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

TRT Lowers HbA1c Through Two Separate Mechanisms

Testosterone improves glycemic control by a real metabolic pathway and a lab-based artifact, and distinguishing between the two matters for clinical decision-making. The metabolic pathway works through androgen receptor activation in skeletal muscle, which increases GLUT4 transporter expression and glucose uptake independent of insulin signaling [1]. The artifact pathway involves TRT-driven erythrocytosis shortening the average age of circulating red blood cells, which reduces the time hemoglobin is exposed to glucose.

The landmark T4DM trial (N=1,007) randomized men with impaired glucose tolerance or newly diagnosed type 2 diabetes to testosterone undecanoate 1,000 mg or placebo every 12 weeks for two years. The testosterone group showed a 40% relative risk reduction in type 2 diabetes diagnosis (oral glucose tolerance test reversion to normal: 52% vs. 43%, P=0.007) [2]. Mean HbA1c fell by 0.32 percentage points more in the testosterone arm than placebo. These men also received a lifestyle intervention, so the effect reflects testosterone added to diet and exercise, not testosterone alone.

A 2016 meta-analysis of 29 randomized controlled trials (N=1,737) in Endocrine Reviews found that testosterone therapy reduced fasting glucose by 8.1 mg/dL (95% CI: 3.4 to 12.8) and HOMA-IR by 1.53 in men with metabolic syndrome or type 2 diabetes [3]. The effect was largest in men with baseline testosterone below 300 ng/dL. Men with normal testosterone saw minimal glycemic benefit.

Why Erythrocytosis Makes HbA1c Unreliable

HbA1c measures glycated hemoglobin as a percentage of total hemoglobin, reflecting average blood glucose over 90 to 120 days (the typical red blood cell lifespan). Anything that changes red cell turnover changes HbA1c independent of actual glucose levels. This is not a minor technicality. It alters treatment decisions.

TRT stimulates erythropoietin production through direct androgen receptor activation in renal peritubular cells [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. A man whose true average glucose would produce an HbA1c of 6.2% might test at 5.7% simply because his red cells are being replaced faster.

The American Diabetes Association notes in its 2024 Standards of Care that "conditions that increase red blood cell turnover, such as recent transfusion, erythropoietin therapy, or hemolysis, can falsely lower HbA1c results" [5]. TRT-induced erythrocytosis falls squarely in this category. A 2019 study in Clinical Chemistry quantified the effect: each 5-point hematocrit rise above 50% correlated with a 0.15% HbA1c underestimation when compared against fructosamine-derived average glucose [6].

For men on TRT with hematocrit above 50%, cross-referencing HbA1c with fasting glucose, fructosamine, or continuous glucose monitoring (CGM) data provides a more accurate glycemic picture. Relying on HbA1c alone could mask prediabetes or undertreated type 2 diabetes.

Total Testosterone Range: What Labs to Target on TRT

The goal of TRT is to restore testosterone to mid-normal physiologic levels, not to maximize the number. The AUA and Endocrine Society define the normal adult male total testosterone range as 264 to 916 ng/dL based on the Harmonized Reference Range established across four large cohorts [7]. Most TRT protocols target trough levels between 450 and 700 ng/dL, measured at the nadir of the dosing cycle (typically the morning before the next injection for weekly or biweekly protocols).

Dr. Shalender Bhasin, the lead investigator on the Testosterone Trials (TTrials), stated in the 2018 Endocrine Society Clinical Practice Guideline: "We recommend aiming for testosterone levels in the mid-normal range for healthy young men, and adjusting the dose to achieve symptom improvement while minimizing side effects" [8].

The timing of the blood draw matters considerably. For men on weekly intramuscular testosterone cypionate, peak levels occur 24 to 48 hours post-injection and can reach 1,200 ng/dL or higher. Trough levels drawn on the morning of the next injection better reflect steady-state exposure. For topical testosterone gels, levels should be drawn 2 to 4 hours after application [8].

Supraphysiologic levels (total testosterone consistently above 1,000 ng/dL) increase the risk of erythrocytosis, acne, and prostate-related complications without proven additional metabolic benefit. The TRAVERSE trial (N=5,246), which is the largest randomized TRT safety study to date, maintained participants at a mean total testosterone of 530 ng/dL and found no increased cardiovascular risk versus placebo over 33 months of follow-up [9].

Free Testosterone Calculation: Why It Matters for Metabolic Assessment

Total testosterone tells you how much testosterone is circulating. Free testosterone tells you how much is biologically active. Only 2 to 3% of total testosterone circulates unbound; the rest is attached to sex hormone-binding globulin (SHBG, tightly bound, ~44%) or albumin (loosely bound, ~54%) [10]. Albumin-bound testosterone dissociates easily at tissue capillaries, so "bioavailable testosterone" (free plus albumin-bound) is sometimes used as an alternative metric.

SHBG levels vary with age, obesity, liver disease, thyroid function, and medications. A man with a total testosterone of 500 ng/dL and high SHBG might have a free testosterone below 5 ng/dL, well under the reference range of 5 to 21 ng/dL. This explains why some men with "normal" total testosterone still experience fatigue, low libido, and insulin resistance.

Measuring free testosterone directly by equilibrium dialysis is the gold standard, but it is expensive and not widely available. Most labs use a direct analog immunoassay for free testosterone, which the Endocrine Society has described as "unreliable at low concentrations and should not be used" [8]. The preferred alternative is the Vermeulen calculated free testosterone, which uses total testosterone, SHBG, and albumin concentrations in a validated equation [11]. Several online calculators (ISSAM, FT calculator based on the Vermeulen equation) are freely available.

For men on TRT, monitoring calculated free testosterone alongside total testosterone helps identify cases where SHBG shifts (from weight loss, metformin use, or thyroid changes) alter the bioactive fraction despite stable total levels.

Estradiol Sensitive Assay: The Right Test for Men on TRT

Standard immunoassay-based estradiol tests were designed for women and produce unreliable results in men. The estradiol sensitive assay uses liquid chromatography-tandem mass spectrometry (LC-MS/MS), which accurately quantifies estradiol at the lower concentrations found in males (typically 20 to 35 pg/mL) [12].

Testosterone converts to estradiol via the aromatase enzyme, primarily in adipose tissue. TRT increases substrate availability for aromatase, so estradiol levels often rise in parallel with testosterone. Moderate estradiol is beneficial: it supports bone mineral density, lipid metabolism, and sexual function [13]. The problem arises at extremes. Estradiol above 50 pg/mL on the sensitive assay can cause gynecomastia, water retention, and mood changes. Estradiol below 15 pg/mL has been associated with bone loss and joint pain.

A 2014 study in the Journal of Clinical Endocrinology & Metabolism (N=3,014 men from the Framingham Heart Study) found that men in the lowest estradiol quintile had significantly higher rates of vertebral fractures and that estradiol and testosterone had independent and additive effects on bone density [13]. This is why suppressing estradiol aggressively with aromatase inhibitors on TRT is not recommended unless symptoms and lab values both support intervention.

When ordering labs on TRT, specify "estradiol, sensitive, LC-MS/MS." If a standard estradiol result comes back at 42 pg/mL, it could be anywhere from 25 to 60 pg/mL in reality. Clinical decisions about aromatase inhibitor use should never be based on the standard assay.

CBC on TRT: Hematocrit Is the Safety Gate

A complete blood count (CBC) is the single most important safety lab on TRT. Testosterone stimulates erythropoiesis dose-dependently, and hematocrit above 54% increases blood viscosity and the risk of thromboembolic events [14]. The 2018 Endocrine Society guideline recommends: "Measure hematocrit at baseline, at 3 to 6 months, and then annually. If hematocrit exceeds 54%, stop testosterone therapy until hematocrit decreases to a safe level" [8].

In the TRAVERSE trial, erythrocytosis (hematocrit >54%) occurred in 7.5% of men on testosterone gel versus 2.9% on placebo (hazard ratio 2.6, 95% CI: 2.0 to 3.5) [9]. Risk factors for TRT-associated erythrocytosis include injectable formulations (which produce higher peak testosterone levels than gels), obstructive sleep apnea, chronic lung disease, and living at high altitude.

Not every hematocrit elevation requires stopping TRT. The standard clinical approach is:

• Hematocrit 50 to 52%: Increase hydration, recheck in 4 to 6 weeks, consider switching from injection to gel.
• Hematocrit 52 to 54%: Reduce testosterone dose, switch to daily microdose injections or topical formulation, recheck in 4 weeks.
• Hematocrit above 54%: Hold TRT, consider therapeutic phlebotomy, investigate secondary causes.

Beyond hematocrit, the CBC provides hemoglobin, white blood cell count, and platelet count. TRT does not typically affect white cells or platelets, but baseline values help rule out underlying hematologic conditions before starting therapy.

Dr. Abraham Morgentaler, Associate Clinical Professor of Urology at Harvard Medical School, has written: "The hematocrit concern with testosterone therapy is manageable with proper monitoring, and it should not be a reason to withhold treatment from men who would benefit from it" [15].

How to Build an Accurate 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, and monitoring them together paints a complete picture.

The minimum lab panel for men on TRT who are tracking metabolic health should include: total testosterone (trough level), calculated free testosterone (using SHBG and albumin), estradiol sensitive (LC-MS/MS), CBC with differential, comprehensive metabolic panel, HbA1c, and fasting glucose. For men with known prediabetes or diabetes, adding fasting insulin and fructosamine provides a more reliable glycemic assessment when hematocrit is elevated.

Lab timing follows a predictable schedule. Baseline labs before starting TRT establish reference values. The first follow-up at 6 to 8 weeks confirms dose adequacy and screens for early hematocrit rises. A second follow-up at 3 to 6 months evaluates steady-state levels and metabolic changes. Annual labs thereafter are sufficient for stable patients [8].

The Endocrine Society also recommends baseline and periodic PSA testing for men over 40 starting TRT, along with a digital rectal exam. While TRT does not cause prostate cancer (the TRAVERSE trial showed no increased incidence over 33 months [9]), monitoring PSA velocity helps detect pre-existing occult disease that might grow faster with androgen exposure.

Lipid panels deserve attention as well. TRT modestly reduces HDL cholesterol by 2 to 5 mg/dL in most studies [9]. For men already on statin therapy or with borderline cardiovascular risk, tracking lipids at 6 and 12 months after starting TRT is reasonable.