DoctorAIThyroid blood tests measure thyroid-stimulating hormone (TSH) and the thyroid hormones thyroxine (T4) and triiodothyronine (T3). Doctors use them to check whether the thyroid gland is making too little or too much hormone, to look into symptoms such as tiredness or a racing heart, and to monitor treatment.
What thyroid tests measure
Conventional units come first, with SI units in brackets. These are typical adult ranges, and thyroid assays vary between labs, so your report may differ.
| Marker | What it is | Typical adult range | If higher | If lower |
|---|---|---|---|---|
| TSH (thyroid-stimulating hormone) | A signal from the pituitary gland that tells the thyroid how much hormone to make | 0.4 to 4.0 mIU/L | Can suggest an underactive thyroid (hypothyroidism), because the pituitary is pushing harder. It can also mean a thyroid medicine dose is too low. | Can suggest an overactive thyroid (hyperthyroidism) or a thyroid medicine dose that is higher than needed. Less often it reflects a pituitary problem, severe illness or early pregnancy. |
| Free T4 (free thyroxine) | The main hormone made by the thyroid, in its active, unbound form | 0.8 to 1.8 ng/dL (10.3 to 23.2 pmol/L) | Can suggest an overactive thyroid or too much thyroid medicine. | Can suggest an underactive thyroid. Rarely it relates to a pituitary problem or severe illness. |
| Free T3 (free triiodothyronine) | The more active thyroid hormone, in its unbound form | 2.3 to 4.2 pg/mL (3.5 to 6.5 pmol/L) | Can suggest an overactive thyroid. It is often the first hormone to rise in some forms of hyperthyroidism. | Can occur in an underactive thyroid or during severe illness. It is not often needed to check for an underactive thyroid. |
Your doctor may also order thyroid antibodies, such as thyroid peroxidase (TPO) antibodies. These help look for autoimmune causes such as Hashimoto's disease, a common cause of an underactive thyroid, or Graves' disease, a common cause of an overactive one.
Why your lab's range may differ
Free T4 and free T3 are measured with different methods in different labs, and their ranges vary more than most blood tests. Reference ranges for TSH commonly run from 0.4 to 4.5 mIU/L, and the upper limit tends to be a little higher in older adults. During pregnancy, labs and doctors use ranges specific to each trimester. Use the range printed on your report.
What can affect your results
- Biotin (vitamin B7): high-dose supplements, including many hair, skin and nail products, can interfere with some thyroid assays and push results up or down. Ask your doctor or lab whether to pause biotin before the test, and for how long.
- Thyroid medicine timing: if you take levothyroxine, your doctor may suggest taking that day's dose after the blood draw, because free T4 can read higher a few hours after a dose.
- Pregnancy: TSH is usually lower in the first trimester, and ranges change by trimester.
- Illness: a serious illness or hospital stay can temporarily change results without a thyroid problem.
- Medicines and iodine: amiodarone, lithium, steroids, some cancer medicines and iodine-containing contrast dye can affect the thyroid. Calcium and iron supplements can reduce levothyroxine absorption if taken at the same time.
- Time of day: TSH varies over 24 hours and is usually highest at night and early morning.
How the results fit together
- High TSH with low free T4 is the usual pattern of an underactive thyroid.
- High TSH with a normal free T4 is often called subclinical hypothyroidism. Doctors often repeat the test after some weeks before deciding what to do.
- Low TSH with high free T4 or free T3 is the usual pattern of an overactive thyroid.
- Low TSH with normal hormone levels is called subclinical hyperthyroidism and is usually rechecked.
- A low or normal TSH together with a low free T4 can point to a pituitary cause, which needs specialist follow-up.
- An underactive thyroid can raise cholesterol, so doctors sometimes read thyroid tests with a lipid panel. It can also contribute to anemia, which you can see on a CBC.
When to contact a doctor
Thyroid results are best read alongside symptoms, so mention both to your doctor. Contact them if a result is flagged and you have:
- a fast or pounding heartbeat, trembling, unexplained weight loss, heat intolerance or trouble sleeping
- tiredness, weight gain, feeling cold, constipation, dry skin or low mood
- a lump or swelling in the neck, hoarseness or trouble swallowing
Tell your doctor if you are pregnant or planning a pregnancy, since thyroid levels matter for both. Seek urgent care for chest pain, fainting, a very fast or irregular heartbeat, extreme drowsiness or confusion, or a high fever with agitation. In an emergency, call your local emergency number.
Frequently asked questions
What is a normal TSH level?
A typical adult reference range for TSH is about 0.4 to 4.0 mIU/L, though many labs print an upper limit up to 4.5 mIU/L. The range is a little higher in older adults and differs in pregnancy. Compare your result with the range on your own report and ask your doctor what it means for you.
What does a high TSH mean?
A high TSH can suggest that the thyroid is underactive, because the pituitary gland is releasing more TSH to push it. Your doctor usually looks at free T4 and your symptoms too, and may repeat the test, since mildly raised TSH can settle on its own.
What does a low TSH mean?
A low TSH can suggest an overactive thyroid, or that a thyroid medicine dose is higher than needed. It can also occur in early pregnancy, with severe illness, or with some medicines and, less often, with a pituitary problem. Free T4 and free T3 help your doctor sort out the cause.
Do I need to fast for a thyroid test, and does biotin matter?
Fasting is usually not required for TSH and free T4, though check if other tests share the blood draw. Biotin supplements can interfere with some thyroid tests and give misleading results. Tell your doctor about any biotin you take and ask whether and how long to stop before the test.