Still Tired on Levothyroxine? Reasons and What to Check

Feeling tired on levothyroxine is real, and it often has a findable reason: a dose change that hasn't settled yet (about 6 to 8 weeks), a routine that cuts absorption, or something separate, such as low iron, low vitamin B12, poor sleep or low mood.

You are not imagining it, and it is not a lack of willpower. Each of these is worth raising with your doctor, and most have a clear next step.

Give a recent change time to work

Levothyroxine doesn't switch your energy back on overnight. The prescribing information says it may take several weeks before you notice your symptoms improve, and that TSH is usually checked 6 to 8 weeks after a dose change. The hormone builds up slowly: the ATA guideline says levels generally reach a steady state about 6 weeks after starting.

If your dose changed recently, some tiredness may be the in-between period. Note when your last change was, because that matters when you talk to your doctor.

How strong is the evidence? Strong.

Check how you take it

If less of each pill gets in, symptoms can hang on even when the dose on paper is right. The label lists poor absorption, missed doses and other medicines as possible reasons a dose can seem not to work. The usual culprits:

Too close to food or coffee. The label says to take it on an empty stomach, half an hour to an hour before breakfast. See how long to wait before eating and levothyroxine and coffee.
Too close to iron or calcium. The label says not to take it within 4 hours of iron, calcium or antacids. See iron and levothyroxine timing.
Not the same way every day. The American Thyroid Association says the most important thing is to take it the same way, every day.

None of this means changing your dose. Tidy up the routine, and tell your doctor what you changed: if more of each pill starts getting in, they may want to recheck your TSH. The full levothyroxine routine sets it all out.

How strong is the evidence? Strong.

Other common causes of tiredness

Tiredness has many causes, and plenty have nothing to do with your thyroid dose (NHS). The ATA notes that other health problems can cause the same symptoms as an underactive thyroid (ATA). When your thyroid results are steady but the tiredness stays, it is worth looking wider.

Low iron

Iron deficiency anaemia causes tiredness and lack of energy, and heavy periods and pregnancy are very common causes of it (NHS). A blood test can check it, and it is a sensible one to ask about if tiredness is your main symptom. If your periods are heavy, mention that too.

Low vitamin B12

Low vitamin B12 can cause extreme tiredness and a lack of energy (NHS). It is another blood test to ask about.

Sleep

Sleep apnoea, where your breathing stops and starts while you sleep, causes loud snoring and daytime tiredness (NHS). Low thyroid hormone levels are one of its risk factors (NHLBI). If you snore loudly, someone has seen you stop breathing in your sleep, or you wake up unrefreshed, tell your doctor.

Stress and mood

Stress and depression are common causes of tiredness (NHS). Low mood and anxiety deserve attention in their own right, not only as part of the thyroid picture.

Other conditions

Diabetes, infections and many other conditions can also cause tiredness (NHS). That is why a proper check-up matters, rather than assuming the dose is the only lever.

How strong is the evidence? Strong that low iron, low B12 and depression can cause tiredness. Moderate for the link between an underactive thyroid and sleep apnoea.

What to bring to your doctor

When your dose last changed, and the date of your last blood test.
How you take levothyroxine: the time, the wait before food and coffee, and when you take iron, calcium or other medicines.
Which symptoms bother you most, and how long they have lasted.
Notes on your sleep, stress and mood, and on your periods if they are heavy.
Questions worth asking: "Could we check my iron and vitamin B12?" and "Could my sleep be part of this?"

This turns "I'm still tired" into a clear starting point. Your doctor knows your history and has the final say on any change.

Don't start, stop or change your dose on your own to fix the tiredness. Any change goes through your doctor, backed by a blood test (what TSH measures).

When to get help

Make an appointment if you are still tired despite taking levothyroxine as prescribed, if the tiredness is getting worse or affecting daily life, or if your blood test is overdue.
Contact your doctor if feeling cold, slowed thinking or confusion, or extreme tiredness is severe or getting worse.
Tell your doctor about signs that can mean too much thyroid hormone, which the label lists: a fast or irregular heartbeat, shakiness, nervousness, trouble sleeping, sweating or feeling too hot.
Get urgent medical help for chest pain, a very fast or irregular heartbeat, fainting, or severe shortness of breath.
If you are pregnant or think you might be, contact your doctor as soon as you know. It is time-sensitive, because pregnancy can raise how much levothyroxine you need (prescribing information).
If your mood is low most days, talk to your doctor. If you have thoughts of harming yourself, get help now: call 911 (US) or 999 (UK), call or text 988 in the US, or call Samaritans on 116 123 in the UK.

Sources

AbbVie. SYNTHROID (levothyroxine sodium) tablets, prescribing information, revised February 2024. DailyMed, US National Library of Medicine.
Jonklaas J, et al. Guidelines for the treatment of hypothyroidism: prepared by the American Thyroid Association Task Force on Thyroid Hormone Replacement. Thyroid. 2014;24(12):1670–1751.
American Thyroid Association. Thyroid hormone treatment (patient information).
American Thyroid Association. Hypothyroidism (patient information).
NHS. Tiredness and fatigue. Page last reviewed 2 June 2023.
NHS. Iron deficiency anaemia. Page last reviewed 26 January 2024.
NHS. Vitamin B12 or folate deficiency anaemia. Page last reviewed 20 February 2023.
NHS. Sleep apnoea. Page last reviewed 11 May 2026.
National Heart, Lung, and Blood Institute. Sleep apnea: causes and risk factors. Updated 9 January 2025.

Thyroid Coach keeps your doses, how you feel and your test results in one place, and turns them into a one-page report to take to your appointment.

This is general information, not medical advice. Always follow your own doctor's guidance.