In individuals assigned female sex at birth and undergoing gender-affirming testosterone therapy (trans men), hormone concentrations change markedly but the immunological consequences are poorly understood. Most hemoglobin increases occur early, so frequent monitoring helps detect rising levels before they become problematic. Many people see changes within 3–6 months, but timing varies. Many people who need it only require it once or twice until their treatment is stabilized. If hemoglobin or hematocrit becomes too high, therapeutic phlebotomy may be recommended.
These mechanisms can also help explain the divergent immune responses in cis men and women that are regulated dynamically by sex hormones in relation to ever changing needs during the human life course. Evolutionary pressures from pathogens have shaped human immune systems and the risk of vertical transmission is a selective pressure unique to pregnant females55. Male investment into reproduction is much lower than that of females, and males allocate more resources into muscle growth and immune function and testosterone is a key regulator of such resource allocation in vertebrates53. The two ESRs show variable expression across immune cell types but highest protein expression in pDCs (Extended Data Fig. 6b). Performing in vitro experiments using human cells as described herein offers some mechanistic understanding but is limited in that such experiments are disconnected from physiological mechanisms of regulation in vivo through the nervous system or other endocrine pathways of importance.
If hematocrit is 54% or higher, stopping TRT temporarily allows levels to return to a safe range. Still, action is taken based on lab results—even if no symptoms exist—because thickened blood can be risky even without warning signs. Some people feel normal even with elevated levels, while others develop headaches, dizziness, flushing, or vision changes. Not everyone with high hematocrit will develop complications, but the risk becomes higher as the level rises, especially above the 54% threshold. At this level, the blood is much thicker, and the risk of clot-related problems rises. A hematocrit level in the 52–54% range is more concerning and often requires a change in treatment. Many people on TRT fall into this range occasionally, especially early in treatment or after a dose adjustment.
The absolute lymphocyte count did not change significantly from baseline across dose groups. The baseline characteristics of study participants were summarized as mean and standard deviations or median and interquartile ranges for normally and non-normally distributed data, respectively. These findings were verified in another trial in which testosterone treatment was administered to functionally-limited older men23.
After quality control, a total of 143,624 peaks (features) across 12,773 cells remained for further analysis. The 10X Genomics cellranger pipeline (cellranger-atac mkfastq, count and aggr) was followed for 10x sc-ATAC-seq analysis. In this setup, 1N and 2N refer to the DNA insert sequencing, while i5 and i7 sequencing identify the individual barcodes of single cells. Briefly, cells were washed with PBS containing 0.04% BSA and nuclei subjected to isolation as per the manufacturer’s instructions. BTMs68 were used to compare transcriptional patterns before and during testosterone treatment and in response to stimulation. Cells were further filtered using a bimodal distribution-based approach, excluding those with read counts below (considered low quality) or above (considered technical artifacts) cut-off thresholds.
In PBMCs stimulated with mTB, sex-divergent NK cells responses were also found. We found monocyte-derived IL-6, TNF and IL-15 were upregulated by testosterone, and monocyte-released IL-12B as candidate genes to best explain several upregulated transcripts measured in NK and CD8+ T cells during GAHT (Fig. 4a). As testosterone modulated monocyte function strongly during masculinizing GAHT, we performed NicheNet analyses to infer downstream consequences on other immune cell populations. Treg cells are more abundant in postpubertal male participants than in age-matched female participants26 but, during GAHT, frequencies were stable over 12 months (Extended Data Fig. 5d). CD4/CD8 T cell ratios were higher in female in than male participants20,21 but no decrease occurred during testosterone treatment (Extended Data Fig. 5a), indicating that genetic factors rather than steroids are responsible for this.
This is why it’s important to look at the whole picture, not just testosterone levels. Some people respond better to methods that provide steady hormone levels rather than sharp peaks. Lowering the testosterone dose, changing how often you take it, or switching from injections to a gel or patch can help bring levels down. Another key point is understanding that high hemoglobin on TRT does not mean therapy must be stopped forever. Regular blood testing allows you and your healthcare provider to spot changes long before they become a problem. But for some, it can climb higher than it should, which can increase the risk of health problems if not managed.
But when levels rise too high, the blood becomes thicker. With proper evaluation, most patients can continue therapy successfully while keeping hemoglobin in a safe range. When someone on TRT has high hemoglobin, it is important to look beyond testosterone alone. If a patient has extremely high hemoglobin levels that do not improve with TRT adjustment, doctors may test for PV or other bone marrow disorders. Patients living at high altitude may have naturally higher baseline hemoglobin. In all these situations, the body increases hemoglobin to help deliver more oxygen.