Tests guide

Thyroid Function Tests (TSH)

Thyroid function tests are blood tests that show how well the thyroid gland is working. The TSH test is usually the first one done, often followed by free T4 and sometimes T3.

Last updated 3 sources

At a Glance

  • The thyroid is a gland in the neck that makes hormones controlling metabolism, energy, and many body functions.
  • TSH (thyroid-stimulating hormone) comes from the pituitary gland and signals the thyroid to make hormone.
  • TSH is commonly the first screening test; free T4 and T3 add more detail.
  • In general, a high TSH is associated with an underactive thyroid and a low TSH with an overactive thyroid.
  • Reference ranges vary by lab, age, and pregnancy, and a clinician interprets individual results.

What It Is

The thyroid makes two main hormones: thyroxine (T4) and triiodothyronine (T3). The pituitary gland in the brain monitors these levels and releases TSH to prompt the thyroid to make more when needed.

This feedback loop is why TSH often moves in the opposite direction of thyroid hormones. When the thyroid is underactive (hypothyroidism), TSH tends to rise; when it is overactive (hyperthyroidism), TSH tends to fall.

Antibody tests, such as thyroid peroxidase (TPO) antibodies, can help identify autoimmune thyroid conditions like Hashimoto thyroiditis or Graves disease.

Common Terminology

TSH
Thyroid-stimulating hormone, made by the pituitary gland to regulate the thyroid.
Free T4
The portion of thyroxine in the blood that is not bound to proteins and is available to the body.
T3
Triiodothyronine, the more active thyroid hormone, much of which is made from T4 in body tissues.
Hypothyroidism
A condition in which the thyroid does not make enough hormone.
Hyperthyroidism
A condition in which the thyroid makes too much hormone.
TPO antibodies
Antibodies against an enzyme in the thyroid, often found in autoimmune thyroid disease.
Subclinical
A pattern where TSH is outside the reference range but free T4 is within it.

Why the Test Is Done

Thyroid tests are ordered for several reasons.

  • To look into symptoms that can be related to the thyroid, such as tiredness, weight changes, feeling cold or hot, a fast heartbeat, or changes in mood, skin, or hair.
  • To check for a goiter (enlarged thyroid) or thyroid nodules.
  • To monitor people being treated for thyroid conditions.
  • As part of screening in certain groups, such as newborns and sometimes during pregnancy.
  • To evaluate abnormal results from other tests, such as high cholesterol or irregular heart rhythm.

What the Results Generally Describe

General patterns are described below for reference. Reference ranges vary by lab and method (a commonly cited adult TSH range is about 0.4 to 4.0 mIU/L, though many labs use different limits), ranges differ in pregnancy and older age, and a clinician interprets individual results.

  • High TSH with low free T4 is a pattern generally associated with hypothyroidism.
  • Low TSH with high free T4 or T3 is a pattern generally associated with hyperthyroidism.
  • TSH outside the range with normal free T4 is often described as subclinical hypothyroidism or hyperthyroidism.
  • Less common patterns can occur with pituitary conditions, recent illness, or certain medications.
  • Positive TPO antibodies suggest an autoimmune cause but do not by themselves show how well the thyroid is working.

How the Test Is Done

Thyroid tests use a blood sample.

  • Blood is usually drawn from a vein in the arm.
  • Newborn screening uses a few drops of blood from a heel prick.
  • TSH may be tested alone first, with free T4 or T3 added if TSH is outside the range ("reflex" testing).
  • Imaging, such as a thyroid ultrasound, is a separate test sometimes used alongside blood tests.

Preparation and What to Expect

Little preparation is generally needed.

  • Fasting is not usually required for thyroid tests alone.
  • Biotin (vitamin B7) in high amounts, found in some hair, skin, and nail supplements, can interfere with some thyroid tests, so labs and clinicians often ask about supplement use.
  • Some medications, pregnancy, and recent serious illness can affect results.
  • For people taking thyroid hormone, the timing of the blood draw may be discussed with the care team.
  • The blood draw takes a few minutes and may leave a small bruise.

When Results Are Discussed With a Clinician

Thyroid results are generally reviewed with symptoms, history, and other tests.

  • TSH outside the range is often rechecked, sometimes along with free T4, before conclusions are drawn.
  • Mildly abnormal results are commonly followed over time.
  • Results during pregnancy are generally interpreted using pregnancy-specific ranges.
  • For people with thyroid conditions, results are periodically reviewed as part of ongoing care.

Questions to Ask a Healthcare Professional

General questions people often find useful to bring to an appointment:

  • Which thyroid tests were included, and why?
  • What reference range does this lab use for TSH?
  • Could supplements, medications, or recent illness affect these results?
  • Would antibody testing or imaging add useful information?
  • How often are thyroid tests generally repeated?
  • How are thyroid results interpreted during pregnancy?

Sources

This guide was written in plain language from the following authoritative references.

  1. TSH (Thyroid-Stimulating Hormone) Test — MedlinePlus (U.S. National Library of Medicine)
  2. Thyroid Tests — MedlinePlus (U.S. National Library of Medicine)
  3. Thyroid Tests — National Institute of Diabetes and Digestive and Kidney Diseases (NIH)

Last updated October 8, 2026. FreeDoc guides are general educational information and are not a substitute for care from a qualified professional.