Short answer: Thyroid blood tests check how the thyroid gland works. TSH (thyroid-stimulating hormone) comes from the pituitary gland and tells the thyroid how much hormone to make; free T4 and free T3 are the thyroid hormones themselves. TSH is usually the first test, and free T4 (sometimes free T3) is added to understand the result.
At a glance
| TSH | From the pituitary; tells the thyroid how much hormone to make; usually the first test |
|---|---|
| Free T4 and free T3 | The thyroid hormones themselves |
| Units | mIU/L (TSH); ng/dL or pmol/L (free T4); pg/mL or pmol/L (free T3) |
| LOINC | 3016-3 (TSH), 3024-7 (free T4), 3051-0 (free T3) |
| Moves with | Time of day, illness, pregnancy, some medicines, biotin supplements |
| Keep | All three with the date and the dose of any thyroid medicine |
What the test measures
The pituitary and the thyroid form a feedback loop: when thyroid hormone is low, the pituitary raises TSH; when it is high, TSH falls. That is why TSH and free T4 are read together. "Free" means the part of the hormone not bound to proteins in the blood, the part that is active.
Thyroid antibodies (for example anti-TPO) are a separate test, ordered when a doctor looks for an autoimmune cause.
When it is ordered
Common reasons, which your doctor adapts to you:
- when symptoms such as tiredness, weight change, palpitations or feeling cold or hot need checking;
- as part of some routine checks, and in pregnancy in some countries;
- to adjust and follow thyroid medication.
How the result is written on the report
| Result on the report | LOINC code | Usual unit | Also seen as |
|---|---|---|---|
| TSH | 3016-3 | mIU/L | µIU/mL (same number) |
| Free T4 | 3024-7 | ng/dL | pmol/L (×12.87) |
| Free T3 | 3051-0 | pg/mL | pmol/L (×1.536) |
TSH is written in mIU/L or µIU/mL, which are the same value. Free T4 appears as ng/dL in the US and pmol/L in Europe: pmol/L = ng/dL × 12.87 (1.2 ng/dL ≈ 15.4 pmol/L). Free T3 in pg/mL converts with pmol/L = pg/mL × 1.536.
Reading your report
Reference ranges vary between laboratories and methods, and they are different in pregnancy and in older age. TSH can change with illness, some medicines and biotin supplements, which interfere with many assays; tell the lab if you take biotin. If you take thyroid medication, the timing of the dose relative to the blood test matters: ask your doctor.
Keeping the results in your record
Scan or photograph the report in Anpheros Daily: each value is read with its unit and the laboratory's reference range, you check it, and it is stored with its LOINC code as an HL7 FHIR Observation, a format used by many hospitals and national health systems. Results from different laboratories line up on one chart over time, and the lab unit converter helps when two reports use different units. Our guide on how to read lab results explains reference ranges and flags in general; the lab results guide for a new doctor shows how to share them.
What this page does not do
It explains what the test is and how the result is written. It does not interpret your result, diagnose or recommend treatment: the meaning of a value depends on your history, your other results and how the sample was taken, and that is your doctor's job.
Related pages
Sources
Frequently asked questions
Why is TSH high when the thyroid is underactive?
Because the pituitary raises TSH to push a slow thyroid to make more hormone. A low thyroid hormone level and a high TSH go together.
Is mIU/L the same as µIU/mL for TSH?
Yes, they are equal, so the number does not change.
How do I convert free T4 from ng/dL to pmol/L?
Multiply by 12.87. For example, 1.0 ng/dL is about 12.9 pmol/L.
Can supplements affect thyroid tests?
Yes. Biotin (vitamin B7), common in hair and nail supplements, can distort several thyroid assays. Tell your doctor or lab before the test.
Try it in Anpheros Daily
Photograph your lab report and Anpheros Daily lines up each value with its reference range, so you can compare reports from different laboratories.
How it works in the app →
Anpheros builds a medical record that the patient controls and can share with any clinic, on an HL7 FHIR R4 platform, based in the EU. These guides help you understand and organise your health; they do not replace your doctor.
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