Thyroid Calculators: TSH Levels, Free T4 & Free T3

Which thyroid tool do you need?

Thyroid blood tests are reported in several different units around the world, and the same result can look very different on two reports. The tools in this section help you read and convert them:

• [TSH levels checker and chart](/tsh-levels) – our main thyroid guide. Enter a TSH result to see where it sits against typical adult ranges, age-adjusted ranges and trimester-specific pregnancy ranges, and what high or low TSH usually means.
• [Free T4 converter](/free-t4-ng-dl-to-pmol-l) – converts free thyroxine between ng/dL (US) and pmol/L (UK, Europe, Australia).
• [Free T3 converter](/free-t3-pg-ml-to-pmol-l) – converts free triiodothyronine between pg/mL and pmol/L.

How the thyroid tests fit together

The thyroid is controlled by a feedback loop. The pituitary gland releases TSH (thyroid-stimulating hormone), which tells the thyroid to make thyroxine (T4) and a smaller amount of triiodothyronine (T3). When thyroid hormone levels are high, the pituitary makes less TSH; when they are low, it makes more.

That is why TSH moves in the opposite direction to thyroid function:

Because TSH responds so strongly to small changes in thyroid hormone, it is usually the first test ordered. Free T4 is added when TSH is abnormal, and free T3 mainly when hyperthyroidism is suspected.

TSH at a glance

A typical adult reference interval for TSH is about 0.4 to 4.0 mIU/L, although each laboratory sets its own range. TSH tends to rise with age, so a mildly raised value in an older adult may be normal for their age. In pregnancy, especially the first trimester, TSH normally falls, and the American Thyroid Association recommends using trimester-specific ranges. The TSH levels page covers all of these in detail, with sources.

Units: why thyroid results look so different

So a free T4 of 1.2 ng/dL is the same as about 15.4 pmol/L, and a free T3 of 3.0 pg/mL is about 4.6 pmol/L. If you have moved between countries or are comparing results from different labs, convert before comparing.

Things that can affect thyroid results

• Biotin supplements can interfere with some laboratory immunoassays and produce falsely abnormal thyroid results. Tell your clinician if you take biotin, which is common in hair, skin and nail products.
• Acute illness can temporarily change TSH and thyroid hormone levels, so results taken during or shortly after a serious illness are often repeated.
• Timing of thyroid medicine relative to the blood test can affect free T4; follow your clinician's instructions about when to take your dose on the day of the test.
• Pregnancy changes thyroid-binding proteins and TSH, which is why separate ranges are used.

What these tools do not do

These pages convert units and explain ranges. They do not diagnose thyroid disease or suggest doses of thyroid medicine. Any decision about treatment – including changes to a levothyroxine dose – should be made with your clinician, who can consider symptoms, examination, antibody tests and the trend in your results.

Related tools

• How to read a blood test – flags, ranges and units on any lab report.
• Hormone unit converters – testosterone, estradiol, cortisol, prolactin and more.
• Lab unit converter – convert any common lab result in one place.