"Tired all the time" is one of the most common things people bring to a GP. Very often the cause is not in the blood at all. The NHS lists not sleeping enough, stress, an unhealthy diet, little exercise, hormonal changes such as the menopause, and recent illness among the usual causes. Still, when tiredness has gone on for a while without a clear reason, a tiredness blood test is a reasonable step. This guide explains what such a test looks at, and why iron, thyroid, vitamin B12 and vitamin D come up together.
What a tiredness blood test covers
The Tiredness/Fatigue Profile in the BPR catalogue was designed by London Medical Laboratory (LML) to look for physical clues in the blood. It includes:
- a full blood count: haemoglobin, red and white cells, platelets and red cell size;
- iron studies: serum iron, ferritin, transferrin saturation and iron-binding capacity;
- thyroid function: TSH and free T4;
- vitamin D (25-OH).
Because it includes a full blood count, the profile needs a phlebotomy sample rather than a finger-prick. LML reports results one day after the sample reaches the laboratory.
This is close to what GPs request in practice. An audit of nearly 17,000 primary care requests for tiredness in Scotland found that a full blood count was requested in 89% of cases and TSH in 80%, while ferritin was requested in only 9.4%, even though it was the second most likely of the first-line tests to come back outside the range (Handjiev et al., 2025).
Iron and ferritin
Iron is needed to make haemoglobin, which carries oxygen in red blood cells. The NHS lists tiredness and lack of energy, shortness of breath, noticeable heartbeats, paler skin and headaches among the symptoms of iron deficiency anaemia. A review in The Lancet adds that iron deficiency particularly affects children and premenopausal women, and that problems can appear even before anaemia shows on the blood count (Pasricha et al., 2021).
That is why the blood count and the iron studies are read together. Ferritin reflects iron stores; serum iron is the amount in the blood at that moment; transferrin saturation shows how much of the carrying capacity is in use. If you only want this part, the Iron Status Profile covers the iron markers and, according to LML, can be done on a finger-prick sample.
Thyroid
An underactive thyroid can bring tiredness, dry skin, thinning hair, weight gain, intolerance of cold and low mood, as LML lists in its description of the Thyroid Function test. The NHS lists tiredness among the features of an overactive thyroid as well, together with restlessness and muscle weakness. TSH and free T4, both in the fatigue profile, are the first two markers usually checked.
Vitamin B12 and folate
The fatigue profile does not include B12 or folate, so they are worth knowing about separately. The NHS explains that a lack of either can cause extreme tiredness, lack of energy, pins and needles, a sore red tongue and problems with memory, and that some of these can happen even without anaemia. The Vitamin B12 and Folate (Vitamin B9) tests measure each one. LML notes that B12 deserves particular attention for people on a plant-based diet.
