Short answer: Yes, it can. Every testosterone label lists gynecomastia, the medical word for growth of breast tissue in men, as a warning, and some say it may persist. The only trial rate in the labels is small: 1% of men on Testim 50 mg, against 0% on placebo. Testosterone is partly turned into estradiol in the body, and MedlinePlus says an imbalance between estrogen and testosterone is the usual cause of breast growth in men.
Gynecomastia appears in the warnings section of each testosterone label, with slightly different wording:
| Product | Label wording |
|---|---|
| AndroGel 1.62% (5.11) | May develop and persist |
| Xyosted (5.10) | May develop and may persist |
| Testim (5.11) and Aveed (5.12) | Occasionally develops and occasionally persists |
| Testosterone enanthate (Warnings) | Frequently develops and occasionally persists |
Only the Testim label gives a number. In its 90-day placebo-controlled trial (Table 1), gynecomastia was reported in 1% of 103 men on 50 mg, 0% of 149 men on 100 mg and 0% of 99 men on placebo. The AndroGel 1.62% label lists gynecomastia and breast pain (mastodynia) among reports after the product went on the market, where rates cannot be worked out.
The AndroGel and Xyosted labels (section 12.3) say the major active metabolites of testosterone are estradiol and DHT. In the AndroGel 1.62% studies, estradiol levels rose and fell in step with testosterone.
MedlinePlus explains that breast enlargement in males "is usually caused by an imbalance of estrogen (female hormone) and testosterone (male hormone)." It lists low testosterone itself, obesity, ageing past 50, liver and kidney disease, and medicines including anabolic steroids, spironolactone and finasteride among the causes. So breast change on TRT may have more than one source. More on estradiol in men is on the estrogen on TRT page.
MedlinePlus says gynecomastia often starts as a small, sometimes tender lump under the nipple, in one or both breasts, and that the first step is telling breast tissue apart from fat. It says men with enlarged breasts may have a higher risk of breast cancer, which is rare in men, and names signs to see a provider about:
The testosterone labels list breast cancer in men as a reason not to use these products at all (section 4).
The labels do not say how to treat gynecomastia that starts on testosterone. Whether to check estradiol, change the dose or product, or look for another cause is up to the prescriber. For what the evidence shows on estradiol testing and anastrozole in men, see the estradiol on TRT article.
A note in the Dose Track side effects log with the date you first noticed tenderness, next to your dose history and lab results, gives the prescriber a clear timeline.
Read more on Dose Track: Estrogen on TRT · Aromatase inhibitor · Testim side effects · Side effects log in Dose Track