Testosterone Injections and Blood Effects
Testosterone injections do NOT increase A1C (hemoglobin A1C, the diabetes marker), but they DO commonly raise hematocrit, which makes blood thicker and more viscous—particularly with injectable formulations.
The Two-Part Answer
Part 1: A1C (Blood Sugar Control)
Testosterone replacement therapy at physiologic doses has no effect on glucose metabolism, insulin sensitivity, or A1C levels 1. This is a case of confusing two different blood tests:
- A1C (Hemoglobin A1C): Measures average blood sugar over 3 months—used for diabetes monitoring
- Hematocrit: Measures the percentage of red blood cells in your blood
Testosterone does not worsen diabetes control or raise blood sugar.
Part 2: Blood Thickness (Hematocrit/Viscosity)
Yes, testosterone injections significantly increase hematocrit and blood viscosity, creating legitimate clinical concerns 1.
How Testosterone Thickens Blood
Testosterone stimulates red blood cell production (erythropoiesis) through multiple mechanisms 1:
- Increases erythropoietin (EPO) initially
- Establishes a new EPO/hemoglobin "set point"
- Decreases hepcidin (iron regulator)
- This is why boys' hemoglobin increases 15-20% at puberty
Risk Varies Dramatically by Formulation
Injectable testosterone carries the highest risk of erythrocytosis 1:
- Intramuscular injections: 43.8% of patients develop elevated hematocrit (>52%)
- Testosterone gel: 11.3-17.9% (dose-dependent)
- Transdermal patches: 2.8-15.4%
The difference is striking—injections are 3-4 times more likely to cause problematic hematocrit elevation compared to topical preparations 2.
Clinical Consequences of Thick Blood
Elevated hematocrit above normal range increases blood viscosity, which can aggravate vascular disease in coronary, cerebrovascular, or peripheral circulation 1. The risk is particularly concerning in:
- Elderly patients
- Those with pre-existing cardiovascular disease
- Patients with chronic obstructive pulmonary disease (COPD)
- Anyone with conditions that independently raise hematocrit
However, no testosterone-associated thromboembolic events have been definitively reported to date 1, though the theoretical risk remains 3.
Monitoring Requirements
Hematocrit or hemoglobin must be monitored 1:
- Baseline: Before starting therapy
- Follow-up: At 1-2 months, then every 3-6 months for the first year, then yearly
- Most changes occur in the first 3 months 1
Management Algorithm When Hematocrit Rises
If hematocrit exceeds normal range 1:
- Temporarily withhold testosterone
- Reduce dosage
- Consider therapeutic phlebotomy (though evidence for this is actually lacking 3)
- Switch from injections to topical formulations (gel or patch)—this is often the most practical solution given the dramatic difference in erythrocytosis rates
Critical Pitfall to Avoid
Do not confuse A1C with hematocrit. These are completely different blood tests:
- If your doctor is concerned about "thick blood," they're monitoring hematocrit (normal range ~40-52% for men)
- If concerned about diabetes, they're monitoring A1C (normal <5.7%)
- Testosterone affects hematocrit, NOT A1C
The FDA label explicitly states: "Hemoglobin and hematocrit levels (to detect polycythemia) should be checked periodically in patients receiving long-term androgen administration" 4.
Bottom Line for Clinical Practice
If you're on testosterone injections and develop elevated hematocrit, switching to transdermal gel or patches dramatically reduces the risk while maintaining therapeutic testosterone levels 1, 2. This is preferable to stopping therapy entirely, which causes return of hypogonadal symptoms, or repeated phlebotomy, which lacks evidence and may paradoxically increase thrombotic risk through iron depletion 3.