Most guidelines agree that a hematocrit of 54% or higher is the point where TRT should not continue without intervention. A hematocrit level between 50% and 52% is usually considered mildly elevated. When this percentage rises too high, the blood becomes thicker and moves less easily through blood vessels. With proper monitoring and timely adjustments, most people can continue TRT safely without major problems. Regular blood tests help doctors track changes early. The good news is that most complications linked to high hemoglobin on TRT are preventable.
Among the 209 randomized men, 166 men, who completed the 6-month intervention, were evaluated at baseline, after 1, 3, and 6 months of intervention and 3 months after discontinuation of study intervention (Figure 1). Testosterone levels were measured by a radioimmunoassay that has been validated against liquid chromatography tandem mass spectrometry (13) and has a sensitivity of 10ng/dL. We anticipated that trajectories with time would be nonlinear; therefore treatment effects were allowed to vary by visit.
Several endocrine organizations mention therapeutic phlebotomy as a means of treating TTh–induced erythrocytosis, commonly with a hematocrit cutoff around 54%, as an alternative or adjuvant to testosterone dosage reduction (Table 1). Nevertheless, 3 months after diagnosis, patients with PV have a 2.7- and 13.1-fold higher risk of arterial and venous thrombosis, respectively, compared with matched controls (47). The increased risk, however, is likely modest in comparison with myeloproliferative neoplastic disorders such as PV. There might be one or more underlying factors that increase the risk of developing erythrocytosis and thrombosis on TTh. A US multi-institutional database of 74 million patients was used to identify two cohorts of hypogonadal men who either did or did not develop erythrocytosis (defined as hematocrit ≥52%) on TTh. An individual participant dataset meta-analysis of testosterone trials comprising 3431 participants reports no increase in thrombotic events (44). However, hematocrit per se has been challenged as an important determinant of thrombotic risk in erythrocytosis (13).
These observations are similar to those reported in Chuvash polycythemia due to homozygosity for VHLR200W, in which EPO levels are often in the normal range but inappropriately increased in the context of polycythemia (24). The EPO levels were higher than baseline at each hemoglobin level after testosterone administration; this was true even in participants who developed erythrocytosis. Taken together, these changes in hematologic parameters support the hypothesis that testosterone increases iron utilization for erythropoiesis. The ratio of sTR to log10 ferritin, which has been described as an index of iron-dependent erythropoietic activity, increased significantly in men assigned to the testosterone arm but did not change in those assigned to the placebo arm (Figure 4D). Levels of sTR increased markedly in the testosterone group at 1 and 3 months and then returned toward baseline but remained higher than placebo at 6 months (Figure 4B).
Over time, the risk tends to grow if testosterone levels stay high or if the dose is not adjusted. This makes high hemoglobin one of the most frequent lab changes seen with testosterone therapy. High hemoglobin is one of the most common side effects of testosterone replacement therapy (TRT). Understanding these mechanisms explains why doctors monitor hemoglobin and hematocrit during TRT. This is another reason why TRT can increase hemoglobin more than natural testosterone does.
Most risks come from untreated high hematocrit, underlying medical conditions, or lifestyle factors—not TRT alone. Because of this, dose timing and delivery method can affect clotting risk indirectly by controlling how high hematocrit climbs. People using intramuscular injections often see larger swings in testosterone levels. In reality, the risk depends on how high the levels go, how long they stay elevated, and other health factors a person may have. Because of this, many assume that higher hemoglobin automatically means a higher chance of blood clots. Slow blood flow can increase the chance of platelets sticking together, which is the first step in forming a clot. But the relationship between TRT-related erythrocytosis and blood clots is more complex than many think.
While significant progress has been made, our understanding of the mechanism of action behind the stimulation of erythropoiesis by testosterone remains incomplete. It is important to realize that hypoxia is a relative term since pO2 varies widely between tissues and also within a tissue depending on the proximity of a cell to its supplying vasculature and arterial (oxygen-rich) end (31). Consequently, the VHL complex is unable to interact with and ubiquitylate HIF-α, and thus its levels accumulate. Under normoxic conditions, the α-subunits are hydroxylated on two prolyl residues by the iron- and oxoglutarate-dependent HIF prolyl hydroxylases (prolyl hydroxylase domain (PHD) 1 to 3) (29). The α-subunits are principally regulated by two types of enzymes that collaboratively control their levels. A group of transcription factors named hypoxia-inducible factors (HIFs) play a central role in the molecular machinery that senses pO2 and translates this to EPO production in order to maintain red cell mass and thereby O2 delivery to the body (28).
Serum ferritin levels decreased in the testosterone group in parallel with hepcidin but remained unchanged in the placebo group (Figure 4C). The coefficient of variation for hepcidin, hematocrit, and hemoglobin was 0.47 (0.41, 0.53), 0.04 (0.04, 0.05), and 0.04 (0.04, 0.04) SDs, respectively. Testosterone administration significantly increased serum EPO level into the high normal range (13.5 ± 12 to 21.3 ± 17 mIU/mL) at 1 month; this 58% increase from baseline was statistically significant and remained significant at 3 months (Figure 3A).
But for some, the increase can be large enough that it needs medical attention. For many people, this is a normal and expected response. They help your doctor track changes in oxygen-carrying capacity, blood thickness, and overall cardiovascular safety. This test helps your doctor get a full picture of your blood health. They help determine whether the treatment is safe, whether the dose is appropriate, and whether any adjustments are needed. When these markers rise too high, the blood can become thicker than normal. Doctors measure hemoglobin in grams per deciliter (g/dL).

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