Gynecomastia: Hormonal Causes & When to Get Tested

How does low testosterone or high estrogen cause gynecomastia?

Breast tissue growth in men is primarily stimulated by estrogen and restrained by testosterone's counterbalancing effect. Anything that shifts this ratio toward more estrogenic influence can trigger glandular growth, including: falling testosterone with estradiol staying the same (removing the counterbalance), rising estradiol with testosterone staying the same (increasing the stimulus), or changes in sex hormone-binding globulin (SHBG), which binds testosterone more tightly than estradiol in the bloodstream — so a rise in SHBG can lower free testosterone proportionally more than free estradiol, effectively tilting the ratio even if total hormone levels look normal.

This is why gynecomastia is frequently listed as a symptom of low testosterone, and why an evaluation typically includes both a testosterone panel and an estradiol level rather than testosterone alone. See our explainer on SHBG for how this binding protein affects the free-hormone balance.

Obesity and aromatization: a common non-hormone-disorder pathway

One of the most common drivers of an unfavorable estrogen-to-testosterone ratio isn't a gland disorder at all — it's body fat. Adipose (fat) tissue expresses the enzyme aromatase, which converts testosterone into estradiol. The more adipose tissue a man carries, particularly visceral fat, the more aromatase activity is present, which both lowers testosterone and raises estradiol at the same time — a double effect on the ratio. This mechanism is well-established and is a major reason obesity is associated with both low testosterone and a higher likelihood of gynecomastia.

This creates a distinction clinicians look for: true gynecomastia (firm, rubbery glandular tissue, often tender, typically centered under the nipple) versus pseudogynecomastia (soft, diffuse fatty tissue without a distinct glandular disc, common in men carrying excess weight generally). The two can coexist, and only a physical exam — sometimes supported by ultrasound — reliably tells them apart.

Medications and substances that commonly cause gynecomastia

Medication-induced gynecomastia is common enough that reviewing a patient's drug list is a standard part of the workup. Frequently implicated categories include:

• Anabolic steroid or testosterone therapy misuse — supraphysiologic doses are aromatized to estradiol in excess, and abrupt cessation can also cause a temporary estrogen-testosterone imbalance during recovery.
• 5-alpha reductase inhibitors (finasteride, dutasteride) — used for hair loss or prostate conditions, these can shift androgen metabolism.
• Spironolactone — an anti-androgen and diuretic, commonly cited as a cause.
• Certain antipsychotics and gastrointestinal drugs that raise prolactin, which can secondarily suppress testosterone.
• Opioids, which suppress the hypothalamic-pituitary-gonadal axis with chronic use.
• Alcohol and marijuana, both associated with gynecomastia in some studies, plausibly through effects on the HPG axis and hepatic hormone metabolism.

A careful medication and substance history is often one of the highest-yield parts of the evaluation.

When is gynecomastia a sign of something more serious?

Most gynecomastia is benign and related to the common causes above, but certain features warrant prompt evaluation rather than a wait-and-see approach: rapid onset or growth over weeks, a hard or fixed (rather than firm-but-mobile) lump, asymmetric or one-sided growth, skin dimpling or nipple discharge, or a palpable mass in only one breast. These features can, in rare cases, indicate a testicular tumor (some of which secrete estrogen or hCG), a prolactin-secreting pituitary tumor, liver or kidney disease affecting hormone metabolism, hyperthyroidism, or, uncommonly, male breast cancer. These causes are far less common than the benign, ratio-related causes above, but they're the reason a physician, not self-diagnosis, should evaluate persistent or unusual-feeling gynecomastia.

What labs are used to evaluate gynecomastia?

A hormonal workup for gynecomastia typically includes total and free testosterone, estradiol (using a sensitive assay, since standard immunoassays are less accurate at the low estradiol levels seen in men), LH and FSH to distinguish primary from secondary causes, SHBG, and sometimes prolactin, thyroid function (TSH), and liver and kidney function tests. In men with a testicular exam finding or unclear labs, a testicular ultrasound may be added to rule out a hormone-secreting tumor. A complete panel, rather than testosterone alone, is what allows a physician to identify whether the driver is low testosterone, elevated estradiol, an SHBG shift, or a combination. See our overview of hormone testing for what a full panel generally includes.