Alterations in Body Fat and Composition Among Thai Transmasculine Individuals Undergoing Short-Term Testosterone Therapy

Purpose: This study aimed to investigate the early impacts of masculinizing hormone therapy on body composition among testosterone-naive Thai transmasculine (TM) individuals, focusing on changes in fat and lean mass within the initial 3–6 months of treatment. Methods: In total, 32 participants were enrolled in this prospective study, receiving a 6-month course of 250-mg intramuscular testosterone enanthate. The assessment included whole-body dual X-ray absorptiometry, alongside anthropometric and laboratory evaluations. Results: After 6 months of testosterone administration, a significant reduction in total fat mass (FM) was observed (−4.5%; 95% confidence interval [CI] −8.5 to −0.5). Concurrently, there was an increase in total fat-free mass (FFM) (7.0%; 95% CI 3.3 to 10.8). Stratifying participants based on the World Health Organization Asian obesity criteria (body mass index <23 kg/m 2 for normal and ≥23 kg/m 2 for overweight-obesity), an elevation in FFM was observed in both categories, with a significant FM reduction only among individuals with obesity. Additional findings included hemoconcentration, reduced high-density lipoprotein cholesterol (HDL-C), and a decline in sex hormone-binding globulin (SHBG). Conclusions: Within a brief timeframe of testosterone therapy, a substantial decrease in FM and an increase in FFM were noted. Notably, this reduction in FM was more prominent in participants with obesity. Various alterations in laboratory parameters, such as hemoconcentration, reduced HDL-C, and diminished SHBG, were attributed to changes in hepatic metabolism and pituitary suppression induced by exogenous testosterone. These findings contribute valuable insights into the nuanced effects of hormone therapy on body composition among TM population. Clinical Trial Registration Number—TCTR20241004001.