Thyroid Test Normal Ranges
Thyroid test results come as numbers, and each number is compared against a reference range to decide whether it is normal, high or low. Understanding what those ranges mean helps you read your own report.
This page explains normal ranges for the main thyroid tests and why a result must always be interpreted in context.
Last medically reviewed: 15 November 2025
The main ranges
TSH is typically reported against a reference range of roughly 0.4 to 4.0 mIU/L, though this varies slightly between laboratories. Free T4 and free T3 have their own ranges, also laboratory-specific.
A result inside the range is described as normal; above or below it is described as high or low.
Why ranges differ
Different laboratories use different analysers, methods and population data, so their reference ranges are not identical. This is why comparing a result from one laboratory directly with the range from another can be misleading.
Reference ranges also shift in pregnancy, which uses trimester-specific targets, and can be influenced by age and certain medications.
Normal is not always "fine"
A normal TSH with abnormal free T4 — or the reverse — can still indicate disease, such as subclinical thyroid dysfunction. Similarly, a result just inside the range may be abnormal for you personally if your usual level is different.
This is why symptoms, examination and the pattern of results together matter more than any single number.
What to do with your result
Bring your report to a clinician who can interpret it alongside your history. If a result is borderline or puzzling, a repeat test or an additional test often clarifies the picture.
Frequently asked questions
What is a normal TSH level?
TSH is commonly reported against a range of about 0.4 to 4.0 mIU/L, but the exact range is set by each laboratory and can vary slightly.
Why is my result normal but I still feel unwell?
A normal result does not always mean everything is fine — symptoms can have other causes, and the pattern of several results matters more than one number. Discuss this with your clinician.
Can normal ranges change?
Yes — ranges differ between laboratories and change during pregnancy. Always interpret your result against the range printed on your own report.
References & further reading
This page is based on established medical knowledge and guidelines from authoritative sources. Always consult a qualified clinician for personal medical advice.
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