HbA1c on TRT: How Testosterone Therapy Affects Blood Sugar and Lab Accuracy

TRT Can Lower HbA1c Through Two Separate Mechanisms

Testosterone can improve glycemic control through a real metabolic pathway, and it can also lower HbA1c through a lab artifact. Telling the two apart matters, because only one of them means glucose control actually got better.

The metabolic pathway works through androgen receptor activation in skeletal muscle, which increases GLUT4 transporter expression and glucose uptake [1]. The artifact pathway works through TRT-driven erythrocytosis, which shortens the average age of circulating red blood cells and reduces the time hemoglobin spends exposed to glucose before the cell is measured.

The T4DM trial randomized 1,007 men with impaired glucose tolerance or newly diagnosed type 2 diabetes to testosterone undecanoate or placebo, every 12 weeks for two years, with both groups also enrolled in a lifestyle program [2]. The testosterone group had fewer men progress to confirmed type 2 diabetes than the placebo group. The trial's exact between-group percentages and statistical values are in the published report and should be confirmed there before citing a specific figure; this draft could not independently verify the precise numbers against the source text, so they are intentionally omitted here. The trial also showed a somewhat greater reduction in HbA1c in the testosterone arm than placebo, on top of a shared lifestyle intervention, so the effect reflects testosterone added to diet and exercise, not testosterone alone.

Some observational research has reported associations between testosterone therapy and body composition changes in men with metabolic syndrome or type 2 diabetes, and other small trials have suggested reductions in fasting glucose and insulin resistance (HOMA-IR) with testosterone therapy, with the largest effect reported in men whose baseline testosterone was low. Men who started with normal testosterone saw little to no glycemic benefit. Anyone using this article to weigh a specific numeric benefit should read the primary paper rather than relying on a percentage or mg/dL figure quoted secondhand.

Why Erythrocytosis Can Make HbA1c Misleading

HbA1c measures glycated hemoglobin as a percentage of total hemoglobin, reflecting average blood glucose over roughly 90 to 120 days, the typical red blood cell lifespan. Anything that changes red cell turnover changes HbA1c somewhat independent of actual glucose levels.

TRT stimulates erythropoietin production through androgen receptor activation in the kidney [4]. The result is increased red blood cell production, higher hematocrit, and a younger average red cell population. Younger red cells have had less time to accumulate glycated hemoglobin, so a man whose true average glucose would otherwise produce a higher HbA1c can test lower simply because his red cells are being replaced faster.

The American Diabetes Association's 2024 Standards of Care notes that conditions which increase red blood cell turnover, including recent transfusion, erythropoietin therapy, or hemolysis, can lower HbA1c results without a true change in average glucose [5]. TRT-induced erythrocytosis works through the same underlying mechanism, faster red cell replacement, even though it is not one of the specific examples the ADA lists. A study examining anemia and HbA1c in a general adult population found that conditions affecting red cell turnover measurably shift HbA1c readings relative to other markers of average glucose [6]. That population was not TRT-specific, but it supports the general principle that HbA1c is not a stable marker when red cell kinetics change.

Decision framework: is your HbA1c on TRT trustworthy?

A few situations change how to apply this table:

• Starting point matters. An HbA1c drawn before TRT starts, or in the first few weeks, is a reasonable baseline because erythrocytosis has not developed yet. Compare later readings back to that baseline, not just to the general population reference range.
• A rising HbA1c on TRT is not explained away by this artifact. The lab-turnover effect only pushes HbA1c down, not up. A rising HbA1c on TRT should be worked up as you would off TRT.
• Anemia unrelated to TRT creates the same problem in reverse. Iron deficiency anemia slows red cell turnover and can falsely raise HbA1c. If you have anemia from another cause on top of TRT-related erythrocytosis, the two effects can partly cancel out, which is another reason to confirm with fasting glucose rather than reasoning from HbA1c and hematocrit alone.
• After a dose reduction or phlebotomy, hematocrit falls faster than the red cell population fully normalizes. Give it 8 to 12 weeks before trusting a new HbA1c as representative.

Total Testosterone: What Level to Target on TRT

The goal of TRT is to restore testosterone to mid-normal physiologic levels, not to maximize the number. The Endocrine Society and related reference-range work define the adult male total testosterone reference range as roughly 264 to 916 ng/dL, based on a harmonized analysis across several large cohort studies [7]. Most TRT protocols target trough levels between 450 and 700 ng/dL, measured at the low point of the dosing cycle, typically the morning before the next injection for weekly or biweekly protocols.

The 2018 Endocrine Society Clinical Practice Guideline, led by Dr. Shalender Bhasin, recommends aiming for testosterone in the mid-normal range for a healthy young man and adjusting the dose based on symptom improvement and side effects rather than chasing a specific number [8].

Blood draw timing affects the result considerably. For men on weekly intramuscular testosterone cypionate, peak levels occur 24 to 48 hours after injection and can run well above 1,000 ng/dL; a level drawn at that point overstates steady-state exposure. A trough drawn the morning of the next injection gives a more representative picture. For topical testosterone gels, levels are typically checked 2 to 4 hours after application [8].

Consistently supraphysiologic levels, generally described as total testosterone persistently above 1,000 ng/dL, raise the risk of erythrocytosis and acne without established additional metabolic benefit over mid-normal dosing. The TRAVERSE trial, the largest randomized TRT safety study to date at 5,246 participants, kept mean total testosterone around 530 ng/dL and found no increased cardiovascular risk versus placebo over roughly 33 months of follow-up [9].

Free Testosterone: Why the Calculated Value Matters

Total testosterone tells you how much testosterone is circulating. Free testosterone tells you how much is biologically active. Only about 2 to 3% of total testosterone circulates unbound; most of the rest is bound to sex hormone-binding globulin (SHBG, tightly bound) or albumin (loosely bound) [10]. Albumin-bound testosterone dissociates easily at tissue capillaries, so some clinicians use "bioavailable testosterone" (free plus albumin-bound) as an alternative metric.

SHBG varies with age, obesity, liver disease, thyroid function, and certain medications. A man with a total testosterone of 500 ng/dL and high SHBG can still have a free testosterone below the reference range, which helps explain why some men with a "normal" total testosterone still report fatigue, low libido, or signs of insulin resistance.

Direct measurement of free testosterone by equilibrium dialysis is the reference method, but it is expensive and not widely available. Most commercial labs instead run a direct analog immunoassay, which professional guidance describes as unreliable at the low concentrations typical of adult men [8], [12]. The Vermeulen equation, which calculates free testosterone from total testosterone, SHBG, and albumin, is the more commonly recommended alternative [11]. Several free online calculators implement this equation.

Tracking calculated free testosterone alongside total testosterone helps catch cases where a shift in SHBG, from weight loss, metformin use, or a thyroid change, alters the bioactive fraction even when total testosterone looks stable.

Estradiol Sensitive Assay: The Right Test for Men on TRT

Standard immunoassay estradiol tests were designed for the higher concentrations typical in women and are unreliable in men. An estradiol sensitive assay uses liquid chromatography-tandem mass spectrometry (LC-MS/MS), which is accurate at the lower concentrations typically seen in men [12].

Testosterone converts to estradiol through the aromatase enzyme, mostly in fat tissue, so estradiol often rises alongside testosterone on TRT. Moderate estradiol supports bone density, lipid metabolism, and sexual function [13]. Problems tend to show up at the extremes: high estradiol is linked to gynecomastia, water retention, and mood symptoms, while very low estradiol is linked to bone loss and joint pain in some studies.

A study using data from the Framingham Heart Study found that men with the lowest estradiol levels had higher rates of vertebral fracture, and that estradiol and testosterone appeared to have independent, additive effects on bone density [13]. This is part of why aggressively suppressing estradiol with an aromatase inhibitor is generally not recommended unless both symptoms and lab values support it.

When ordering labs on TRT, specify an estradiol sensitive or LC-MS/MS assay by name. A standard estradiol result in a man should be treated as unreliable, and decisions about starting or stopping an aromatase inhibitor should not be based on it.

CBC on TRT: Hematocrit Is the Safety Gate

A complete blood count is the most important safety lab on TRT. Testosterone stimulates red blood cell production in a dose-dependent way, and hematocrit above roughly 54% increases blood viscosity and thromboembolic risk [14]. The 2018 Endocrine Society guideline recommends checking hematocrit at baseline, at 3 to 6 months, and then annually, and holding testosterone therapy if hematocrit exceeds 54% until it comes back down to a safer level [8].

In the TRAVERSE trial, erythrocytosis (hematocrit above 54%) occurred in about 7.5% of men on testosterone gel versus 2.9% on placebo [9]. Risk factors for TRT-associated erythrocytosis include injectable formulations, which produce higher peak testosterone than gels, obstructive sleep apnea, chronic lung disease, and living at high altitude.

Not every hematocrit elevation requires stopping TRT outright. A common clinical approach, rather than a single formally quoted guideline threshold below 54%, is to scale the response to how high the number is:

• Hematocrit 50 to 52%: increase hydration, recheck in 4 to 6 weeks, consider switching from injection to a topical formulation.
• Hematocrit 52 to 54%: reduce the testosterone dose, consider more frequent smaller injections or a topical formulation, recheck in about 4 weeks.
• Hematocrit above 54%: hold TRT, consider therapeutic phlebotomy, and evaluate for other causes of elevated red cell count.

Beyond hematocrit, a CBC also provides hemoglobin, white blood cell count, and platelet count. TRT does not typically affect white cells or platelets, but a baseline CBC helps rule out unrelated hematologic issues before starting therapy.

Dr. Abraham Morgentaler, a urologist at Harvard Medical School who has written on testosterone and cardiovascular risk, has argued that the hematocrit effect is manageable with routine monitoring and should not, by itself, be a reason to withhold treatment from men who could benefit [15]. That is a clinical opinion from an author in the field, not a guideline requirement, and it does not override the hold-at-54% threshold above.

Building a Complete Lab Panel on TRT

A single lab value in isolation tells you very little. Glucose metabolism, testosterone levels, estrogen balance, and hematologic safety interact with each other, so monitoring them together gives a more complete picture than any one test alone.

A reasonable panel for men on TRT who want to track metabolic health includes: total testosterone (trough), calculated free testosterone (using SHBG and albumin), estradiol sensitive (LC-MS/MS), CBC with differential, a comprehensive metabolic panel, HbA1c, and fasting glucose. Men with known prediabetes or diabetes may benefit from adding fasting insulin and fructosamine, particularly if hematocrit is elevated and HbA1c reliability is in question.

Lab timing typically follows: baseline labs before starting TRT, a follow-up around 6 to 8 weeks to confirm dose adequacy and screen for early hematocrit rises, a second follow-up around 3 to 6 months to evaluate steady-state levels, and annual labs after that for stable patients [8].

The Endocrine Society guideline also supports baseline and periodic PSA testing, along with a digital rectal exam, for men over 40 starting TRT. TRAVERSE did not find an increased incidence of prostate cancer over its follow-up period [9], but PSA monitoring still helps catch pre-existing disease that could grow faster under androgen exposure.

Lipid panels are worth tracking too. TRAVERSE and other studies have found a modest reduction in HDL cholesterol on testosterone therapy [9]. Men already on statin therapy or with borderline cardiovascular risk may want lipids checked around 6 and 12 months after starting TRT.