Fatigue: causes and which tests to ask for

There is a tiredness that explains itself: a deadline week, a cold, a baby who does not sleep. And there is one that does not: months without energy, you sleep what you can, you had tests done and “everything came back fine”. This text is for the second kind.

The bad news is that fatigue is the least specific symptom there is: it shows up in almost anything, from sleeping too little to a slow thyroid. The good news is that a small number of tests and a two-week log usually sort the problem out enough for the appointment with your doctor to be short and useful.

Why fatigue almost never has a single cause

When someone comes to primary care with fatigue lasting over a month, UK primary care guidance (NICE) recommends a fairly stable first block: complete blood count, inflammation markers, kidney, liver, thyroid function and glucose or HbA1c, with ferritin when iron deficiency is suspected.^[1] Not because the doctor suspects all of those at once, but because fatigue does not let you choose: very different causes produce the same feeling.

Add to that the causes that never show up in any blood test: chronically sleeping under seven hours, sustained stress, low mood, medicines that cause drowsiness, an infection your body is still recovering from. In practice, two or three factors are usually stacked, and fixing just one is not enough.

First, what is not a test: sleep, schedule and recovery

Before ordering anything, it is worth answering one honest question: how many hours do you actually sleep? The consensus of the sleep medicine societies is that an adult needs seven hours or more on a regular basis; below six, daytime tiredness, trouble concentrating and low mood are the body’s expected response, not a mystery.^[4]

If you wear a watch or a band, you already have data worth gold for this question: sleep hours per night, resting heart rate and how they change after an effort. A resting heart rate that climbs a few beats and takes days to return to your usual, after a hard workout or a cold, is a sign that the body is still recovering even if you already want to be at full speed.

• Real sleep hours for two weeks, not the ones you think you sleep.
• Resting heart rate every morning: what matters is your trend, not a number from a table.
• How many days you take to return to your normal after something that pulls you out of routine.
• Caffeine and alcohol: how much and at what time. Both fragment sleep even when they do not wake you.

The tests that are usually checked first

This is the block most doctors order as a starting point when fatigue lasts more than a month with no obvious cause.^[1] It does not replace your doctor’s judgement, which will adapt the list to your history; it helps you know what each test is looking at.

Iron and ferritin: the part a blood count does not see

Haemoglobin is the last thing to drop when iron is missing. The stores empty first, and that is what ferritin measures. The World Health Organization sets the cut-off for depleted stores in a healthy adult at 15 µg/L, and reminds us that ferritin rises with inflammation, so a “normal” value during an infection can hide a deficiency.^[2] In clinical practice many doctors treat any value below 30 µg/L as suggestive of deficiency.^[7] That is why ferritin should be ordered together with the blood count rather than afterwards, and why someone with heavy periods can be exhausted with an impeccable haemoglobin.

In practice, the symptoms that most often accompany low ferritin are tiring with little effort, breathlessness on stairs, dizziness on standing quickly, brittle nails and hair shedding.^[7] If you recognise yourself, the useful log is simple: the heaviness and length of your last periods if you menstruate, and how much effort now costs what used to cost none.

Thyroid: TSH answers the first question

A thyroid working slowly produces a very characteristic tiredness: even, lasting months, not improved by sleeping more. It usually comes with feeling cold when others do not, dry skin, constipation, weight going up without changing habits and low mood.^[3] The two entry tests are TSH and free T4;^[3] TSH is usually ordered first and, if it is off, your doctor will decide whether to add free T4 and antibodies. Ordering a “full thyroid panel” up front rarely adds more than a well-interpreted TSH.

Glucose and insulin: tiredness after meals

If the crash always comes after meals, with mid-afternoon sleepiness and sugar cravings, the question is how your body handles sugar. Two very common tests describe it: fasting glucose and HbA1c, which summarises the average of the last two or three months. The American Diabetes Association criteria place the intermediate zone at fasting glucose of 100 to 125 mg/dL or HbA1c of 5.7 to 6.4 %; above 126 mg/dL or 6.5 % on two occasions they speak of diabetes.^[6] A person can spend years in the intermediate zone feeling tired without anyone explaining why, because each value on its own “is still fine”.

Vitamin B12 and vitamin D

B12 enters the list when there is tingling, memory or mood changes, a low-meat diet, long-term use of acid-reducing medicines or metformin, or stomach surgery.^[5] Vitamin D is often ordered for fatigue; the evidence that supplementing it relieves tiredness in people without a deficiency is weak, so its value lies in ruling out a clear deficiency, not in explaining every exhaustion.

When fatigue should not wait

Everything above is for a bothersome but stable tiredness. Some signs change the priority and call for a prompt appointment: unintended weight loss, fever that comes and goes, night sweats, breathlessness at rest or when talking, chest pain, bleeding, growing lymph nodes, or fatigue that worsens week by week. None of them is solved by a log or by a test you order on your own.

How to log fatigue so the appointment is useful

“I am tired” is a sentence a doctor hears twenty times a day and it tells them almost nothing. “Six weeks of fatigue at 7 out of 10 that worsens after meals, I sleep 6 hours and take four days to recover from a workout” is different: half the work is already done.

1. Every night, a number from 0 to 10 for the day’s fatigue. Two weeks are enough to see a pattern.
2. Sleep hours, from your watch or written down. And the time you actually go to bed.
3. How you feel an hour after eating: the same, worse, sleepy. One word is enough.
4. Times of day when fatigue is worst and what happened before.
5. If you menstruate, the cycle day and how heavy the period is.
6. Medicines and supplements you take, including those you do not think of as medicines.

With that log and the basic block of tests, the first appointment usually ends with a working hypothesis instead of “let’s see”. And if everything comes back normal, the log keeps being useful: it is what lets you compare in three months whether anything changed.