Effect of androgen receptor polymorphism on hypogonadism severity and efficacy of testosterone replacement therapy

Introduction

Normal serum testosterone levels encompass a wide range, with most labs citing 300 ng/dL up to 1,000 ng/dL. This wide range makes clinical diagnosis of symptomatic hypogonadism challenging. Hypogonadism is a medical condition characterized by low serum testosterone (1). Hypogonadism causes an array of symptoms that significantly affect patients’ daily lives, including: fatigue, changes in mood, osteoporosis, gynecomastia, loss of libido, and erectile dysfunction (ED) (1,2). Testosterone replacement therapy (TRT) is the preferred treatment for hypogonadism (3). TRT is most commonly administered via subcutaneous or intramuscular injection or transdermal gel (3). TRT has consistently been shown to improve libido, erectile function, mood and affect, and bone mass and strength (4). Recent data demonstrate that, contrary to popular belief, TRT does not adversely affect cardiovascular health (5) or promote prostate cancer growth (6,7). However, TRT is not a perfect treatment for hypogonadism. TRT does not permanently change testosterone levels; it cannot cure hypogonadism. It is, therefore, a lifelong therapy to maintain optimal serum levels. Additionally, TRT varies significantly between patients in efficacy, dosage, and treatment course to a degree that is not well understood (3).

A potential explanation for this variation in patient response to TRT involves androgen receptor (AR) transactivational activity. Closely related to testosterone levels, the AR is a nuclear receptor that, once bound to testosterone, acts as a nuclear transcription factor to regulate expression of various androgen-related genes (8,9). Located on the X chromosome, the AR gene possesses a sequence on exon 1 with a CAG (glutamine-coding) trinucleotide repeat of varying length, between 10 and 36 codons (10). Polymorphism of this CAG repeat region is associated with a large number of health complications, including spinobulbar muscular atrophy (Kennedy’s disease) (11), elevated high density lipoprotein (HDL) cholesterol levels (12), prostate cancer (13-15), male infertility (16), and late-onset hypogonadism (17). There is no evidence that CAG repeat length correlates with endogenous testosterone levels (17,18). Evidence does, however, suggest that CAG repeat length inversely correlates with activity of the AR, and removal of the polyglutamine tract has been shown to increase transactivation (19).

Because of this relationship between AR activity and CAG repeat length, we theorize that polymorphism of the AR causes the unexplained variance in patient response to TRT. In a population of 162 males undergoing TRT, we compared AR genotype to baseline serum testosterone (T) levels, self-reported symptoms of hypogonadism, and change in serum T levels following TRT. We present this article in accordance with the STROBE reporting checklist (available at https://tau.amegroups.com/article/view/10.21037/tau-2026-0182/rc).

Methods

Participants

One hundred sixty-two adult males diagnosed with hypogonadism at Baylor College of Medicine (Houston, TX, USA) were selected for this study upon consultation with provider. The patient recruitment period was conducted from April 2023 through October 2025. The study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. This study was approved by the Institutional Review Board for Baylor College of Medicine and Affiliated Hospitals (protocol H-53114), and all subjects were informed of the study’s purpose and provided informed consent prior to enrollment.

Data collection

Patients’ demographic information, medical history, medication use, and laboratory and hormone values were collected. Hypogonadism was diagnosed by provider based on serum testosterone levels, patient history, consultation with provider, and severity of symptoms noted in the American Urological Association guidelines, including ED, reduced libido, infertility, fatigue, decrease in energy, depression, and increased irritability. Symptom severity of sexual and psychological symptoms was determined via patient scores on the International Index of Erectile Function (IIEF-15) and the Patient Health Questionnaire-9 (PHQ-9). Blood samples were collected and sent to the lab to determine CAG repeat length. Upon each follow-up visit, laboratory and hormone values were collected again. Final testosterone levels (end of data collection period) were defined as the values taken at the first follow-up visit during which each patient reported significant symptomatic improvement. Patients who did not return to clinic for a follow-up visit, or for whom follow-up testosterone levels were not available, were considered lost to follow-up.

DNA sequencing

A total of 10 mL of blood was extracted from each patient, and DNA extraction was performed using QIAGEN DNeasy Blood & Tissue Kit (QIAGEN). PCR was performed using the primers and methods of Zitzmann et al. (12) to amplify the CAG repeat region located on exon 1 of the AR. The presence of PCR products was confirmed using agarose gel electrophoresis and visualized in UV light with ethidium bromide. Yield (ng/µL) was calculated using spectrophotometry. Gel extraction products were then sequenced (Azenta), and CAG repeat numbers were manually scored.

Statistical analysis

Mean and standard error were calculated for each parameter measured. Hypogonadism severity was determined based on initial testosterone levels. Linear regression modeling was performed on CAG repeats vs. parameters to determine statistical significance (P<0.05). Two-tailed t-tests were performed (RStudio) to compare CAG lengths between groups.

Results

Baseline information

This study sought to compare the CAG repeat length of patients undergoing TRT to the treatment’s efficacy. Of the 162 patients, the average number of CAG repeats found was 21.7±3.1 codons (Figure 1), which falls within the typical range of 10–36 codons. Mean age was 51.9±14.8 years. The most common patient comorbidities were ED (n=72), type 2 diabetes (n=25), and obstructive sleep apnea (n=17) (Table 1).

Figure 1 CAG repeat distribution among hypogonadal patients. The average number was 21.7±3.1 codons, and the range was 11–29 codons.

Of the 162 patients who presented with symptoms of hypogonadism, the majority (n=112) started therapy. Approximately half (n=75) of these patients had complete follow-up data at time of statistical analysis [n=39 on only testosterone; n=12 on testosterone, human chorionic gonadotropin (hCG), and anastrozole; n=12 on a selective estrogen receptor modulator (SERM); n=1 on monotherapy hCG]. CAG repeat length did not vary significantly depending on which treatment modality each patient was on. Of those missing full follow-up data, 11 reported no significant improvement in symptoms upon initial follow-up visit, at which point their treatment plan was modified upon consultation with provider. None of these patients had returned for a follow-up visit within the study period. The other 76 cases were lost to follow up and had no data regarding changes to symptoms and testosterone levels. There was no significant difference in CAG repeat length between the 11 patients with incomplete follow-up data and the 75 with complete follow-up data (P=0.74). All patients with complete follow-up data (n=75) noted significant symptomatic improvement. Average time between initial visit and significant symptomatic improvement was 5.4±2.7 months.

Hypogonadism severity correlates with CAG repeat length

Patients presenting with symptoms of hypogonadism had a wide range of initial testosterone levels, with an average of 413.7±274.1 ng/dL. Over half (n=104) presented with symptoms of hypogonadism despite baseline testosterone levels exceeding 300 ng/dL. Comparing these two groups, hypogonadal patients with a baseline testosterone value above 300 ng/dL demonstrated a significantly higher average CAG repeat length (22.1 codons) than those below 300 ng/dL (20.9 codons) (P=0.02). That is to say, patients presenting with symptoms of hypogonadism despite serum testosterone levels higher than 300 ng/dL possessed significantly longer CAG repeat regions than those presenting with hypogonadal serum testosterone levels (<300 ng/dL). For the entire study group (n=162), presence of each symptom recorded had no significant effect on CAG repeat length, including ED (P=0.93), osteoporosis (P=0.88), obstructive sleep apnea (P=0.81), type 2 diabetes (P=0.57), and cardiovascular disease (P=0.07) (Table 1).

In the cohort with complete follow-up data (n=75), patients whose testosterone levels superseded 700 ng/dL following therapy demonstrated a higher average CAG repeat length (22.5 codons) than those who did not (21.4 codons), though this result did not reach significance (P=0.11) (Table 2).

Symptom severity and CAG repeat length correlate with final testosterone values

A weak, statistically significant correlation was found between final testosterone levels and CAG repeat length (R²=0.0631, P=0.02) (Figure 2). An adjusted linear regression model, in which CAG repeat length, type 2 diabetes, and ED are used as predictors for final/ideal testosterone levels, was much stronger compared to the simple linear regression model (R²=0.178, adjusted R²=0.143, P=0.002) (Table 3). Each other symptom (cardiovascular disease, obstructive sleep apnea, and osteoporosis) decreased adjusted R², suggesting reduced regression model strength.

Figure 2 Final testosterone levels loosely correlate with CAG repeat length (R²=0.0631, P=0.02). Gray shading denotes 95% confidence interval. T, testosterone; TRT, testosterone replacement therapy.

No other significant correlations were found between CAG repeat length and other parameters, including: PHQ-9 (P=0.58), IIEF (P=0.89), baseline testosterone levels (P=0.77), change in testosterone levels following treatment (P=0.18), and duration of therapy (P=0.59).

Discussion

CAG repeat length and hypogonadism severity

This study sought to establish a correlation between CAG repeat length, hypogonadism severity, and efficacy of TRT based on patient-reported symptomatic relief. The data from this cohort of 162 hypogonadal males suggest that there exists a relationship between CAG repeat length and hypogonadism severity. Current AUA guidelines denote 300 ng/dL as the serum testosterone cutoff for diagnosis of hypogonadism (20), but clinical data demonstrate that this criterion fails to account for a subset of the population who experiences hypogonadal symptoms despite serum testosterone levels exceeding the 300 ng/dL threshold. The data from this study provide a plausible explanation for why this subset exists. Hypogonadal males with baseline testosterone levels greater than 300 ng/dL demonstrated a significantly higher average CAG repeat length (22.1 codons) than those below 300 ng/dL (20.9 codons) (P=0.02). Previous research has demonstrated that CAG repeat length inversely correlates with the AR’s transcriptional activity (19). Therefore, these data suggest that males with longer CAG repeat regions exhibit a higher androgen insensitivity that manifests in hypogonadal symptoms at higher testosterone levels.

CAG repeat length and TRT efficacy

Adjusted linear regression analysis from this study suggests that a relationship exists between CAG repeat length, comorbidities of hypogonadism, and final testosterone levels. The significance (R²=0.178, adjusted R²=0.143, P=0.002) of CAG repeat length, type 2 diabetes, and ED as predictors for final testosterone levels suggests the importance of a holistic approach to treatment of hypogonadism. The role of ED as a predictor of optimal serum testosterone levels is further corroborated by previous research, which suggests different symptoms are alleviated at different testosterone thresholds (400 ng/dL for nighttime erections, 500 ng/dL for sexual intercourse, and 600 ng/dL for sexual desire) (21). Additionally, evidence suggests that TRT in patients with type 2 diabetes mellitus (DM2) increases insulin sensitivity (22). This study found that patients with DM2 reported symptomatic relief at lower serum testosterone levels. Therefore, it is likely that TRT’s role in modulating insulin sensitivity provided more significant symptom relief in this diabetic cohort.

When comorbidities are removed from regression analysis, a significant, albeit weak, correlation remains between CAG repeat length and final testosterone levels (R²=0.0631, P=0.02). Additionally, patients with final testosterone levels exceeding 700 ng/dL following therapy demonstrated a higher average CAG repeat length (22.5 codons) than those who did not (21.4 codons) (P=0.11). It should also be noted that, in an earlier cohort (n=44), the difference between those who required testosterone levels exceeding 700 ng/dL to achieve symptomatic relief and those who did not (23.9 vs. 21.8 codons) was significant (P=0.02) (23). These data complicate the aforementioned notion that alleviation of hypogonadal symptoms can be categorized by universal thresholds, such as 600 ng/dL to fully improve symptoms of sexual dysfunction (21). Rather, these data demonstrate that supplementing testosterone past 700 ng/dL is necessary for some patients to achieve meaningful symptomatic relief. The particular threshold of 700 ng/dL chosen for this study demonstrated the greatest, most significant difference in CAG repeat length between the two groups. Therefore, further research should seek to more accurately determine where the cut-off lies and how CAG repeat length may be useful as a predictor for each patient’s ideal testosterone levels to achieve symptomatic relief.

Given that the average testosterone levels of the hypogonadal patients in this study exceeded 300 ng/dL, these data are useful to practicing physicians evaluating patients with hypogonadal symptoms yet seemingly “normal” (i.e., >300 ng/dL) testosterone levels. Specifically, this regression suggests that patients with longer CAG repeat lengths and more severe symptoms (specifically, type 2 diabetes and ED) require different levels of testosterone supplementation to effectively treat their hypogonadism. Therefore, based on these data, physicians should consider treatment for hypogonadal patients with baseline testosterone levels exceeding 300 ng/dL, in addition to supplementing testosterone levels past 700 ng/dL based on CAG repeat length and symptoms. Furthermore, recent large-scale trials such as TRAVERSE have demonstrated the safety of testosterone supplementation in relation to risk of cardiovascular events and prostate cancer (5-7), which further suggests that traditional notions of TRT should be revised.

Modern research demonstrates that each hypogonadal patient’s therapeutic testosterone level, at which they feel significant symptomatic improvement, varies based on etiology (24). Based on the results of this study, CAG repeat length may further explain this variation. Linear regression analysis demonstrated that men who require more testosterone to achieve symptomatic relief had longer CAG repeat regions (P=0.02). The CAG repeat region’s role in modulating transcriptional activity of the AR (19) suggests that men with longer CAG repeat regions respond less strongly to TRT and, therefore, require more exogenous testosterone to effectively treat their symptoms. Clinically, males with shorter CAG repeat regions are more likely to exhibit sexual dysfunction symptoms due to late-onset hypogonadism (LOH), which suggests that ARs with shorter CAG repeat regions are more sensitive to changes in testosterone (25). Conversely, males with LOH who possess longer CAG repeat regions report less dramatic sexual improvement following TRT when compared to those with shorter repeat regions (26).

Despite the significance of the adjusted linear regression data in this study, it should be noted that the relatively low R² (0.178) suggests that many other factors should be considered when diagnosing and treating hypogonadism. Hypogonadism is undoubtedly a multifaceted condition; therefore, providers must holistically consider each patient’s unique response to therapy. Future research should seek to further refine the predictive value of CAG repeat length and symptoms, which may reveal more information regarding the mechanisms behind hypogonadism severity and response to TRT.

Conclusions

Based on the results of this study, increased CAG repeat length correlates with symptoms of hypogonadism at serum testosterone levels exceeding 300 ng/dL. When used as a predictor alongside diabetes mellitus type 2 and ED, CAG repeat length demonstrates a significant linear correlation with serum testosterone levels needed for symptomatic relief. Therefore, providers should consider usage of CAG repeat length alongside patient reported symptoms when creating a treatment plan for hypogonadism.

Acknowledgments

Earlier versions of this manuscript were presented at the 2024 and 2025 meetings of the Sexual Medicine Society of North America, the 2025 meeting of the South Central Section of the American Urological Association, and the 2026 meeting of the American Urological Association.

Footnote

Reporting Checklist: The authors have completed the STROBE reporting checklist. Available at https://tau.amegroups.com/article/view/10.21037/tau-2026-0182/rc

Data Sharing Statement: Available at https://tau.amegroups.com/article/view/10.21037/tau-2026-0182/dss

Peer Review File: Available at https://tau.amegroups.com/article/view/10.21037/tau-2026-0182/prf

Funding: This work was supported by Baylor College of Medicine (Internal Funding Only).

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://tau.amegroups.com/article/view/10.21037/tau-2026-0182/coif). M.K. reports consulting fees from Endo Pharmaceuticals, Marius, Besins, Boston Scientific, AbbVie, Coloplast and Verity; and holds stock in Sprout. The other authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. The study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. This study was approved by the Institutional Review Board for Baylor College of Medicine and Affiliated Hospitals (protocol H-53114), and all subjects were informed of the study’s purpose and provided informed consent prior to enrollment.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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