Your thyroid is a small gland in the neck. Its hormones set how fast your body uses energy. Thyroid tests check whether it is making too little or too much.
TSH comes from the pituitary gland in the brain and tells the thyroid how hard to work. That is why TSH usually moves in the opposite direction to the thyroid hormones.
Every lab sets its own normal range, based on its equipment and the people it tests. Always compare your result with the range printed on your own report.
Why this test is done
- Tiredness, weight change, feeling cold or hot, a fast heartbeat
- Checking thyroid medicine
- Pregnancy planning, or a family history of thyroid disease
What each result means
TSH(thyroid-stimulating hormone)
The signal from the brain to the thyroid.
Typical adult range: Roughly 0.4 to 4.0 mIU/L in adults; labs differ, and ranges change in pregnancy.
If high: Often means the thyroid is underactive (hypothyroidism), because the brain is pushing it harder.
If low: Often means the thyroid is overactive (hyperthyroidism), or that the dose of thyroid medicine is too high.
Free T4(free thyroxine, FT4)
The main hormone the thyroid makes, the part free to act in the body.
Typical adult range: Roughly 0.8 to 1.8 ng/dL (about 10 to 23 pmol/L).
If high: With a low TSH, points to an overactive thyroid.
If low: With a high TSH, points to an underactive thyroid.
Free T3(FT3)
The more active thyroid hormone, made mostly from T4.
Typical adult range: Roughly 2.3 to 4.2 pg/mL (about 3.5 to 6.5 pmol/L).
If high: Mostly checked when an overactive thyroid is suspected.
Thyroid antibodies(TPO antibodies)
Antibodies that can attack the thyroid.
Typical adult range: Reported as negative or below the lab's cut-off.
If high: A positive result suggests an autoimmune cause such as Hashimoto's thyroiditis or Graves' disease.
Before the test
- Biotin (vitamin B7) supplements can upset some thyroid tests. Tell the lab or stop them for 2 days before if your doctor agrees.
- If you take levothyroxine, ask whether to take it before or after the blood test.
Questions to ask your doctor
- Does this point to an underactive or overactive thyroid, or neither?
- Should the test be repeated before deciding anything?
- Do I need antibody tests or a scan?
- If I take thyroid medicine, is my dose right?
When to get help fast: Get urgent care for a very fast or irregular heartbeat, high fever with confusion, or extreme drowsiness and feeling very cold. In an emergency, call your local emergency number.
Sources
- MedlinePlus: TSH (Thyroid-stimulating hormone) Test
- MedlinePlus: Thyroxine (T4) Test
- NIDDK: Thyroid Tests
This guide is general information written in plain words. It is not medical advice and has not been reviewed by a doctor for your situation. Read our medical disclaimer.