TSH levels and what they say about your thyroid
Last updated: 24 September 2026
TSH tracker
Add your results to see them against the typical range. They are saved in this browser only — nothing is sent anywhere.
TSH stands for thyroid stimulating hormone, and the confusing part is that it is not made by your thyroid. The pituitary makes it, to tell the thyroid how hard to work. So TSH is the instruction, not the output.
That is why the result reads backwards from what people expect. A high TSH means the pituitary is shouting because thyroid hormone is low — an underactive thyroid. A low TSH means it has gone quiet because there is plenty — an overactive one.
Typical TSH ranges
The upper cut-off is genuinely debated, and labs differ between 4.0, 4.5 and 5.0 mIU/L. Pregnancy uses different and lower ranges, and the normal range drifts upward with age.
| Result | Commonly reported range | Usually suggests |
|---|---|---|
| Low | below 0.4 mIU/L | Overactive thyroid (hyperthyroidism) |
| Normal | about 0.4–4.0 mIU/L | Thyroid functioning as expected |
| Mildly raised | about 4.0–10 mIU/L | Often subclinical hypothyroidism |
| High | above 10 mIU/L | Underactive thyroid (hypothyroidism) |
Why TSH alone is not the whole answer
A raised TSH with a normal free T4 is called subclinical hypothyroidism. It is common, it often causes no symptoms, and a good proportion of cases return to normal on their own — which is why a single mildly raised TSH is usually repeated a few weeks or months later rather than treated immediately.
TSH also moves for reasons unrelated to thyroid disease. It is highest overnight and in the early morning and lowest in the afternoon, so the time of your blood draw shifts the number. Recent illness, some medications including biotin supplements, and steroids all interfere.
What usually gets checked next
- Free T4, the hormone the thyroid actually produces, which separates subclinical from overt.
- Free T3, in some cases, particularly when hyperthyroidism is suspected.
- Thyroid peroxidase antibodies (TPO), which point to Hashimoto’s as the cause of an underactive thyroid.
- A repeat TSH after a few weeks, because single abnormal results frequently normalise.
Symptoms that tend to go with each direction
An underactive thyroid classically causes fatigue, cold intolerance, weight gain, dry skin, constipation and low mood. An overactive one tends toward weight loss, heat intolerance, palpitations, tremor, anxiety and loose stools.
Both lists overlap heavily with ordinary life, which is exactly why the bloods matter — and why treating a borderline number without symptoms or a repeat test is generally avoided.
Tracking it over time
Thyroid results are a case where your own history beats the reference range. A TSH of 3.8 is inside most lab ranges, and means something different if your previous four results clustered around 1.2. Keeping the values together with their dates is what makes that visible.
Common questions
Is a TSH of 4.9 bad?
It is mildly above most lab ranges and, on its own, rarely means anything is urgently wrong. The usual next steps are a free T4, thyroid antibodies, and a repeat test after a few weeks, because a single mildly raised result often settles by itself.
Does a high TSH mean an overactive thyroid?
No, the opposite. High TSH means the pituitary is pushing a thyroid that is underproducing, so it points to an underactive thyroid.
Does the time of day matter?
Yes. TSH peaks overnight and in the early morning and is lowest in the afternoon, so results can differ meaningfully depending on when blood was drawn. Testing at a consistent time makes your results easier to compare.
Keep your results in one place
Dosely stores every blood test you have had, checks each marker against its reference range, and charts how it moves between tests — which is what turns a single number into something you can read. Free to use, no sign-up screen.