Hormone therapy being taken by thousands of men could increase the risk of a dangerous heart rhythm problem that can cause a stroke, scientists have warned.
Researchers say taking testosterone can cause atrial fibrillation if the hormone reaches abnormally high levels.
The condition, which causes the heart to beat irregularly, can lead to heart failure and a fatal stroke.
American scientists made the discovery after reviewing evidence from existing studies linking testosterone therapy and atrial fibrillation.
So-called testosterone replacement therapy (TRT) works by supplementing testosterone, the main male sex hormone, in men whose bodies do not produce enough of it – medically known as hypogonadism.
Although sex hormone levels decline with age, it is estimated that only two to five percent of men suffer from a clinical deficiency.
Very low levels can cause fatigue, bad mood, concentration and memory problems and a reduced sex drive.
In recent years, however, the therapy has become popular among men without a diagnosis of hypogonadism, amid claims that it can increase energy levels and improve exercise performance.
Researchers say testosterone replacement therapy (TRT) can cause atrial fibrillation if not tailored to a man’s individual needs
This has led to some men seeking treatment privately, rather than through the NHS, where TRT is only prescribed to people who meet specific criteria.
Experts have warned about this trend, noting worrying side effects such as weight gain and prostate enlargement.
Now researchers at the Kansas City Veterans Affairs Medical Center in the US have discovered that this can also put patients at risk for heart problems.
The team found that men with very high levels are at greater risk of developing heart disease, along with men with abnormally low levels.
Men with testosterone levels in the middle of the normal range are at the lowest risk, she added.
Scientists say the findings suggest TRT may need to be personalized for each patient.
“Both too little and too much testosterone appear to increase the risk of atrial fibrillation…which means dose and target level matter,” says co-author Rajat Barua, chief of interventional cardiology at the Kansas City VA Medical Center.
‘What the review claims is that treatment should be individualized and controlled.’
There are many ways to take TRT, and some of them cause testosterone levels to fluctuate more than others.
The first is testosterone gel, which is rubbed on the skin every day and gradually absorbed into the bloodstream.
In the meantime, injections are given every few weeks, releasing a large first dose of testosterone.
Levels may rise sharply initially but gradually fall before the next dose, causing dramatic fluctuations.
In contrast, longer-acting injections release testosterone more slowly.
They can be administered every 10 to 14 weeks and provide stable levels for longer.
The researchers suggested that men at increased risk for heart rhythm problems may be better suited to this, more gradual, type of TRT.
Testosterone levels decline as men age, by about one to two percent after age 30.
But having lower levels with age does not automatically mean someone has hypogonadism or needs TRT.
It is thought that thousands of men have been given TRT in private clinics, some of whom are not deficient.
Researchers in the new review said doctors could assess TRT patients for risk factors for atrial fibrillation in simple ways.
This includes monitoring for obesity, sleep apnea, hypertension, and alcohol consumption before initiating TRT.
Patients should also regularly monitor their heart rhythm and blood pressure to ensure the treatment remains safe, they said.
The authors emphasized that their conclusions are based on a review of existing evidence, and that their findings are open to interpretation.
