When you have signs and symptoms suggesting underactive thyroid (hypothyroidism) or overactive thyroid (hyperthyroidism) due to a condition affecting the thyroid; when you have an enlarged thyroid (goiter) or a thyroid nodule (a small lump on the thyroid gland that may be solid or a fluid-filled cyst)
A blood sample drawn from a vein
No test preparation is needed. However, certain medications, multivitamins and supplements can interfere with thyroid testing, so tell your healthcare practitioner about any prescribed or over-the-counter drugs and/or supplements that you are taking. If you take thyroid hormone as treatment for thyroid disease, it is recommended that your blood sample be drawn before you take your dose for that day. Acute illness may affect thyroid hormone test results. It is generally recommended that thyroid testing be avoided in hospitalized patients or postponed until after you have recovered from the illness.
A thyroid panel is a group of tests that may be ordered together to help evaluate thyroid gland function and to help diagnose thyroid disorders. The tests included in a thyroid panel measure the level of thyroid hormones in the blood.
A thyroid panel usually includes tests for:
Although rarely used these days, sometimes a T3 resin uptake (T3RU) test is included. T3RU and T4 can be used to calculate a free thyroxine index (FTI). This is another method for evaluating thyroid function. It corrects for changes in certain proteins that can affect total T4 levels.
The thyroid is a small butterfly-shaped organ that lies flat across the windpipe at the base of the neck that produces thyroid hormones, primarily T4 and some T3. These hormones travel throughout the body and regulate the metabolism by telling the cells in the body how fast to use energy and produce proteins. Most of the hormone produced by the thyroid is T4. This hormone is relatively inactive, but it is converted into the much more active T3 in the liver and other tissues.
Almost all of the T3 and T4 circulating in the blood is bound to protein. The small portions that are not bound are called "free" and are the biologically active forms of the hormones. Lab tests can measure the amount of free T3 or free T4 or the total T3 or total T4 (bound plus unbound) in the blood.
- The total T4 and total T3 tests have been used for many years, but they can be affected by the amount of protein available in the blood to bind to the hormone.
- The free T4 and free T3 tests are not affected by protein levels and are thought by many to be more accurate reflections of thyroid hormone function.
- In most cases, the free T4 test has replaced the total T4 test. However, some professional guidelines recommend the total T3 test, so either the total T3 or free T3 test may be used to assess thyroid function.
The body has an elaborate feedback system to control the amount of T4 and T3 in the blood.
- When blood levels of the thyroid hormones decrease, the hypothalamus releases thyrotropin-releasing hormone (TRH), which in turn causes the pituitary gland to release thyroid-stimulating hormone (TSH). TSH stimulates the thyroid gland to produce and release T4 (primarily) and T3.
- As thyroid hormone levels increase in the blood, the pituitary gland produces less TSH and the thyroid produces less T4 and T3.
Under normal circumstances, this feedback system regulates thyroid activity to maintain relatively stable levels of thyroid hormones in the blood.
If your thyroid gland does not produce enough T4 and T3 (underactive thyroid), due to thyroid dysfunction or to insufficient TSH, then you may have signs and symptoms of hypothyroidism such as weight gain, dry skin, cold intolerance, irregular menstruation, and fatigue. Hashimoto thyroiditis is the most common cause of hypothyroidism in the U.S. (See the article on Hashimoto Thyroiditis for more details.)
If your thyroid gland produces too much T4 and T3, you may have signs and symptoms associated with overactive thyroid (hyperthyroidism), such as rapid heart rate, anxiety, weight loss, difficulty sleeping, tremors in the hands, and puffiness around dry, irritated eyes and in some cases, bulging eyes. Graves disease is the most common cause of hyperthyroidism. (See the article on Graves Disease for more details.)
How is the test used?
A thyroid panel may be used to evaluate thyroid function and/or help diagnose thyroid disorders.
Typically, the preferred initial test for thyroid disorders is a TSH test. If the TSH level is abnormal, it will usually be followed up with a test for free T4. Sometimes a total T3 or free T3 will also be performed. Often, the laboratory will do this follow-up testing automatically. This is known as reflex testing and it saves the healthcare practitioner time from having to wait for the results of the initial test and then requesting additional testing to confirm or clarify a diagnosis. Reflex tests are typically performed on the original sample that was submitted when the initial test was requested.
As an alternative, a thyroid panel may be requested by your healthcare practitioner. This means that all three tests will be performed at the same time to get a more complete initial picture of thyroid function.
When is it ordered?
- Slowed heart rate
- Weight gain
- Enlarged thyroid (goiter)
- Dry skin
- Trouble tolerating cold
- Muscle and joint pain
- Puffy skin
- Thinning hair, hair loss
- Heavy or irregular menstrual periods or infertility in women
Signs and symptoms of an overactive thyroid may include:
- Rapid heart rate
- Weight loss
- Difficulty sleeping
- Tremors in the hands
- Muscle weakness
- Increased sweating
- Trouble tolerating heat
- Sometimes more frequent bowel movements
- Some uncommon problems that can affect the eyes: puffiness around the eyes, dryness, irritation, excessive tearing, light sensitivity, blurry double vision
- In some cases, bulging of the eyes
- Less frequent or lighter menstrual periods in women
What does the test result mean?
If the feedback system involving the thyroid gland is not functioning properly due to one of a variety of disorders, then increased or decreased amounts of thyroid hormones may result.
- When TSH levels are increased, the thyroid will make and release inappropriate amounts of T4 and T3 and you may experience symptoms associated with hyperthyroidism.
- If there is decreased production of thyroid hormones, you may have symptoms of hypothyroidism.
The test results alone are not diagnostic but will prompt your healthcare practitioner to perform additional testing to investigate the cause of the excess or deficiency and thyroid disorder. As examples, the most common cause of hyperthyroidism is Graves disease and the most common cause of hypothyroidism is Hashimoto thyroiditis. (See the condition article on Thyroid Diseases for more on these and other related diseases.)
The following table summarizes some examples of typical test results and their potential meaning.
Note: Laboratory results must always be correlated with the clinical findings of the patient. TSH Free T4 Total or Free T3 Most likely diagnosis Normal Normal Normal Normal thyroid function (e.g., "euthyroid") Normal or decreased Normal or decreased Decreased Normal adjustment in thyroid function due to illness (nonthyroidal illness or sick euthyroid syndrome) Increased Normal Normal Subclinical hypothyroidism1; in a person with hypothyroidism on treatment, not enough thyroid hormone is being given Increased Decreased Normal of decreased Hypothyroidism resulting from a problem with the thyroid gland itself (primary hypothyroidism) Normal or increased Increased Increased Hyperthyroidism resulting from a problem with the pituitary gland signals (central hyperthyroidism) or from a problem with the thyroid hormone receptor (thyroid hormone resistance) Decreased Normal Normal Subclinical hyperthyroidism2; in a person with hypothyroidism, too much thyroid hormone is being given Decreased Normal Increased Hyperthyroidism resulting from the thyroid gland making too much active thyroid hormone T3 (uncommon, also known as T3 toxicosis) Decreased Increased Increased Hyperthyroidism resulting from the gland making too much thyroid hormones (primary hyperthyroidism) Decreased Decreased Decreased Hypothyroidism resulting from a problem with the hypothalamus or pituitary signals that govern the thyroid gland (central hypothyroidism)
1In affected adults, the diagnosis of subclinical hypothyroidism is applied when the TSH level is elevated and the free T4 level is normal on repeat testing over a number of weeks or months. Adults with subclinical hypothyroidism may have few or no overt symptoms of hypothyroidism. However, subclinical hypothyroidism places affected adults at somewhat increased risk for an elevated LDL cholesterol level, increased risk for cardiovascular disease, and reduced mental acuity.
2In affected adults, the diagnosis of subclinical hyperthyroidism is applied when the TSH level is decreased and the free T4 level and T3 levels are normal on repeat testing over a number of weeks or months. Adults with subclinical hyperthyroidism may have few or no overt symptoms of hyperthyroidism. However, subclinical hyperthyroidism places affected persons at somewhat increased risk for atrial fibrillation and osteoporosis.
What conditions are associated with hypothyroidism and hyperthyroidism?
What medications can affect the thyroid panel tests?
Many multivitamins, supplements (especially hair and nail), and over-the-counter and prescription medications may affect thyroid test results and their use should be discussed with your healthcare practitioner prior to testing. For example, biotin (vitamin B7) can interfere with some lab tests, so your healthcare practitioner may advise you to refrain from taking biotin or supplements that contain biotin for a few days before having blood drawn for a thyroid panel.
What other tests may be ordered in addition to a thyroid panel?
Blood tests that may be performed in addition to a thyroid panel may include:
- Thyroid antibodies - to help diagnose autoimmune thyroid disease and distinguish it from other thyroid conditions (e.g., Graves disease, Hashimoto thyroiditis)
- Calcitonin - to help diagnose C-cell hyperplasia and medullary thyroid cancer
- Thyroglobulin - primarily to monitor treatment and recurrence of thyroid cancer
- Thyroxine-binding globulin (TBG) - to evaluate patients with abnormal T4 and T3 levels
What is reverse T3?
Reverse T3 (RT3 or REVT3) is a biologically inactive form of T3. Normally, when T4 is converted to T3 in the body, a certain percentage of the T3 is in the form of RT3. When the body is under stress, such as during a serious illness, thyroid hormone levels may be outside of normal ranges even though there is no thyroid disease present. RT3 may be elevated in non-thyroidal conditions, particularly the stress of illness. It is generally recommended that thyroid testing be avoided in hospitalized patients or deferred until after a person has recovered from an acute illness. Use of the RT3 test remains controversial, and it is not widely requested.
Is there anything else I should know?
In the past, panels of tests were more common. More recently, however, the practice has been to order, where possible, one initial or screening test and then follow up with additional testing, if needed, to reduce the number of unnecessary tests. With thyroid testing, one strategy is to screen with a TSH test and then order additional tests if the results are abnormal or if clinical suspicions warrant.
Your thyroid hormone test results can be affected by:
- Increases, decreases, and changes (inherited or acquired) in the proteins that bind T4 and T3 (This is important for tests that measure total T3, but it is not likely important for free T4 and free T3 tests, which measure the thyroid hormones that are not bound to protein.)
- Estrogen and other drugs
- Liver disease
- Systemic illness
- Rarely, resistance to thyroid hormones