What Causes Gynaecomastia? Puberty, Hormones, Medicines, Steroids and Weight

Gynaecomastia is caused by an imbalance between oestrogen and androgen activity in the male breast, where oestrogen’s effect on the tissue outweighs testosterone’s, and anything that tips that ratio can set it off: the natural hormone swings of puberty and older age, certain medicines, anabolic steroids, some medical conditions, and, as fatty fullness rather than true gland, weight gain. It is a symptom with a cause behind it, not a single disease, which is why the cause is always looked into before anyone talks about surgery¹.

I spent a long time assuming mine was just fat I had failed to shift. I lost weight, I lifted for two years, and the firm fullness right behind each nipple stayed exactly where it was, and it was only when a GP pressed the tissue and asked, plainly, whether I had ever used supplements or steroids that I understood there might be a reason rather than a personal failing. This is the plain account of what actually drives it. If you are trying to work out whether your fullness is gland or fat, start with gynaecomastia versus pseudogynaecomastia; for the operation itself, see gynaecomastia surgery.

What causes gynaecomastia?

Gynaecomastia is driven by a shift in the balance of oestrogen to androgen activity in the breast tissue, so that the oestrogen signal, which tells the tissue to grow, outweighs the testosterone signal that normally holds it in check. An imbalance in the ratio of oestrogen to androgens at tissue level is the mechanism reviews point to, whether that comes from more oestrogen, less androgen, or breast tissue that is simply more sensitive to normal levels².

That single mechanism is why the cause list looks so varied. Puberty, ageing, a medicine, a steroid, an overactive thyroid, or a liver problem can each nudge the same hormonal balance in the same direction, and the chest responds the same way. The job at consultation is to work out which lever is being pulled in you, because that is what decides whether the fullness is likely to settle on its own, needs a medicine reviewing, or is an established gland that will only go with surgery.

Puberty and older age: the physiological causes

The commonest causes are entirely physiological: newborns pick up their mother’s hormones, boys develop it around puberty as the hormone balance swings, and older men develop it as testosterone falls, all without any disease behind it. Pubertal gynaecomastia is the big one, and it is self-limited in 75 to 90% of adolescents, regressing over 1 to 3 years².

That resolution figure is the reason a teenager is usually asked to wait rather than operated on, and why adolescent gynaecomastia is given about two years to settle before surgery is even considered. In older men the driver is the slow fall in testosterone with age, sometimes on a background of a little extra weight, which is why the NHS lists hormone changes in those aged 50 and over among the ordinary causes³. Physiological does not mean trivial to live with, but it does change the plan.

Medicines that can cause it

A long list of medicines can cause gynaecomastia, most of them by shifting the testosterone to oestrogen balance: antiandrogens used for prostate disease, spironolactone, some heart and blood-pressure drugs, certain stomach-ulcer and HIV medicines, and others. Medicines and drugs are estimated to account for around 10 to 25% of cases, with antiandrogens and, to a lesser extent, 5-alpha-reductase inhibitors and spironolactone among those most consistently linked to it in the literature⁴.

This matters because a drug-related cause is the one most likely to be fixable without an operation. If a medicine is the suspected trigger and it has not been present long, reviewing it with the prescribing doctor can settle the tissue before it becomes an established gland. The key word is reviewing: a prescribed medicine is a conversation with the doctor who started it, not something to stop on your own, and it is part of why the candidacy assessment always goes through your medicines first.

Anabolic steroids and gynaecomastia

Anabolic steroids are a common and specific cause: the body converts the extra androgen into oestrogen through aromatisation, and that oestrogen stimulates the breast tissue to grow, often leaving a firm disc behind the nipple. Because a continuing cause is a route to recurrence, surgeons expect anabolic steroid use to have stopped before operating, and stopping first is what protects the result⁵.

The honest headline is that timing decides everything with steroid-related gyno. Caught early, before the tissue is established, the fullness can settle when the drugs stop; left to organise into a firm gland, it usually stays and needs excising, and operating while use continues is how chests come back. The full picture, including the difference between a temporary flare and an established gland, is set out in steroids and gynaecomastia.

Hormonal and medical conditions

Less often, gynaecomastia is the sign of an underlying medical condition that alters the hormone balance: an overactive thyroid, kidney disease, cirrhosis of the liver, low testosterone (hypogonadism), the genetic condition Klinefelter syndrome, and, rarely, hormone-producing tumours. These are why a doctor takes a history and may order blood tests rather than assuming every enlarged chest is cosmetic³.

Most men do not have one of these, and finding the cause is often reassuringly ordinary. But the possibility is exactly why a website cannot tell you what is behind your chest, and why the cause is checked before any cosmetic plan is made¹. A one-sided, firm, or rapidly growing lump in particular is taken seriously, because it is the pattern least likely to be simple physiological gynaecomastia, and because male breast cancer, though it makes up fewer than 1% of all breast cancers, can present this way³.

Weight, and the fatty kind that is not really gland

Being overweight causes fatty fullness of the chest that looks like gynaecomastia but is pseudogynaecomastia: fat rather than true gland, which can partly settle with weight change in a way an established gland never does. More body fat also raises oestrogen production, so weight can both add fat directly and tip the hormonal balance¹.

This is the trap I fell into for years, assuming the whole thing was fat I could train away. Many men, me included, actually have a mix: some fat that does respond to weight loss, and a firm disc of gland behind the nipple that does not. A surgeon feels for that disc directly to tell the two apart, which is the whole point of gynaecomastia versus pseudogynaecomastia. If yours is genuinely fatty, the honest first step is weight, as set out in gynaecomastia surgery versus weight loss, not an operation.

When a lump needs investigating first

Any new lump in the breast or nipple should be seen by a GP, and a firm, one-sided, tender, growing, or discharging lump is investigated before anyone plans cosmetic surgery, because the cause is always established first. The NHS advice is straightforward: see a GP if you have a lump in your breast or nipple³.

For most men this ends in reassurance and a diagnosis of ordinary gynaecomastia, sometimes after an examination, blood tests, or a scan. But the order matters: the cause comes first, the plan second. That was true for me, and it is why the candidacy assessment begins with your history and your hormones rather than a price and a date. Understanding what is driving your chest is not a delay before the real decision; it is the real decision.