Fatigue is one of the best-known complaints of an underactive thyroid, but far from the only explanation for tiredness. An underactive thyroid often also makes you feel cold, slow and low, and sometimes goes together with weight gain. If your fatigue instead comes mainly with paleness and breathlessness, iron deficiency is more likely. A few blood values make the distinction.
The tricky part is that tiredness has so many causes. Below you will read when your thyroid is likely involved, and when you should look elsewhere.
How does an underactive thyroid cause fatigue?
In an underactive thyroid the gland makes too little hormone, which slows your metabolism. Your cells get less of a "fuel signal", and you feel that as persistent tiredness that does not lift with rest (Chaker et al., 2017). The fatigue rarely comes alone.
Typical of thyroid-related tiredness is the combination with other underactive-thyroid complaints. For the full picture, see our explanation of underactive thyroid symptoms in women.
Which signs point to the thyroid?
A few accompanying complaints make a thyroid cause more likely. They fit a slowed system, not ordinary tiredness from a busy spell.
- Feeling cold quickly, especially cold hands and feet
- Weight gain without eating differently
- Dry skin, brittle hair or hair loss
- Constipation and slow digestion
- A low mood and foggy thinking
- Sometimes a swollen feeling in the neck, also under stress
If you notice hair loss alongside your tiredness, read also which hormones to test with hair loss.
Thyroid, or something else?
Fatigue often has a cause other than the thyroid. The table below helps you tell the most common candidates apart. It remains an aid, not a diagnosis.
| Possible cause | What fits with it | Which value |
|---|---|---|
| Underactive thyroid | Cold, weight gain, slow thinking | TSH + free T4 |
| Iron deficiency | Paleness, breathlessness, palpitations on exertion | Ferritin + Hb |
| Menopause | Hot flushes, poor sleep, cycle change | FSH + oestradiol |
| Vitamin B12 or D deficiency | Tingling, muscle weakness, low mood | B12 + vitamin D |
Because the causes overlap, a broader check is sometimes more useful than one value. Fatigue from iron deficiency, for example, calls for an iron status rather than a thyroid test.
When should you test your thyroid for fatigue?
If your fatigue lasts longer than a few weeks, or you recognise several underactive-thyroid complaints, a thyroid test is a logical step. The Thyroid Function measures TSH and free T4, without a GP referral.
If nothing points to the thyroid, it is worth looking more broadly. This article belongs to our overview of thyroid symptoms in women.
Frequently asked questions
Can stress swell my thyroid?
Stress does not cause thyroid disease, but a swollen or lumpy feeling in the neck under stress is common without a thyroid problem. If the complaints persist, or a visible swelling appears, have it checked.
Does an underactive thyroid cause restless legs?
Restless legs are more often associated with iron deficiency than with the thyroid. Still, an underactive thyroid can contribute to a general feeling of sluggishness. A blood test helps separate the cause.
How long until I feel better after treatment?
That differs per person and depends on the cause and the treatment. Discuss a realistic expectation with your GP; a thyroid test is the starting point, not the end point.
References
- NHG-Standaard Schildklieraandoeningen (M31). Nederlands Huisartsen Genootschap. Available via nhg.org.
- Chaker L, Bianco AC, Jonklaas J, Peeters RP. Hypothyroidism. Lancet. 2017;390(10101):1550-1562. PMID: 28336049.
- Taylor PN, Albrecht D, Scholz A, et al. Global epidemiology of hyperthyroidism and hypothyroidism. Nat Rev Endocrinol. 2018;14(5):301-316. PMID: 29569621.
Every blood test result through Lunara includes a professional assessment by a BIG-registered doctor. For treatment decisions, discuss your results with your GP.
Author
