Testosterone Cypionate and Gynecomastia: The Biology of Why It Happens

Does testosterone cypionate cause gynecomastia?

The label says it can. In its warnings, Depo-Testosterone (testosterone cypionate) states that gynecomastia may develop and occasionally persists in patients being treated for hypogonadism, and the adverse reactions list includes gynecomastia among reactions that have occurred with some androgens [3]. The Depo-Testosterone adverse reactions section lists it without a percentage, and without tying it to a dose or injection interval.

How does testosterone become estradiol?

Aromatase, the enzyme made by the CYP19A1 gene, converts androgens (testosterone and androstenedione) into the estrogens estradiol and estrone [7]. It is active in several tissues in men, including adipose tissue, breast, brain, skin and bone, so estrogen production is not confined to the testes [7].

In men, testosterone is the major source of plasma estradiol, and only about 20% of plasma estradiol is secreted by the testes; the rest is produced by conversion in other tissues [5]. After an injection, estradiol rises along with testosterone [4].

What happens to estradiol after a 200 mg injection?

One study of 11 hypogonadal men given 200 mg testosterone cypionate intramuscularly found a threefold rise in serum testosterone, peaking on days 2 to 5, and a threefold rise in estradiol during days 2 to 7 after the injection [4]. Many of the men reached androgen levels above the normal range between days 2 and 7, and values fell back to baseline by days 13 to 14, so biweekly 200 mg injections produced wide swings in circulating androgens [4]. Luteinizing hormone was suppressed after the first injection and stayed suppressed in men on chronic treatment [4].

The label puts the half-life of intramuscular testosterone cypionate at about 8 days and the replacement range at 50 to 400 mg every 2 to 4 weeks [3].

Why the estrogen-to-androgen balance matters

Reviews describe gynecomastia as a problem of balance rather than of estradiol alone: estrogens stimulate breast tissue growth, androgens inhibit it, and most cases result from deficient androgen action or excessive estrogen action in the breast [2]. Another review frames it as an alteration in the androgen-estrogen ratio, alongside growth hormone, IGF-1, prolactin and other factors that affect the aromatase enzyme [8].

Does body fat change estradiol levels?

Body fat is one of the places where conversion happens. In a review of estradiol in men, plasma estradiol was strongly related to body fat mass and more specifically to subcutaneous abdominal fat, but not to visceral fat; aromatase activity in omental fat was one-tenth of the activity in gluteal fat [5]. The same review notes that aromatase activity increases with age, alongside an age-related increase in fat mass [5]. Adipose tissue is also one of the listed sites of aromatase expression in men [7].

Can gynecomastia on testosterone be treated?

The European Academy of Andrology guideline is cautious. It recommends watchful waiting after the underlying cause is treated or the substance associated with gynecomastia is discontinued, states that SERMs, aromatase inhibitors and non-aromatizable androgens are not justified in general for gynecomastia, and calls surgery the treatment of choice for patients with long-lasting gynecomastia [6].

The label's own wording is that gynecomastia may develop and occasionally persists [3], so some cases do not resolve on their own.

Anti-estrogen drug data come from other populations. A retrospective chart review of 38 patients with persistent pubertal gynecomastia, who received reassurance alone or a course of tamoxifen or raloxifene, found a mean reduction in breast nodule diameter of 2.1 cm with tamoxifen and 2.5 cm with raloxifene, with some improvement in 86% and 91% respectively [9]. That study did not involve men on testosterone.

Anastrozole does lower estradiol in men: in a 12-week randomized trial of 37 men aged 62 to 74 with low or borderline testosterone, 1 mg daily (12 men) took mean estradiol from 26 to 17 pg/mL [10].

What should be checked if breast tissue enlarges?

The European Academy of Andrology guideline says the purpose of assessment is to find underlying pathology and reversible causes such as drugs, and to tell gynecomastia apart from other breast lumps, particularly breast cancer. Assessment includes a breast and genital examination. Depending on the findings, tests may include testosterone, estradiol and other hormone levels, along with liver and kidney function; breast imaging may be used when the clinical examination is equivocal [6]. Breast cancer in men is much less common than benign gynecomastia, and a careful physical examination can usually distinguish the two [2]. The Depo-Testosterone label lists breast carcinoma in men as a contraindication to the product [3].