Many men remain undiagnosed, partly because of a lack of knowledge and a reticence to discuss hormonal issues. “Women are happier to talk about their menopause or sexual health with each other or their doctor, but hormones are not something men discuss,” says Hackett.

GPs might diagnose depression, but fail to check testosterone first

Another obstacle is that many GPs lack specialist knowledge about testosterone. Dr Channa Jayasena, a specialist in reproductive endocrinology at Imperial College London, believes that while many healthy men seek unnecessary testosterone treatment, some men with genuine hormonal deficiencies struggle to get diagnosed.

One major long-term challenge for GPs is that symptoms of low testosterone are quite vague and confusingly overlap with those of other health issues. According to the NHS, symptoms include fatigue, depression, anxiety, irritability, low libido, erectile dysfunction, reduced strength, night sweats, poor concentration or memory and needing to shave less often. As a result, many GPs diagnose men with depression or stress, without ever checking their testosterone levels.

The three key signs of low testosterone

However, the European Male Ageing study has identified three robust clues to low testosterone: loss of morning and evening erections, low libido and erectile dysfunction. “The study took 3000 men, and asked them questions about every symptom under the sun,” says Jayasena. “And hardly any symptoms were related to testosterone, apart from those three.” While there are many reasons why a man could be tired or depressed, testosterone is a sex hormone, so these sexual symptoms offer the most specific and accurate signs of low testosterone.

1. Loss of morning erections

A loss of spontaneous erections in the morning or evening is the strongest sign of low testosterone. “Erections can be triggered any time by emotion or arousal, but morning and evening erections should exist even in a man who isn’t in a sexual relationship, because it’s a physiological protective mechanism,” says Hackett. “This is nature’s way of keeping the [penis] tissues perfused. Morning erections are how the body preserves sexual function, even when sex is not possible. And testosterone is usually highest in the morning, which is why a loss of morning erections, in particular, is a very strong sign of low testosterone.”

2. Erectile dysfunction

Erectile dysfunction is another key warning sign. “Testosterone has important actions on the system that operates the penis and allows erections,” says Jayasena. “So when you lose testosterone, you lose that response.”

3. Low libido

A dwindling sexual desire is also a vital clue. “Testosterone controls libido in men, which is a mysterious biological process,” says Jayasena. “Testosterone is a key cog in that machine, not just for making the penis work, but also for your brain and wanting to have sex.” A man’s libido will naturally decline with age, and sexual desire can be affected by stress or relationship issues, but a long-term disinterest in sex that affects your current relationship or prevents you finding a new one could signify a hormonal disruption.

How to get your testosterone checked

If you have these sexual symptoms, speak to your GP and ask for a testosterone check. “If you have symptoms, it is entirely fair to get a health check,” says Jayasena. A “normal” testosterone level typically ranges from 8-31 nanomoles per litre of blood, depending on your age and health. Most endocrinologists adhere to the “enough is enough” theory: if your testosterone is within the normal range, variation within this range doesn’t matter much.

Some medical bodies also suggest men with conditions linked to low testosterone, such as infertility, diabetes, obesity, osteoporosis and unexplained anaemia, should be screened too. This doesn’t always happen in practice, so be prepared to raise the question with your GP or consultant. “Two-thirds of men with diabetes have erectile dysfunction, but although you put that issue on the nurses’ templates when they do their routine checks, I have watched some skip over it because they’re going out of their comfort zone,” says Hackett.

The dangers of unnecessary testosterone

A serious caveat is that healthy men who seek out unnecessary testosterone treatment, whether in pursuit of a muscular physique or youthful vigour, will switch off their natural testosterone production. This can cause infertility, shrunken testicles and heart problems. “Many private clinics will prescribe it for men who don’t need it,” says Jayasena.

Furthermore, after the age of 40, a man’s testosterone levels will drop, slowly and naturally, by around 1% per year, with no serious harm to their health. This is part of the natural ageing process. Only if your levels dip below the normal range does it become a medical issue.

If you are one of the 2.1% of men who have medically low testosterone, you may be prescribed testosterone injections or transdermal patches. But if you are among the 20% of men who have sub-optimal testosterone levels – low but not low enough to warrant treatment – scientific evidence confirms that you can reverse the trend with positive lifestyle changes. “For most men, this is largely in your own hands,” says Hackett.

How to protect your testosterone

Lose weight

An American study found that every one-point increase in a man’s body mass index (BMI) is linked to a 2% drop in testosterone. “Fat works against you by making oestrogen which stops your testicles working properly,” says Jayasena. Gynecomastia – the medical term for “man boobs” – is one sign of this imbalance of oestrogen and testosterone. But research proves that overweight men who lose 7.8kg can slash their risk of low testosterone by almost 50%.

Lift dumbbells

Lifting weights three days a week will elevate your testosterone levels after just four weeks, according to a Spanish study. Big compound multi-muscle movements like squats and deadlifts seem to be particularly effective, according to Pennsylvania State University research.

Run or cycle three times a week

Japanese researchers discovered that men who do aerobic exercise for up to 90 minutes, three times a week, gain a rise in testosterone after 12 weeks.

Follow the Mediterranean diet

Switching to a healthy Mediterranean diet – high in fruit, veg, whole grains, fish, organic meat, green leafy veg and healthy fats from olive oil, avocados and nuts, and low in refined carbs, dairy and saturated fats – helps to protect a man’s testosterone levels, according to an Italian research paper.

Limit beer to six pints a week

Don’t break the NHS guidelines of 14 units of alcohol (six pints of 4% beer) per week. Australian research found that heavy drinking causes hormonal disruption, inflammation and oxidative stress, leading to a drop in testosterone.

Get seven to eight hours of sleep

Poor sleep releases the stress hormone cortisol, which blocks testosterone production. Research in the journal Andrology found that for men aged 41-64 a healthy target is seven to eight hours per night. Sleeping for less or more than this is linked to low testosterone.

Take vitamin D

An Austrian study confirmed that vitamin D supplementation boosts testosterone levels. Aim for 100mg (4000 IU) per day, especially over winter. Most pharmacies sell rapid vitamin D tests, offering results in 10 minutes.