How to read the reference range on your lab report
Every thyroid lab report has two numbers that belong together: your result, and the reference range your lab printed next to it. The range is easy to skip past, but it is the only thing that tells you how to read the result on that particular report.
Where to find it
Look for a column next to your result called "reference range", "reference interval", "normal range" or "expected values". It is usually written as two numbers, for example "0.4 to 4.0". That example is only an illustration. The range printed on your own report is the one that counts.
Some tests only have one limit. Antibody tests are often written as "less than 35" or "<35", with no lower number at all.
Check the units
The unit sits beside the result, and it changes how the number looks.
- TSH is usually reported in mIU/L. You may also see µIU/mL, which gives the same number.
- Free T4 is often reported in ng/dL in the United States and in pmol/L in the UK, Canada and Australia.
- Free T3 is often reported in pg/mL in the United States and in pmol/L elsewhere.
The same blood sample gives very different looking numbers in different units, so a result only makes sense with its unit and its range attached.
What the flags mean
Many reports mark a result with an "H", an "L", an asterisk or bold text. That flag means the result is outside the range that lab uses. It is a prompt for you and your doctor to look at it together. It is not a diagnosis.
What a result means for you depends on your history, your symptoms and your treatment. That is a conversation to have with your doctor.
A habit worth keeping
Each time you record a result, record the range and the unit from the same report. Ranges differ between labs and can change over time, so last year's range may not match this year's.
thyro.health saves each result with its own range for this reason, and describes results in plain terms such as "within your lab's range" without judging them.
This article is for general information only and is not medical advice. Always talk to your doctor about your own results and treatment.