Introduction

Treatment with testosterone represents the principal medical therapy for many transgender men (also referred to as trans men or female‐to‐male transsexuals) to promote masculinization. In addition to identifying as male, most transgender men desire to be perceived as male by society. One of the most desired effects of testosterone therapy is a lowering of the mean fundamental frequency (MF0) of the voice to that of cisgender (also referred to as non‐transgender or natal) males. Although the widespread belief that transgender men taking testosterone achieve a desired voice without voice therapy is not without exception (Davies et al., 2015), the high rate of success contributes to an assumption that the same physiological changes that occur in the larynx during puberty occur when transgender men take testosterone. The effects of testosterone are considered irreversible.

Although the effects of testosterone therapy on the transgender male voice are well recognized, there is sparse prospective data regarding the timing and degree of the voice deepening. A few small studies suggested that testosterone's effects on the transgender male voice begin within 2–4 months of initiation of therapy (Van Borsel et al., 2000; Damrose, 2009). A longitudinal study of two transgender men treated with long‐acting intramuscular testosterone undecanoate for 12–13 months found that MF0 during reading dropped most significantly between 2 and 4 months. The MF0 of one subject decreased but leveled at 155 Hz (D#3) by 8 months and it decreased to 132 Hz (C3) but had not yet leveled off by 12 months in the other subject (Van Borsel et al., 2000). For comparison, the MF0 while reading is approximately 200–225 Hz (G#3) for cisgender females and 100–130 Hz (A2) for cisgender males (Brown et al., 1993; Morris et al., 1995; Howard & Murphy, 2008; Colton et al., 2011). Similarly, a 16‐month study of a 33‐year‐old semiprofessional singer found that the transgender man experienced a marked decline in fundamental frequency between the third and fourth months of testosterone therapy (Damrose, 2009). This subject, however, experienced no voice deepening after that period. The earliest that voice changes from testosterone therapy have been reported was in a 22‐year‐old hypogonadal man who happened to be a professional singer (King et al., 2001). He recalled that he began to perceive changes to his voice within 2 weeks of starting testosterone therapy.

Up until recently, the lack of a prospective voice study of three or more transgender men with testosterone therapy likely relates to the paucity of transgender patients among most endocrinologists and the need to collaborate with a speech and voice center (Irwig, 2016). A study by Deuster et al. (2016) prospectively assessed acoustic voice deepening in nine of 15 transgender men who completed 1 year of study while receiving testosterone. The subjects averaged a decrease in almost nine semitones (ST) over primarily the first 9 months and achieved a level equitable to cisgender males (Deuster et al., 2016). The first statistically significant changes in speaking fundamental frequency occurred within the first 2 weeks of testosterone therapy. Percentile curves generated from this small data set assumed voice deepening minimized over time and calculated the few missing values using a linear trend. The authors noted individual differences in response to treatment (e.g. slow performer, sigmoid function, rapid change).

The largest longitudinal voice study on transgender men is a retrospective study at Karolinska University Hospital of patients who had voice assessments before and during testosterone therapy for 24 months (Nygren et al., 2016). Fifty subjects had baseline voice data with follow‐up data available for 36 subjects at 12 months and 22 subjects at 24 months. Voice assessments were grouped at 3 (2–5), 6 (5–10), 12 (11–15), 18 (16–21), and 24 (22–36) months and there was some missing data as not all subjects came to all visits. The majority were treated with long‐acting intramuscular testosterone undecanoate (1000 mg every 8–12 weeks) with the remainder treated with intramuscular esters, transdermal gels, or a combination of therapies. The average MF0 during reading decreased by eight ST within the first year: 192 Hz before treatment, 155 Hz at 2–5 months, 134 Hz at 5–10 months, and 125 Hz at 11–15 months. The largest drop (i.e. 4 ST) was seen during the first 2–5 months and no significant decrease was seen between 12 and 24 months. Objective data correlated well with questionnaire data regarding perception of male by others when speaking on the phone. Although the majority of transgender men were satisfied with their voice at 12 months, 24% received vocal therapy because of symptoms of vocal fatigue, vocal instability, strained voice quality, insufficient lowering of pitch, voice projection difficulties, and problems with the voice sounding younger than the subjects’ chronological age.

The effects of long‐term administration of testosterone on voice parameters has been described in a cross‐sectional study of 38 transgender men in Belgium with a control group of heterosexual cisgender males matched for age and smoking status (Cosyns et al., 2014). The mean duration of testosterone therapy for the group of transgender men was more than 10 years and the majority were current (n = 13) or former smokers (n = 12). As compared to the controls, transgender men showed no differences in any of the variables studied which included median F0 during reading, counting, and sustained vowel production; pitch variation, jitter, and shimmer. However, the median F0 during reading revealed more heterogeneity in the transgender men group. Although most transgender men could not be distinguished acoustically from cisgender men, the study found that 10% of transgender men had pitch‐lowering difficulties. Two subjects had a MF0 in a gender ambiguous range when reading and two subjects underwent voice therapy and/or surgery.

Given the paucity of prospective data on the effects of testosterone on the transgender male voice, we sought to investigate the timing and degree of voice deepening over 12 months among an ethnically diverse sample of transgender men who did not receive any speech or voice therapy.