This is the biggie: a syndrome defined by the existence of ongoing, dramatic, and life-impacting fatigue. Chronic fatigue syndrome is defined not only as exhaustion, but as heightened levels of fatigue after ordinary activities or exercise, leading to a massive "crash" where you can't get out of bed or move at all for tiredness. It's a long-lasting illness, but fatigue isn't the only symptom necessary for diagnosis; doctors will also enquire about sleep, pain levels, appetite, brain function and a host of other issues.

The thyroid is a gland in the body that produces hormones to regulate the body's metabolism and the health of its cells. However, it can both under- and over-perform; a sluggish thyroid is known as hypothyroidism, while an overactive one is called hyperthyroidism. Both problems can produce fatigue, and are serious enough to require medical assistance and treatment. Hypothyroidism is particularly notorious for causing fatigue, and it's sometimes mistaken by older women as a normal symptom of the menopause, rather than an individual issue. Thyroid issues are pretty easy to test (doctors just look for the levels of thyroid hormones in the blood), and treatments are comprehensive.

Lyme disease is normally caused by a tick bite, and often causes a distinctive bulls-eye rash around the bite itself. It can be treated by antibiotics if caught quickly, but it can also manifest as chronic lyme disease, which does not have one prescribed treatment. There are a host of long-term effects that can develop from having Lyme, and serious, dragging fatigue is one of them. The fatigue may be the only symptom you notice, but if you've been in a tick-heavy area (even if you didn't notice any bites at the time), mention that to your doctor when you go checked out, as you may need to have a Lyme test.

Iron-deficiency anemia, which means that the body doesn't have sufficient iron levels in the blood, is well-known for causing tiredness and exhaustion. The low levels of iron mean that the blood is less capable of carrying oxygen around the body, leading to an absence of energy. If you're a woman who experiences heavy periods, this may be the first question on a GP's list when you come in complaining of fatigue symptoms, as the two are known to be linked.

This is a slightly lesser-known cause of fatigue, most commonly experienced in pregnant women, people who have gained weight quickly, diabetics, or people who regularly wear extremely heavy clothing around their legs. It refers to a nerve problem; the nerve that goes to the surface of the outer thigh is compressed, resulting in nerve tingling and pain on the skin. Without that symptom, it's exceptionally unlikely that this is your issue, but if you do, it's important to note, as you may be suffering from damage that could become permanent without intervention.

Perhaps you don't immediately connect inflammatory bowel disease (IBD) with fatigue, but the two have a long-established relationship. The pain and disturbed sleep of IBD, combined with a tendency towards anemia in sufferers, seem to contribute to a high risk of fatigue, but it also shows up in ways that we're still trying to understand. Fortunately, fatigue is not the only symptom of IBD and its related syndromes, so it may prove easier to get a diagnosis when you see your medical specialist.

What we now know as "gluten intolerance," celiac disease, the body's chronic inability to digest gluten of any kind, can cause fatigue because you just aren't getting the nutrients you need. The intestinal damage caused by attempting to digest gluten in celiacs means that they may become malnourished, leading to fatigue. If there's a history of celiac disease in your family or you've noticed any symptoms after eating gluten, ask to be tested.

Heart conditions of any kind can lead to tiredness, as the body's restricted ability to pump oxygen-rich blood through organs and limbs results in impaired energy levels. Fatigue is particularly linked to heart failure and issues with the heart's valves, and is considered a pretty serious medical emergency — but women who have heart attacks are much less likely to experience chest pain and much more likely to complain of excessive tiredness and shortness of breath, which is an important thing to be aware of.

Depression has a long-established relationship with fatigue (fatigue raises your likelihood of depression by three times), partially through sleep disturbance, but also through other issues. It's a mistake to think that depression is "all in the mind", as it's well-known to also show physical symptoms in some people, and fatigue can be one of them. Your doctor will do a comprehensive study of your mood over the last few months and any traumatic or saddening events in your life before they reach any conclusions about suffering from depression.