Serum Testosterone Concentrations Remain Stable Between Injections in Patients Receiving Subcutaneous Testosterone

Abstract

Purpose: Intramuscular (IM) testosterone is the most common modality for testosterone therapy of both male hypogonadism and female-to-male (FTM) gender transition. However, IM injections can be painful and often are not self-administered by the patient. The objective of this study was to further characterize subcutaneous (SC) administration of testosterone as an effective and safe alternative to IM injections by evaluating the pharmacodynamics of serum total and free testosterone concentrations between weekly testosterone injections.

Methods: Eleven FTM transgender patients already receiving weekly SC testosterone cypionate with documented therapeutic levels prior to enrollment had free and total serum testosterone levels measured at eight different time points during a 1-week dosing interval.

Results: Mean levels of total and free testosterone were stable and remained well within the normal range between injections. Overall mean ± standard deviation levels for the seven samples taken between injections were 627 ± 206 ng/dL (range, 205 to 1410) for total testosterone and 146 ± 51 pg/mL (range, 38 to 348) for free testosterone. No adverse effects were encountered.

Conclusions: The results of this study support use of SC testosterone to achieve therapeutic and stable serum testosterone levels for the purpose of gender transition. It is anticipated that these results can be extended to hypogonadal men. This route may be preferred over IM testosterone because it is relatively painless and easy to self-inject thus allowing for the convenience and economy of patient self-administration.

Keywords: pharmacokinetics; subcutaneous testosterone.

Figure 1.

Serum total and free testosterone over a 1-week treatment cycle. Serum measurements taken at 0 and 7 days were from samples drawn immediately prior to SC testosterone injection. Dashed lines represent the lower and upper values of normal range for each total free testosterone. Data are shown as mean ± standard deviation.

Figure 2.

Individual hormone levels over time after SC testosterone injection: 11 female-to-male transgender patients. Individual patients are identified by number in the key. Horizontal dashed lines represent the lower and upper limits of the normal male range for total and free testosterone. The three patients with the lowest serum testosterone concentrations were all receiving 50-mg doses, and the patient with the highest concentration was receiving a 100-mg dose. FT, free testosterone; TT, total testosterone.

Figure 3.

The relationship between testosterone dose and mean serum total testosterone. Data are shown as mean serum total testosterone among measurements taken between testosterone injections. The closed triangle represents a patient with one serum total testosterone measurement below the normal male range during the study; the closed square represents one patient with four serum total testosterone measurements above the normal male range during the study.