Transgender individuals are those with a gender identity opposite the sex they were assigned at birth. Approximately 1% of the population is transgender, equating to ~60,000 transgender Wisconsinites. A transgender boy or man is someone with a 46,XX karyotype and typical female genitalia but a male gender identity and desire for more male-typical gender expression. Gender-affirming testosterone (hormonal) treatment (GAHT) is the cornerstone of masculinizing therapy for transgender men and boys, resulting in estrogen (E2) suppression and circulating testosterone (T) levels equivalent to cisgender males. Historically, GAHT was initiated after an E2-driven puberty, but the last decade has seen an explosion of referrals for GAHT in transboys, many of whom are exposed to only low E2 levels before puberty is halted with blocker therapy. Knowledge of risks incurred by GAHT rely on low-quality studies, precluding conclusive assessment of GAHT's long-term impact on cardiometabolic outcomes. Data on transboys receiving GAHT before completion of E2-driven puberty are sparser and no studies have addressed mechanisms by which GAHT may affect vascular physiology. The investigators aim to determine the cardiometabolic impact of GAHT in transboys/men and to determine if any differences identified are mechanistically dependent on the timing of GAHT relative to puberty.

Gender-affirming testosterone (hormonal) treatment (GAHT) is the cornerstone of masculinizing therapy for transgender men, resulting in circulating testosterone (T) levels equivalent to cismen and estrogen (E2) suppression. Historically, GAHT was initiated after an E2-driven puberty, but the last decade has seen an explosion of referrals for GAHT in transboys, many of whom are exposed to only low E2 levels before puberty is halted with blocker therapy. Knowledge of risks incurred by GAHT rely on low quality studies, precluding conclusive assessment of GAHT's long-term impact on cardiometabolic outcomes. Data on transboys receiving GAHT before completion of E2-driven puberty are sparser and no studies have addressed mechanisms by which GAHT may affect vascular physiology. The investigators aim to determine the cardiometabolic impact of GAHT in transboys/men and to determine if any differences identified are mechanistically dependent on the timing of GAHT relative to puberty. The vaso-protective benefits of E2 are well-described; higher circulating E2 begins in female puberty, stimulating a long-term increase in the vascular expression of the E2 receptor's alpha isoform (ERα). ERα activation directly stimulates nitric oxide (NO) production from endothelium-derived NO synthase and is critical to maintaining a healthy vascular endothelium. Pre-menopausal women have an increased capacity to produce endothelial NO due to higher E2 levels and vascular ERα expression, resulting in lower cardiovascular disease risk vs age-matched cismen. These data suggest a novel paradigm for the impact of GAHT on vascular health in transmen that differs depending on when GAHT is initiated relative to puberty. The investigators hypothesize that, in transmen who have completed E2-driven puberty, GAHT will induce a regression of vascular endothelial function towards that of cismen by suppressing circulating E2 and ERα expression and that similar regression will not be seen in transboys initiating GAHT prior to pubertal progression. The investigators propose, to our knowledge, a first-of-its-kind longitudinal study of the cardiometabolic impact of 1-year of GAHT in 40 transboys and transmen as compared to cisgender controls with endogenous sex hormones. The impact of GAHT initiation and continuation in transboys/men on in vivo and ex vivo vascular endothelial function, estrogen-stimulated endothelial vasorelaxation, and expression of vascular ERα expression will be determined by measuring changes in conduit vessel endothelium-dependent vasodilation and vascular E2 receptor expression and responsiveness in transboys on puberty blockers and in transmen. The investigators will also determine whether GAHT suppresses the human arteriolar vascular expression of ERα receptors and reduces E2-stimulated endothelial vasorelaxation in transboys/men relative to ciswomen. Secondarily, The investigators will characterize the metabolic, body composition and inflammatory impact of GAHT. This study will provide a novel mechanistic framework for the cardiometabolic impact of GAHT and make available critical information for care providers and patients on how GAHT may affect their cardiovascular risk.

Inclusion Criteria:

• INCLUSION CRITERIA FOR TRANSGENDER BOYS AND TRANSGENDER MEN

  1. Sex assigned female at birth and a male gender identity (i.e., transgender male)
  2. Aged 12-30 years
  3. Have a care provider in the Children's Wisconsin Gender Health Clinic or Froedtert Hospital Inclusion Clinic who has determined the individual meets clinical criteria to start testosterone therapy and plans to initiate this therapy
  4. Naïve to testosterone or other masculinizing medical therapy
  5. Cisgender girls/women and transgender boys/men of child-bearing potential (defined as having achieved menarche (first menses), excluding those who have had a hysterectomy or are on GnRH agonist therapy (puberty blockers)) must be willing to use effective birth control (which may include abstinence)) from screening visit until final study visit
  6. Willing and able to give informed consent or have parent or legal guardian provide informed consent if the subject is < 18 years of age

INCLUSION CRITERIA FOR CISGENDER INDIVIDUALS

1. Gender identity concordant with sex assigned at birth (i.e., cisgender)
2. Aged 12-30 years
3. Not related to a transgender participant enrolled in the study
4. Naïve to any sex hormone or pubertal blocker therapy or, specific to cisgender girls/women only, must be ≥ 1 year from use of oral contraceptive or other contraceptive technique (intrauterine device, medroxyprogesterone injections)
5. Willing and able to give informed consent or have parent or legal guardian provide informed consent if the subject is < 18 years of age

INCLUSION CRITERIA FOR OPTIONAL NITROGLYCERIN ENDOTHELIUM-INDEPENDENT VASCULAR FUNCTION STUDY

1. Aged ≥ 18 years
2. No history of migraine headaches
3. Systolic blood pressure ≥ 110 mm Hg
4. No prior past adverse reaction to nitroglycerin
5. Has not used a PDE5 inhibitor (i.e., sildenafil, tadalafil, vardenafil) in the past 7 days

Exclusion Criteria:

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Transgender individuals must have been assigned female at birth but identify as transgender/non-binary and interested in gender-affirming testosterone therapy.

EXCLUSION CRITERIA FOR TRANSGENDER AND CISGENDER INDIVIDUALS

1. Any disease affecting glucose, sex steroid, or fat metabolism such as diabetes, polycystic ovary syndrome, or clinical suspicion of insulin resistance)
2. Any medication use that may affect glucose, sex steroid, or fat metabolism such as metformin or glucocorticoids
3. Known cardiovascular disease such as hypertension or atherosclerosis
4. Raynaud's phenomenon or other vascular disease
5. BMI > 40 kg/m2 for those aged ≥ 18 years or > 95% percentile for age- and sex-assigned at birth in those aged < 18 years
6. Pregnancy or lactation
7. Any condition that, in the investigator's opinion, may compromise study participation or may confound the interpretation of the study results.