Aromatase Inhibitors on TRT: What the Trials Actually Show

In online forums the aromatase inhibitor is treated as standard equipment: take testosterone, add an aromatase inhibitor, "just to be safe". In medical practice it is the exception. Not out of excess caution, but because the question has been studied.

The trials are unusually clear. They show that an aromatase inhibitor pushes the testosterone level up. And they show that this is exactly why it is so easy to be misled by one.

What an aromatase inhibitor does

The enzyme aromatase turns part of your testosterone into estradiol. An aromatase inhibitor blocks that enzyme. Two things follow: estradiol drops, and testosterone climbs, partly because less of it is converted and partly because the brain receives a weaker signal to hold production back.

On a lab report that looks like a win. The question that matters is whether the man is any better off.

The head-to-head trial

One randomized, double-blind trial put the two routes side by side. Forty-three men aged 65 to 82, average age 71, with total testosterone below 350 ng/dL (about 12 nmol/L) were assigned to one of three groups for twelve months: testosterone, an aromatase inhibitor, or placebo [1]. Measurements were taken at three, six and twelve months, and the primary outcome was bone density in the lumbar spine.

Both active treatments lifted testosterone above 500 ng/dL (about 17 nmol/L), and the level held steady for the whole trial. So far, a draw. Then the results part ways:

• Lumbar spine bone density improved only with testosterone.
• Fast walking speed improved only with testosterone.
• Knee strength improved with both treatments.
• Lean body mass increased only with the aromatase inhibitor, by an average of 1.24 kg at twelve months.

The authors' conclusion is the single most important sentence on this subject: converting testosterone to estradiol is required to maintain bone density in older men [1]. Block the conversion and you raise the testosterone number while depriving bone of what it needs.

The authors describe their work as a proof-of-concept study, and they flag the walking-speed result as new and in need of confirmation [1]. The bone finding, by contrast, fits everything else that is known about estradiol's role in men.

What happens to bone

A second randomized, double-blind trial, run in Boston, looked at bone on its own. Sixty-nine men aged 60 and over with borderline or low testosterone and symptoms of deficiency took an aromatase inhibitor or placebo for one year [2].

By the end of the year testosterone had eased slightly to 474 ng/dL (about 16 nmol/L), while estradiol stayed at its lowered level. The gap in spine bone density between the groups was statistically clear. Other skeletal sites moved in the same direction without reaching significance, and blood markers of bone turnover did not change [2].

The authors' verdict: aromatase inhibition raises testosterone, lowers estradiol and appears to reduce bone density. It does not improve skeletal health in aging men with low or low-normal testosterone [2].

A note on units. American trials report testosterone in ng/dL and estradiol in pg/mL. Swiss laboratories usually report nmol/L and pmol/L, which is why the table shows both. If you want to place your own results, the reference ranges are on our lab values page. Keep in mind that each laboratory sets its own limits, depending on the method it uses.

What happens to symptoms

That leaves the obvious objection. Perhaps bone pays the price, but symptoms improve in return? A larger trial answered exactly that.

Across 70 sites in Europe and the United States, 2,103 men were screened and 271 enrolled; 81 of them did not complete the trial. To qualify, men needed a body mass index between 30 and 50, a morning testosterone below 10.41 nmol/L and at least two deficiency symptoms, one of them sexual dysfunction. On average, BMI was 38.1 and testosterone 7.97 nmol/L. Participants received a newer aromatase inhibitor, still under investigation at the time, at one of three strengths, or placebo, for 24 weeks [3].

Judged by the lab value alone, the treatment looked like a success. Testosterone normalized in at least three out of four men in every treatment group, reaching averages of 15.89, 17.78 and 20.35 nmol/L against 8.04 nmol/L on placebo. LH and FSH, the pituitary hormones that signal the testes, rose markedly [3].

Then comes the line that decides it: no improvements in body composition or sexual dysfunction were observed [3].

Side effects that showed up more often than on placebo included raised hematocrit, high blood pressure, rising PSA and headache. Lumbar spine bone density fell by 1.24 to 2.09 percent, while it rose by 0.66 percent on placebo. Hip bone density did not change [3].

Put simply: the number on the report improved. The man did not.

Why this is no accident

Estradiol is not a contaminant in male physiology. It is part of the machinery. A landmark study in the New England Journal of Medicine showed how. Four hundred healthy men aged 20 to 50 received a drug that temporarily shut down their own hormone production. Testosterone was then replaced at different levels, and half of the men also received an aromatase inhibitor that blocked conversion to estradiol [4]. That design made it possible to separate which effect depends on which hormone.

The result after 16 weeks:

A second finding from the same study matters just as much in practice: the amount of testosterone a man needed to preserve muscle, fat mass, strength and sexual function varied widely from one man to the next [4]. That is one reason testosterone therapy is set individually by a physician rather than by formula.

Remove the estradiol and you remove part of what therapy was started for in the first place. The full picture is in estradiol in men.

That is also why an aromatase inhibitor has no place as a permanent add-on. It works against the goal of treatment.

These trials also suggest how an estradiol level pushed too low can show itself: fading sex drive despite a good testosterone value and creeping body fat [4], and over the longer term a loss of bone density [2]. If you notice changes like these while on an aromatase inhibitor, the problem may be too little estradiol rather than too little testosterone. That belongs in a conversation with your treating physician.

Three situations that need their own assessment

Three situations regularly raise questions about estradiol in men with low testosterone. In all three, a medical assessment comes first, not an additional medicine, and none of it is something to take into your own hands.

Breast symptoms during therapy. Breast tenderness and enlargement of glandular breast tissue (gynecomastia) are recognized side effects of testosterone therapy; our article on TRT side effects gives an overview. The first step here is assessment. Is it glandular tissue or fat? How high is estradiol, and which method measured it? Is the testosterone therapy set appropriately? Body weight belongs on that list as a possible cause. What follows from these findings is for the physician to decide.

Wanting children. Here the starting point is different. The Endocrine Society guideline recommends against starting testosterone therapy in men who plan to father children in the near term [5]. Testosterone taken from outside suppresses, via feedback, the LH and FSH signals the testes need to make sperm. Which route suits a man who wants children is a matter for medical assessment; TRT and fertility gives an overview.

Marked obesity. This is precisely the group studied in the largest of the three aromatase inhibitor trials in men with low testosterone [3]. Its results argue for restraint: lab values normalized, body composition and sexual function did not. In severe obesity, the lever that moves testosterone and estradiol together remains weight loss.

The rule across all three situations: assess first and measure properly, rather than acting on a hunch.

More inhibition is not better, and measuring is hard

Two points where things most often go wrong in everyday practice.

How hard the enzyme is blocked. A lower estradiol is not a goal in itself. In the trial in men with obesity, lumbar spine bone loss rose with the strength of treatment: 1.24 percent in the weakest group, 1.30 in the middle group and 2.09 percent in the strongest [3]. No group saw an improvement in body composition or sexual function. Stronger inhibition meant more bone loss and no gain in symptoms.

How estradiol is measured. Estradiol is hard to measure accurately in men. The standard immunoassay was built for the much higher concentrations found in women, and in the low male range it tends to read high because related compounds get picked up too. Mass spectrometry is more reliable at those levels, but not every laboratory offers it. A treatment decision based on a single immunoassay result may be treating a measurement artifact rather than a real finding. There is more on this in estradiol in men.

The practical consequence for every check-up: measure estradiol at the same laboratory with the same method each time, alongside testosterone. For the initial diagnosis, the Endocrine Society guideline recommends measuring testosterone in the morning, fasting, and confirming a low result with a repeat test [5]. Once you are on therapy, your physician decides when the blood should be drawn. Keep a note of your symptoms in parallel. One value is a snapshot; the trend is what tells you something.

When estradiol rises on therapy

If your estradiol rises on therapy, there is a sensible order of steps:

1. Confirm the finding. A single high value, possibly from an immunoassay, is not yet a finding. A repeat measurement at the same laboratory shows whether it is consistently raised.
2. Have your physician review how your testosterone therapy is set. More testosterone means more raw material for conversion. The details of your therapy are decided individually by your physician; do not change anything on your own. Hematocrit also responds to how therapy is set and is reviewed at the same time.
3. Tackle body weight. Fat tissue is the main site of aromatase activity, and this lever moves both values at once.
4. Judge symptoms and trend together. Whether anything needs doing at all is for your physician to decide, based on symptoms and the trend, not on a single value.

An elevated estradiol without symptoms is not a reason to treat. The Endocrine Society guideline applies the same principle to testosterone itself: deficiency is diagnosed only when symptoms coincide with unequivocally and consistently low levels [5]. The patient is treated, not the lab value. It is telling, too, what the guideline's standardized monitoring plan covers: symptoms, side effects, testosterone, hematocrit and prostate cancer risk [5].

What the trials do not answer

The direction of the evidence is clear. Its scope is limited, and an honest summary says so.

• The two trials in older men enrolled 43 and 69 participants and each ran for one year [1][2].
• The trial in men with obesity lasted 24 weeks, and almost one in three participants dropped out [3].
• All of them measured bone density. None counted fractures, the outcome that ultimately matters.
• None of the three trials in older men and men with obesity tested an aromatase inhibitor added to ongoing testosterone replacement therapy.

The closest to that situation is the study in healthy men that gave testosterone with and without an aromatase inhibitor, and there the gain in body fat was driven mainly by the lack of estradiol [4]. So a benefit from the combination has not been shown, while the downsides of blocked conversion are measurable. As long as that remains the case, restraint is the sensible position.

How the assessment works at Swiss TRT

At Swiss TRT the process starts with questions and measurements, not with a prescription:

1. The online self-test records your symptoms and gives a first indication of whether low testosterone is likely.
2. A fasting morning blood draw at a partner laboratory near you provides testosterone and the other values needed for a proper assessment. Which ones those are is covered in blood work for TRT.
3. In a video consultation, a physician goes through your values, symptoms and history, rules out contraindications, and decides with you whether and how to treat.
4. Once on therapy, regular check-ups follow. If estradiol turns out high, it is interpreted in context: your symptoms, the measurement method and the trend over time.

If you are unsure whether your symptoms point to a hormonal cause, the self-test is the simplest place to start. Questions about your estradiol level and your symptoms belong in a conversation with a physician, which telemedicine lets you have from home, anywhere in Switzerland.

FAQ

Do I need an aromatase inhibitor on TRT?

No, not as a precaution. An aromatase inhibitor is not part of standard testosterone therapy. A raised estradiol without symptoms is not a reason to treat, and if symptoms do appear, the first step is a medical review of your current therapy.

Can an aromatase inhibitor replace testosterone?

It does raise the testosterone value, in one comparative trial as much as testosterone itself [1]. But bone density improved only on testosterone [1], and in another trial it actually fell on an aromatase inhibitor [2]. In 271 men with obesity, body composition and sexual function did not improve despite normalized levels [3].

Does an aromatase inhibitor harm bone?

The trials point that way. In a one-year trial, spine bone density fell on an aromatase inhibitor while it rose on placebo [2]. In a second trial it fell by up to 2.09 percent, depending on the strength of treatment [3]. Male bone needs estradiol.

My estradiol is high but I have no symptoms. What should I do?

As a rule, recheck before treating. A repeat measurement at the same laboratory makes sense, ideally with an accurate method. If the value stays high, your physician reviews how your testosterone therapy is set. What counts are the symptoms, not a single lab value.

I want to have children. Does that change things?

Yes, the starting point is different. The Endocrine Society guideline advises against starting testosterone therapy if you plan to father children in the near term [5]. Testosterone from outside the body dampens LH and FSH, the signals the testes need to make sperm. Which route fits your situation is settled in a medical assessment, and TRT and fertility covers what to consider in detail.

How is estradiol measured reliably in men?

Most accurately by mass spectrometry. The widely used immunoassay was designed for the higher levels found in women and tends to read high in the low male range. For tracking over time, what matters most is measuring at the same laboratory with the same method every time.

Further Reading

• Estradiol in men: why the hormone matters
• TRT and fertility: what men need to know
• Hematocrit and TRT: when the number matters