How to use this tool
- Choose adult or pregnancy ranges.
- Enter TSH and free T4 from your report, and free T3 if it was tested. Pick pmol/L if your lab uses it.
- Adjust the limits to your lab's printed range and read the pattern shown.
How TSH and free T4 work together
The pituitary gland in the brain releases TSH to tell the thyroid to make hormone. When thyroid hormone is low, the pituitary pushes TSH up; when it is high, TSH falls. That is why a high TSH usually points to an underactive thyroid and a low TSH to an overactive one. Free T4 confirms the picture, and free T3 is sometimes added when an overactive thyroid is suspected.
Common patterns
| TSH | Free T4 | Usually called |
|---|---|---|
| High | Low | Underactive thyroid (hypothyroidism) |
| High | Normal | Subclinical hypothyroidism |
| Low | High | Overactive thyroid (hyperthyroidism) |
| Low | Normal | Subclinical hyperthyroidism |
| Normal | Normal | Normal thyroid function |
Units on your report
TSH is reported in mIU/L, which is the same number as µIU/mL. Free T4 is given in ng/dL in the US and much of South Asia, and in pmol/L in the UK, Europe and the Gulf; multiply ng/dL by 12.87 to get pmol/L. Free T3 in pg/mL multiplied by 1.536 gives pmol/L.
Things that can change thyroid results
Biotin supplements (often in hair and nail products) can interfere with many thyroid assays, so labs advise stopping them for a few days before testing. Recent illness, steroids, amiodarone, lithium and pregnancy all change thyroid results. If you take thyroxine, your doctor may ask you to have blood taken before your morning dose.
Frequently asked questions
What is a normal TSH level?
About 0.4–4.0 mIU/L in most adult reference ranges, though labs vary. Pregnancy uses lower, trimester-specific limits.
What does high TSH with normal T4 mean?
It is called subclinical hypothyroidism. It is common and is often rechecked before any treatment is considered.
How do I convert free T4 from ng/dL to pmol/L?
Multiply by 12.87. For example, 1.2 ng/dL is about 15.4 pmol/L.
Can biotin affect thyroid tests?
Yes. High-dose biotin can give falsely abnormal results on some assays. Tell the lab or stop it for a few days before testing if your doctor agrees.
Sources and references
- MedlinePlus: TSH (Thyroid-stimulating hormone) Test
- American Thyroid Association: Thyroid Function Tests
- NICE NG145: Thyroid disease, assessment and management
- ATA 2017 guidelines for thyroid disease during pregnancy