Why am I always tired? Usually not a hidden disease — but low iron is worth checking

Most persistent tiredness is not the sign of a hidden disease. When Dutch researchers followed 571 adults who came to their family doctor with a new complaint of fatigue, only 47 of them — 8.2% — turned out to have clear physical (somatic) disease within the following year, and about half received no diagnosis that explained the tiredness at all [s1]. The one common, genuinely treatable and often-missed physical cause worth ruling out is low iron, which can flatten energy before it ever shows up as anaemia [s2].

That is the honest shape of the problem, and it is almost the opposite of how tiredness is sold online, where the answer is usually a supplement, a "hormone reset" or a food to cut out.

What "always tired" usually turns out to be

In the Dutch cohort, 268 of the 571 patients (46.9%) eventually received at least one diagnosis that could be linked to the fatigue, but these were mostly not diseases [s1]. The largest categories were musculoskeletal problems (19.4%) and psychological problems such as low mood, stress and anxiety (16.5%) [s1]. Clear somatic pathology accounted for the minority already noted, and most diagnoses were not even made at the visit where fatigue was first raised [s1]. The researchers' conclusion is the useful one: only a minority of fatigued patients have serious disease, yet fatigue still deserves attention because so many different, often ordinary, things feed into it [s1].

The everyday drivers sit behind that statistic — too little or poor-quality sleep, low mood, a recent illness, alcohol, and the accumulated load of stress. Sleep is the first place to look, and how much adults actually need is set out in a joint sleep-medicine consensus; waking through the night, and why that is often normal, is covered separately.

The iron exception, and its limits

The best-documented reversible cause hides in plain sight. In a double-blind randomised trial, 144 non-anaemic women aged 18 to 55 with unexplained fatigue were given either 80 mg a day of elemental iron or a placebo for four weeks [s2]. Fatigue fell by 29% in the iron group against 13% on placebo — a difference of 0.95 points on the scale used (95% confidence interval 0.32 to 1.62; P=0.004) [s2]. Crucially, the benefit was confined to women whose ferritin, the marker of iron stores, was at or below 50 micrograms per litre; just over half the group (51%) had a ferritin of 20 or below [s2]. In other words, these women were iron-deficient without being anaemic, and topping up iron helped.

A later trial pushed the same finding harder. PREFER randomised 290 fatigued, iron-deficient women with normal or borderline haemoglobin (at least 115 g/L) to a single intravenous iron infusion or saline [s3]. Fatigue was reduced in 65.3% on iron versus 52.7% on placebo (odds ratio 1.68, 95% CI 1.05 to 2.70; p=0.03), and a halving of the fatigue score was reached by 33.3% versus 16.4% (p<0.001) [s3]. The effect is real but modest, the trials were in women, and iron carries its own harms if taken when it is not needed. Finding low iron is also only half the job: in a man or a postmenopausal woman it can point to bleeding that itself needs investigating, which is why iron is a reason to see a doctor rather than to self-prescribe. Menstrual iron loss is covered in our piece on iron deficiency in menstruating women.

The thyroid people reach for, and what a trial found

"It must be my thyroid" is a common self-diagnosis, and mild, borderline underactivity — subclinical hypothyroidism, where the thyroid-stimulating hormone is raised but thyroid hormone itself is still normal — is genuinely common in older people. Whether treating it lifts tiredness was tested directly. The TRUST trial randomised 737 adults aged 65 or over with persisting subclinical hypothyroidism (TSH 4.60 to 19.99) to levothyroxine or placebo [s4]. After a year there was no benefit on a dedicated Tiredness score (between-group difference 0.4; 95% CI −2.1 to 2.9) or on a Hypothyroid Symptoms score (difference 0.0; 95% CI −2.0 to 2.1), against a threshold of 9 points set as the smallest change a patient would notice [s4]. The authors concluded that levothyroxine provided no apparent benefit in this group [s4]. Overt thyroid disease is a different matter; the specific trial is examined in our report on levothyroxine for subclinical hypothyroidism.

What it means for a reader

Tiredness that is new, persistent, worsening, or paired with other symptoms — breathlessness, unintended weight loss, drenching night sweats, fever, or a lump — belongs with a clinician, not a supplement aisle, because those are the situations in which the small share of serious causes concentrates [s1]. For the far larger group, the evidence points somewhere less dramatic and more demanding: sleep, mood and the ordinary strain of life, with a blood test for iron as the one cheap, high-yield check that a genuine and treatable cause is not being missed [s2]. This article is informational and is not medical advice.

Sources

• Diagnoses during follow-up of patients presenting with fatigue in primary care — Canadian Medical Association Journal, 2009-10-26
• Iron supplementation for unexplained fatigue in non-anaemic women: double blind randomised placebo controlled trial — BMJ, 2003-05-22
• Evaluation of a Single Dose of Ferric Carboxymaltose in Fatigued, Iron-Deficient Women — PREFER — PLoS ONE, 2014-04-21
• Thyroid Hormone Therapy for Older Adults with Subclinical Hypothyroidism — New England Journal of Medicine, 2017-04-03