Thyroid function tests are a group of blood tests used to evaluate how well the thyroid gland is working. The thyroid is a small butterfly-shaped gland in the front of the neck that produces hormones controlling how the body uses energy. When the thyroid is producing too much or too little hormone, almost every organ system in the body can be affected, often in subtle or easily mistaken ways.
This article explains what each thyroid blood test measures, what abnormal results may mean, and how the tests are used together to diagnose and monitor thyroid conditions.
The reference range that applies to your result is the one printed on your laboratory report, not the typical ranges shown here. Reference ranges vary between laboratories based on the equipment used, the population tested, and individual factors such as age, sex, and pregnancy status. Always compare your result to the reference range printed on your own report, and discuss any abnormal result with your doctor.
How the thyroid works
The thyroid produces two main hormones: thyroxine (T4) and triiodothyronine (T3). These hormones are released into the bloodstream and travel throughout the body, where they regulate metabolism, heart rate, body temperature, growth, and many other functions.
The thyroid does not work on its own. Its activity is controlled by the pituitary gland, a small gland at the base of the brain. The pituitary releases thyroid stimulating hormone (TSH), which tells the thyroid how much T4 and T3 to make:
- When thyroid hormone levels in the blood are low, the pituitary releases more TSH to stimulate the thyroid.
- When thyroid hormone levels are high, the pituitary releases less TSH to slow the thyroid down.
This feedback loop is the key to understanding thyroid function tests. Because the pituitary is so sensitive to small changes in thyroid hormone, TSH is usually the first test to change when something is wrong with the thyroid — often before T4 and T3 fall outside their reference ranges.
Why are thyroid function tests done?
Thyroid function tests are ordered for many reasons:
- To investigate symptoms. Symptoms of an overactive thyroid (hyperthyroidism) include unexplained weight loss, rapid or irregular heartbeat, anxiety, tremor, heat intolerance, and frequent bowel movements. Symptoms of an underactive thyroid (hypothyroidism) include fatigue, weight gain, cold intolerance, dry skin, constipation, depression, and slowed thinking.
- For routine screening. TSH is included in many routine blood test panels, particularly in women, older adults, and people with risk factors for thyroid disease.
- During pregnancy and pre-pregnancy planning. Thyroid function affects fertility and is critical to fetal development. Many guidelines recommend TSH testing during pregnancy and in women planning to conceive.
- To monitor known thyroid disease. Patients with hypothyroidism, hyperthyroidism, or treated thyroid cancer have regular thyroid testing to monitor treatment.
- To monitor medications. Some medications — including lithium, amiodarone, interferon, and certain immunotherapies — can affect thyroid function and require monitoring.
- To evaluate a thyroid nodule. When a lump is found in the thyroid, blood tests are part of the workup to determine whether the nodule is producing excess hormone.
How are the tests performed?
Thyroid function tests are performed on a small sample of blood drawn from a vein in the arm. No fasting is required. TSH levels follow a small daily rhythm — slightly higher overnight and slightly lower in the afternoon — but this variation is usually not clinically important. If you are taking thyroid hormone replacement medication, follow your doctor’s instructions on whether to take it before or after the blood draw; the timing can affect free T4 results.
The thyroid blood tests
Thyroid stimulating hormone (TSH)
TSH is the most useful single thyroid function test and is almost always the first one ordered. Because the pituitary releases TSH in response to thyroid hormone levels, TSH levels move in the opposite direction of thyroid hormone activity:
- High TSH usually means the thyroid is underactive (hypothyroidism). The pituitary is sending strong signals to the thyroid because thyroid hormone levels are too low.
- Low TSH usually means the thyroid is overactive (hyperthyroidism). The pituitary is suppressing TSH production because thyroid hormone levels are too high.
A typical reference range for adults is approximately 0.4–4.0 milli-international units per litre (mIU/L). The reference range varies somewhat by laboratory, age, and pregnancy status. In pregnancy, the upper limit is usually lower, particularly during the first trimester.
Causes of high TSH (suggesting hypothyroidism):
- Hashimoto thyroiditis (chronic lymphocytic thyroiditis), an autoimmune condition that is the most common cause of hypothyroidism in countries with adequate iodine
- Iodine deficiency (rare in countries with iodized salt)
- Previous thyroid surgery or radioactive iodine treatment
- Certain medications, including lithium and amiodarone
- Insufficient thyroid hormone replacement in patients already being treated for hypothyroidism
- Subclinical hypothyroidism (mildly elevated TSH with normal free T4) — a common finding that may or may not require treatment
Causes of low TSH (suggesting hyperthyroidism):
- Graves disease, an autoimmune condition that is the most common cause of hyperthyroidism, often associated with diffuse papillary hyperplasia in the thyroid
- Thyroid nodules that produce hormone independently of TSH control (toxic adenoma or toxic multinodular goiter)
- Thyroiditis, in which inflammation causes stored hormone to leak out, often after pregnancy or a viral illness
- Excess thyroid hormone replacement
- Pituitary disease that reduces TSH production (rare)
- Subclinical hyperthyroidism (mildly low TSH with normal free T4)
Free thyroxine (free T4 or FT4)
Most of the T4 in the blood is bound to carrier proteins and is not biologically active. Only the small unbound fraction — called free T4 — is available to enter cells and have an effect. For this reason, free T4 is the more clinically useful measurement than total T4, which includes both the bound and unbound fractions.
A typical reference range for adults is approximately 0.7–1.8 nanograms per decilitre (ng/dL).
Free T4 is usually ordered together with TSH or as a follow-up when TSH is abnormal. The combination of TSH and free T4 results allows the type and severity of thyroid disease to be classified:
- High TSH, low free T4: overt hypothyroidism — the thyroid is significantly underactive and treatment is usually recommended.
- High TSH, normal free T4: subclinical hypothyroidism — the thyroid is starting to fail but is keeping hormone levels in the normal range. Whether to treat depends on how high the TSH is, the patient’s symptoms, and other factors.
- Low TSH, high free T4 (or high free T3): overt hyperthyroidism — the thyroid is significantly overactive and treatment is usually recommended.
- Low TSH, normal free T4 and free T3: subclinical hyperthyroidism — early or mild overactivity that may or may not need treatment.
- Normal TSH, normal free T4: normal thyroid function.
Total thyroxine (total T4)
Total T4 measures both the protein-bound and free forms of T4. It was once a primary thyroid test but has been largely replaced by free T4 because total T4 is significantly affected by changes in the carrier proteins, which can shift up or down without any actual change in thyroid function.
A typical reference range for adults is approximately 4.5–11.2 micrograms per decilitre (mcg/dL). Total T4 may still be ordered in specific clinical situations.
Triiodothyronine (T3) and free triiodothyronine (free T3 or FT3)
T3 is the more biologically active of the two thyroid hormones. Most T3 in the body is produced by converting T4 to T3 within tissues, but the thyroid also produces a small amount of T3 directly.
Like T4, T3 in the blood is mostly bound to carrier proteins, with only a small free fraction available to enter cells. Free T3 measures the biologically active fraction and is generally preferred over total T3.
Typical reference ranges for adults:
- Total T3: approximately 80–200 ng/dL
- Free T3: approximately 2.3–4.2 picograms per millilitre (pg/mL)
T3 testing is most useful in suspected hyperthyroidism, where T3 levels can rise significantly. In hypothyroidism, T3 levels often remain in the normal range until the condition is severe, so T3 is less useful for diagnosing an underactive thyroid.
A specific pattern called T3 toxicosis — a low TSH and high T3 with a normal free T4 — can occur in some forms of hyperthyroidism and can only be detected if T3 is measured.
Thyroid antibodies
Several blood tests measure antibodies the body may produce against its own thyroid tissue. These antibodies are markers of autoimmune thyroid disease and help identify the underlying cause of an abnormal thyroid function test.
- Thyroid peroxidase antibodies (anti-TPO). Elevated levels are common in Hashimoto thyroiditis and Graves disease. Anti-TPO can also be elevated in healthy individuals, particularly women, and a positive result alone does not mean a person has thyroid disease.
- Thyroglobulin antibodies (anti-Tg). Elevated levels are also seen in autoimmune thyroid disease, often alongside anti-TPO. Anti-Tg testing is also important when monitoring patients with thyroid cancer because the antibodies can interfere with thyroglobulin level interpretation.
- TSH receptor antibodies (TRAb). Highly specific for Graves disease. These antibodies bind to the TSH receptor on thyroid cells and stimulate excess hormone production. A positive TRAb in someone with hyperthyroidism essentially confirms the diagnosis of Graves disease.
Thyroglobulin (Tg)
Thyroglobulin is a protein produced only by thyroid cells. It is not part of routine thyroid function testing but is critically important in monitoring patients who have been treated for differentiated thyroid cancer (papillary or follicular). After complete removal of the thyroid (and usually radioactive iodine treatment), thyroglobulin levels should be very low or undetectable. A rising thyroglobulin level after treatment may suggest residual or recurrent thyroid cancer.
Thyroglobulin antibodies (anti-Tg) must always be measured at the same time, because their presence can falsely lower thyroglobulin results and make interpretation unreliable.
For more information on thyroid cancer pathology, see papillary thyroid carcinoma and our thyroid gland diagnosis guides.
Calcitonin
Calcitonin is a hormone produced by specialized cells in the thyroid called C-cells (or parafollicular cells). It is not part of routine thyroid function testing but is the most important blood marker for medullary thyroid carcinoma, a relatively uncommon thyroid cancer that arises from C-cells. Elevated calcitonin levels in someone with a thyroid nodule strongly suggest medullary thyroid carcinoma, and calcitonin is also used to monitor patients after treatment for this cancer.
Calcitonin testing is not generally done as part of the workup for hyperthyroidism or hypothyroidism — it is reserved for specific situations involving suspected or confirmed medullary thyroid carcinoma, often together with carcinoembryonic antigen (CEA) testing.
How thyroid tests work together: common patterns
The combination of TSH, free T4, free T3, and antibody results helps identify the type of thyroid problem:
- Primary hypothyroidism: high TSH, low free T4, often low free T3. If anti-TPO antibodies are positive, Hashimoto thyroiditis is the most likely cause.
- Subclinical hypothyroidism: high TSH, normal free T4, normal free T3.
- Graves disease (hyperthyroidism): low TSH, high free T4, high free T3. TRAb is typically positive.
- Toxic nodule or toxic multinodular goiter: low TSH, high free T4 or T3, with negative TRAb.
- Thyroiditis (transient hyperthyroidism from inflammation): low TSH and high thyroid hormones, sometimes followed weeks or months later by hypothyroidism. Antibodies depend on the underlying cause.
- T3 toxicosis: low TSH, normal or only slightly elevated free T4, but high free T3.
- Central hypothyroidism (rare): low or inappropriately normal TSH with low free T4. The problem is in the pituitary or the brain rather than in the thyroid itself.
- Sick euthyroid syndrome (non-thyroidal illness): abnormal thyroid tests in someone with a serious non-thyroid illness. The thyroid itself is usually working normally; the abnormalities reflect the body’s response to illness and typically normalize with recovery.
What happens after thyroid function testing?
If your thyroid tests are within reference ranges and you are not on thyroid medication, no further investigation is usually needed. If a result is abnormal, the next steps depend on the type and severity of the abnormality:
- Repeat the test. Mildly abnormal TSH levels often normalize on repeat testing, particularly in patients with recent illness. A repeat test in 4–8 weeks is a common next step.
- Add free T4 and free T3. If only TSH was initially measured, free T4 (and sometimes free T3) is added to clarify the type and severity of thyroid disease.
- Test thyroid antibodies. Anti-TPO and TRAb help identify autoimmune causes of thyroid dysfunction.
- Imaging. A thyroid ultrasound is the standard imaging test and can identify nodules, inflammation patterns, and structural abnormalities. A radioactive iodine uptake scan may be used in some cases of hyperthyroidism to distinguish between Graves disease, toxic nodules, and thyroiditis.
- Start or adjust treatment. Confirmed hypothyroidism is typically treated with daily levothyroxine. Confirmed hyperthyroidism is treated with antithyroid medications, radioactive iodine, or surgery, depending on the cause and severity.
- Refer to an endocrinologist. Complex, severe, or unusual thyroid problems are often referred to an endocrinologist (hormone specialist).
- Investigate a thyroid nodule. If thyroid testing was prompted by a nodule, the next step is usually ultrasound and possibly a fine needle aspiration (FNA) biopsy. Our thyroid diagnosis guides discuss the various findings that may result from FNA and surgery.
Questions to ask your doctor
- Are my thyroid tests within the reference range?
- If my TSH is abnormal, what is my free T4 result?
- Do I have hypothyroidism, hyperthyroidism, or a more subtle pattern?
- Should I be tested for thyroid antibodies?
- Could any of my medications be affecting my thyroid results?
- If I have hypothyroidism, do I need treatment, and if so, with what dose?
- If I am already on thyroid medication, is my dose correct?
- Should I have a thyroid ultrasound?
- How often should my thyroid tests be repeated?
- If I am pregnant or planning a pregnancy, what TSH target should I aim for?
- Should I be referred to an endocrinologist?
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