TSH Test: What It Means and the Normal Range
TSH is the hormone the brain uses to instruct the thyroid. A high result usually points to an underactive gland, not an overactive one.
The easiest way to understand TSH is to read it as an instruction. The pituitary gland sends TSH to tell the thyroid to make hormone.
Why the report reads backwards
If the thyroid is underactive, the brain asks louder — so TSH goes up. If the gland is overactive, the demand drops and TSH comes down.
So a high TSH generally signals an underactive thyroid, not an overactive one. Reading the report the other way round is the most common mistake made with it.
The ranges
| Category | TSH (mIU/L) |
|---|---|
| Normal | 0.4 – 4.0 |
| Raised (towards hypo) | Above 4.0 |
| Low (towards hyper) | Below 0.4 |
Ranges vary slightly between laboratories, so always read the range printed on your own report.
Pregnancy uses different, lower ranges. A result inside the ordinary range can still concern a doctor then.
Is TSH alone enough?
For a first check it usually is. If it comes back outside range, doctors generally add Free T4, and sometimes Free T3 and antibody tests, to establish why.
Before the test
- Fasting is not required, though many doctors prefer a morning sample
- If you take a thyroid tablet, take it after the sample, not before
- Biotin supplements can interfere with some assays — tell the laboratory
- Testing at the same time each time keeps results comparable
Frequently asked questions
My TSH is high — does that mean an overactive thyroid?+
Usually the opposite. A high TSH generally means the gland is underactive and the brain is asking for more.
Do I need to fast?+
Not necessarily, though many doctors prefer a morning sample so results stay comparable.
Should I take my thyroid tablet before the test?+
Generally you are asked to take it after the sample. Confirm with your own doctor.
Why is the pregnancy range different?+
Requirements change during pregnancy, so a result inside the ordinary range can still need attention.