Low Estradiol in Men: Symptoms, Causes and Effects on Bone and Libido

In men, estradiol (E2) is made mainly outside the testes: the enzyme aromatase converts testosterone (and androstenedione) into estradiol in fat tissue, bone, brain, liver and skin. A smaller share comes directly from the testes. A simple relationship follows: the less testosterone and the less aromatase-active tissue, the less estradiol — and conversely, the more fat tissue, the more aromatization. That is why estradiol in men is usually linked both to testosterone and to BMI.

What does this estradiol do? Three areas are best documented. The first is bone: in young men estrogens are responsible for closure of the growth plates in long bones, and in adult and older men they restrain excessive bone resorption. The second is metabolism and body composition, including the amount of fat tissue. The third is sexual function — estrogens work together with androgens in regulating desire and, to a lesser extent, erections. Estradiol also acts on the central nervous system and blood vessels, but clinical evidence in those areas is much weaker.

The most striking proof that estrogens matter in men comes from congenital aromatase deficiencies (CYP19A1 mutations) and estrogen resistance. The men described are tall, with unfused growth plates, osteopenia and metabolic disturbances despite normal or high testosterone. These are extreme and very rare cases, but they show that testosterone alone does not substitute for estradiol.

Estradiol results are reported in pg/ml or pmol/l. Conversion: 1 pg/ml is about 3.67 pmol/l, so 20 pg/ml is roughly 73 pmol/l. Many laboratories use a male range from a few to several tens of pg/ml (approximately 40–160 pmol/l), but ranges differ by assay manufacturer — so what counts is the range printed on your own result, not a number copied from the internet.

The second issue is the method. Standard immunoassays used by most commercial laboratories were designed for the concentrations found in women. At the very low concentrations typical of men, their accuracy is lower and results from different laboratories can be hard to compare. The gold standard is mass spectrometry (LC-MS/MS), but it is not routinely available. In practice this means a difference between '12 pg/ml' and '18 pg/ml' on an immunoassay may fall within method error and biological variability.

The third issue is context. A man's estradiol changes from day to day, depends on the amount of fat tissue and body weight and — above all — on testosterone. Interpreting a result in isolation from testosterone, LH and SHBG has limited value; how to read the whole panel is covered in separate knowledge-base entries on diagnosing hypogonadism and on interpreting testosterone with SHBG.

To separate the effects of testosterone from those of estradiol, you have to switch them off independently. Researchers at Massachusetts General Hospital did this in a randomized trial published in the New England Journal of Medicine — the most important study on this topic.

The significance of this study is twofold. First, it shows that estradiol is not a 'useless by-product' — its absence in healthy young men produced measurable changes in body composition and sexual function. Second, it has important limitations: it involved healthy men aged 20–50 whose hormones were switched off artificially for 16 weeks. It is not known how far the results transfer to older men with moderately reduced estradiol lasting for years, or what exact concentration threshold is 'safe'.

Bone is the area where the role of estradiol in men has the strongest support. In a Mayo Clinic study of 59 elderly men (mean age 68), endogenous testosterone and estradiol production was suppressed, and the two hormones were then given at physiological doses or withdrawn in different combinations.

A consistent result comes from an interventional trial with an aromatase inhibitor. In a one-year, double-blind, placebo-controlled trial in 69 men aged 60+ with borderline or low testosterone, anastrozole 1 mg daily was given. Testosterone rose (from 319 to 524 ng/dl at 3 months, and to 474 ng/dl at one year), estradiol fell from 15 to 12 pg/ml — and lumbar spine bone mineral density decreased in the anastrozole group compared with placebo (p = 0.0014). The authors concluded that aromatase inhibition does not improve skeletal health in aging men with low or low-normal testosterone.

For completeness: a case report of a man with congenital aromatase deficiency suggested that closure of the growth plates, normalization of bone density and bone turnover markers required a serum estradiol above about 73 pmol/l (about 20 pg/ml). That is one person and no threshold for all men can be built on it, but it indicates the order of magnitude at which bone begins to 'feel' a deficiency.

Libido. In the NEJM trial cited above, the decline in sexual desire depended on the testosterone dose, and estrogen deficiency also contributed to worse sexual function. In clinical practice this means that in a man with low libido and normal testosterone it is worth considering — besides checking sleep, mood, medications and prolactin — whether estradiol is extremely low, especially if he takes an aromatase inhibitor. It does not mean that 'raising estradiol' is the solution: far more often the more helpful step is reducing the dose of the drug suppressing aromatase.

Body fat. According to the NEJM data, estrogen deficiency promotes fat gain. Practical reasoning can be misleading, because fat tissue is also a source of aromatase — obesity usually raises estradiol rather than lowering it. Low estradiol with a large waist circumference is therefore more often a sign of very low testosterone or drugs than of 'ordinary' obesity.

Cholesterol. The frequently repeated claim that low estradiol in men 'ruins the lipid panel' has weak support. In a randomized trial of 74 healthy men (mean age 65) in whom the hypothalamic–pituitary–testicular axis was temporarily switched off, testosterone was then controlled and estradiol varied over a very wide range (1.2–82 pg/ml, measured by mass spectrometry), the extreme estradiol values did not change total cholesterol, LDL, HDL, triglycerides, apoB or Lp(a). However, the hypogonadal state itself — without testosterone and estradiol add-back — was associated with higher LDL. The study lasted only about three weeks, so it does not rule out long-term effects, but it gives no basis for claiming that low estradiol alone 'wrecks' lipids.

Other symptoms attributed to low estradiol — joint pain, 'dry' skin, mood swings, fatigue — are nonspecific and come mostly from clinical observation and forum case reports. They may have many other causes, including low testosterone, hypothyroidism, vitamin D deficiency or simply lack of sleep. They should be treated as a hypothesis to check, not as a diagnosis.

The first item deserves attention. In studies of older men with low testosterone, anastrozole effectively raised testosterone but did not bring the expected clinical benefits: in a one-year trial of 88 men aged 60+, neither body composition nor strength improved, and estradiol fell from about 56 to about 42 pmol/l. Similarly, in a six-month trial in obese men (BMI > 35), low-dose letrozole lowered estradiol from about 119 to about 59 pmol/l and raised testosterone from 8.6 to 21.5 nmol/l, yet no effect on somatic or psychological parameters was shown. Raising testosterone at the expense of estradiol is therefore not a free benefit.

The same principle applies to the opposite mistake: 'raising' estradiol on your own with supplements or drugs. Giving estrogens to men outside strictly defined indications is not standard care and carries its own risks (including thrombotic risk and gynecomastia). If a result is low, the first step is finding the cause, not 'topping up' the hormone.

The limitations of the data described are significant. The best evidence comes from short experiments in healthy men in whom hormones were switched off artificially (which does not reflect years-long mild deficiency), and from studies of markers rather than fractures. Aromatase-inhibitor trials involved older or obese men with low testosterone and give no answer for young healthy men using them in bodybuilding. There are also no randomized trials comparing different target estradiol concentrations in men with hard endpoints such as fractures.

Estradiol in men is a victim of its own image: labelled a 'female hormone', it is sometimes treated as a necessary evil to be reduced. The data show the opposite — it is a hormone required for bone health and part of sexual function, and attempts to lower it artificially have not produced benefits in randomized trials that would justify doing so. The most sensible attitude is to treat low estradiol as a diagnostic clue: we ask why it is low, check testosterone, medications and the measurement method, and only then consider action.