Experts said that while many people go to their GP with new symptoms of fatigue, working out what is causing it can be tricky.
The study examined 237 diseases and found that conditions such as depression, respiratory tract infection, insomnia and thyroid problems (in women) were commonly linked to fatigue, but varied by patient age and sex.
Cancer was the disease with the fourth highest excess risk in men aged 80 with fatigue compared with men without fatigue, but was only 13th highest in women of the same age.
Researchers said doctors should prioritise investigating cancer in men aged 70 and over who complain of tiredness, "but not in women at any age, if based solely on the presence of fatigue".
Writing in the British Journal of General Practice (BJGP), they added: "These findings provide GPs with information about risk of a comprehensive range of diagnoses in patients with fatigue, and their relative likelihood, so they can prioritise which tests and referral routes to consider."
The study, which included researchers from University College London (UCL), examined data for 304,914 people going to their GP with fatigue and 423,671 without fatigue who acted as a control group.
In total, 127 of 237 diseases studied were more common in men who had fatigue than in men who did not, while 151 were more common in women.
Claire Knight, senior health information manager at Cancer Research UK, said: "There are over 200 types of cancer with lots of different possible symptoms and it's impossible to know them all. Like fatigue, some symptoms can be general and aren't specific to an area of the body.
"You know your body best, so it's important to get your doctor's advice if you notice anything that's not normal for you or isn't going away. Don't ignore it or put it down to 'just getting older'.
"It probably won't be cancer. But if it is, spotting it at an early stage means that treatment is more likely to be successful."
It comes as a separate small study from UCL found that the short-term boost to the brain from exercising may last into the following day.
The study, published in the International Journal of Behavioral Nutrition and Physical Activity, found that on average people aged 50 to 83 who did more moderate to vigorous physical activity than usual did better in memory tests the following day.
Less time spent sitting and six hours or more of sleep were also linked to better scores, researchers said.
They looked at data from 76 men and women who wore activity trackers for eight days and took cognitive tests each day.
Lead author Dr Mikaela Bloomberg, of the UCL Institute of Epidemiology & Health Care, said: "Our findings suggest that the short-term memory benefits of physical activity may last longer than previously thought, possibly to the next day instead of just the few hours after exercise.
"Getting more sleep, particularly deep sleep, seems to add to this memory improvement.
"Moderate or vigorous activity means anything that gets your heart rate up - this could be brisk walking, dancing or walking up a few flights of stairs. It doesn't have to be structured exercise.
"This was a small study and so it needs to be replicated with a larger sample of participants before we can be certain about the results."