below is each additional marker with the clinical rationale.

PSA (Prostate-Specific Antigen). Testosterone does not cause prostate cancer, but it may accelerate subclinical disease. The Endocrine Society recommends PSA at baseline and at 3-6 months after starting TRT, then per age-appropriate screening guidelines [3]. A rise of more than 1.4 ng/mL within the first 12 months, or a PSA above 4.0 ng/mL at any point, warrants urology referral before continuing therapy.

Comprehensive Metabolic Panel (CMP). Liver function tests (ALT, AST) are relevant primarily for men on oral or buccal testosterone formulations, which undergo hepatic first-pass metabolism. Injectable, transdermal, and subcutaneous testosterone bypasses the liver and carries minimal hepatotoxic risk. Kidney function (creatinine, BUN) matters because erythropoiesis-related polycythemia increases uric acid load, and some TRT patients are also using other compounds that are renally cleared.

SHBG. Already discussed under free testosterone. Draw SHBG at baseline and with each follow-up panel.

LH and FSH. Draw at baseline to document hypogonadism type (primary vs. secondary), then confirm suppression at the first follow-up (3 months). Continuing to draw LH and FSH quarterly on a stable TRT patient adds no management value. They will be suppressed. Redraw them only if compliance is questioned or if the patient is transitioning off TRT.

Prolactin. Measure at baseline, particularly in men with secondary hypogonadism. Elevated prolactin suppresses GnRH and may explain low LH and FSH before TRT is started. A prolactinoma is a treatable cause of hypogonadism that does not require TRT.

Thyroid-stimulating hormone (TSH). Hypothyroidism raises SHBG and can mimic many symptoms of low testosterone. Checking TSH at baseline prevents treating thyroid disease with testosterone.

LH, FSH, and Fertility on TRT: The Critical Conversation

Suppressed FSH means suppressed spermatogenesis. Men who want to father children during TRT need a different approach. A study published in the Journal of Urology found azoospermia in approximately 40% of men after 6 months of exogenous testosterone, with the remainder showing severe oligospermia [9].

Options for fertility preservation on TRT include:

Human chorionic gonadotropin (hCG). hCG is an LH analog that binds LH receptors on Leydig cells, maintaining intratesticular testosterone production and partial spermatogenesis even during exogenous testosterone use. Standard dosing ranges from 500-1 to 000 IU subcutaneously two to three times per week, co-administered with testosterone. A 2005 study by Coviello et al. (N=29) demonstrated that 500 IU hCG every other day maintained intratesticular testosterone concentrations during exogenous testosterone administration, compared to a 94% reduction in the control arm [10].

FSH (recombinant FSH or urofollitropin). For men with severe oligospermia or azoospermia on hCG alone, adding exogenous FSH can stimulate Sertoli cells directly. This combination is used in fertility-focused protocols and is guided by reproductive endocrinology.

Clomiphene citrate or enclomiphene. These selective estrogen receptor modulators (SERMs) block estrogen feedback at the hypothalamus, raising endogenous LH and FSH. They work as an alternative to TRT rather than alongside it, maintaining fertility but typically achieving lower testosterone levels than injectable TRT.

Cessation of TRT. LH and FSH recovery after stopping TRT is variable. A 2015 meta-analysis in the Journal of Sexual Medicine found median recovery time to baseline gonadotropin levels ranged from 3 to 24 months depending on duration and dose of prior TRT [11]. Spermatogenesis recovery lags gonadotropin recovery by an additional 1-3 months.

Any man under 45 who has not completed his family should discuss fertility implications with a reproductive urologist before starting TRT.

Interpreting Abnormal Results: A Practical Decision Tree

When a TRT lab panel comes back abnormal, the response depends entirely on which marker is out of range and by how much.

Total testosterone above 1 to 000 ng/dL at trough (for injectable protocols): Reduce weekly dose by 10-20 mg and recheck in 6 weeks. If the patient is on a daily topical, confirm application technique and site rotation.

Total testosterone below 300 ng/dL at trough: Confirm the draw was truly a trough (just before the next injection). If confirmed, increase weekly dose by 10-20 mg or shorten injection interval. Recheck in 6 weeks.

Free testosterone (Vermeulen) below 12 pg/mL despite adequate total T: Check SHBG. If SHBG is above 50 nmol/L, consider dose increase, more frequent smaller injections, or switch to daily subcutaneous protocol. Some protocols add anastrozole 0.25 mg twice weekly to reduce estradiol-driven SHBG production, but only after confirming elevated sensitive E2.

Sensitive estradiol above 50 pg/mL with symptoms: Lifestyle measures first (weight loss reduces aromatase activity). Anastrozole 0.25-0.5 mg twice weekly if BMI exceeds 30 and symptoms are prominent. Recheck E2 in 4-6 weeks.

Hematocrit 50-54%: Reduce dose or increase injection frequency to flatten peaks. Treat sleep apnea if present. Donate blood if eligible. Recheck CBC in 6-8 weeks.

Hematocrit above 54%: Hold testosterone. Therapeutic phlebotomy. Recheck CBC in 4 weeks before resuming at a lower dose or different formulation.

PSA rise above 1.4 ng/mL in 12 months: Hold TRT. Refer to urology before resuming.

Monitoring Schedule: When to Draw Which Labs

A standardized monitoring schedule reduces both under-detection of problems and unnecessary testing. The Endocrine Society 2018 guideline forms the foundation, with HealthRX clinical additions for SHBG and sensitive E2.

Baseline (before first dose): Total testosterone (two morning draws on separate days), LH, FSH, SHBG, sensitive estradiol, prolactin, TSH, CBC with hematocrit, CMP, PSA, and a digital rectal exam (DRE) in men over 40.

3 months after starting TRT: Total testosterone (trough for injectables), free testosterone (Vermeulen), sensitive estradiol, SHBG, CBC with hematocrit, PSA. LH and FSH once to confirm suppression. CMP if on oral/buccal formulation.

6 months: Total testosterone (trough), free testosterone, sensitive estradiol, SHBG, CBC with hematocrit, PSA, CMP.

Annually (once stable): Full panel as at 6 months, plus renewal of PSA per age-appropriate screening, TSH if symptoms of thyroid dysfunction arise, and prolactin only if hypogonadism returns despite adequate testosterone levels.

Men with hematocrit trending upward should be checked every 3 months regardless of overall stability. Men on hCG for fertility preservation should add semen analysis every 3-6 months.