What are thyroid function tests?
Thyroid function tests are a series of blood tests used to an extent how well your thyroid gland is operational. Available tests include T3, T3RU, T4, and TSH.
The thyroid is a small gland located in the lower front of the neck. It is liable for helping to control many of the body’s processes, such as metabolism, energy generation, and mood.
The thyroid produces two chief hormones: triiodothyronine (T3) and thyroxine (T4). If your thyroid gland does not produce enough of these hormones, you may experience symptoms such as weight gain, lack of energy, and depression. This condition is called hypothyroidism.
If your thyroid gland produces too many hormones, you may experience weight loss, high levels of anxiety, tremors, and a feeling of euphoria. This is called hyperthyroidism.
Generally, a doctor who is concerned about your thyroid hormone levels will order extensive screening tests, such as the T4 or the thyroid-stimulating hormone (TSH) test. If those results are abnormal, your doctor will order more tests to determine the reason for the problem.
The main thyroid hormone secreted by the thyroid gland is thyroxine, also called T4 because it contains four atoms of iodine. To exert its effects, T4 is converted to triiodothyronine (T3) by removing an iodine atom. This occurs mainly in the liver and in certain tissues where T3 acts, such as the brain. The amount of T4 produced by the thyroid gland is controlled by another hormone, which is produced in the pituitary gland located at the base of the brain, called thyroid-stimulating hormone (TSH for short).
The quantity of TSH that the pituitary sends into the bloodstream rest on the amount of T4 the pituitary sees. If the pituitary sees too little T4, then it makes more TSH signal the thyroid gland to make more T4. Once the T4 in the bloodstream exceeds a certain level, the production of TSH by the pituitary stops. In fact, the thyroid and pituitary act in many ways as a heater and a thermostat. When the heater is off and cools down, the thermostat reads the temperature and turns the heater on. When the heat rises to an appropriate level, the thermostat senses it and shuts the heater off. Therefore, the thyroid and pituitary, like a heater and a thermostat, turn on and off.
Understanding the thyroid function tests result
Blood tests to measure these hormones are obtainable and widely used, but not all are helpful in all circumstances. To assess the thyroid function tests include the following:
The best way to initially assess thyroid function is to measure the level of TSH in a blood sample. Changes in TSH can serve as an “early warning system,” which often occurs before the actual level of thyroid hormones in the body is too high or too low. A high TSH level indicates that the thyroid gland is not making enough thyroid hormone (primary hypothyroidism).
The opposite condition, in which the TSH level is low, usually designates that the thyroid is producing too much thyroid hormone (hyperthyroidism). Occasionally, a low TSH can be the result of an abnormality in the pituitary gland, preventing it from producing enough TSH to stimulate the thyroid (secondary hypothyroidism). In most healthy people, a normal TSH value means that the thyroid is working properly.
T4 is the main form of thyroid hormone that circulates in the blood. A total T4 measures hormone bound and free and can change when the binding proteins differ (see above). Free T4 measures what is not bound and can enter and affect body tissues. Tests that measure free T4, either a free T4 index (FT4) or a free T4 index (FTI), more accurately reflect how the thyroid gland is working when it is checked with a TSH.
The finding of elevated TSH and low FT4 or FTI indicates primary hypothyroidism due to thyroid gland disease. Low TSH and low FT4 or FTI designate hypothyroidism allocated to a problem affecting the pituitary gland. A low TSH with an elevated FT4 or FTI is found in people who have hyperthyroidism.
T3 tests are often helpful in diagnosing hyperthyroidism or determining the severity of hyperthyroidism. Hyperthyroid patients will have an elevated T3 level. In some individuals with low TSH, only T3 is elevated and FT4 or FTI is normal. The T3 test is rarely helpful in the hypothyroid patient, as it is the last test to become abnormal. Patients may present with severe hypothyroidism with high TSH and low FT4 or FTI, but with normal T3.
Measurement of free T3 is possible, but it is often unreliable and therefore not usually useful.
Reverse T3 is a biologically inactive protein that is structurally very similar to T3, but the iodine atoms are placed in different locations, rendering it inactive. Some reverse T3 is normally produced in the body, but then it breaks down rapidly. In healthy, non-hospitalized people, reverse T3 measurement does not help determine whether or not hypothyroidism exists, and it is not clinically useful.
The body’s immune system normally protects us from foreign invaders like bacteria and viruses by destroying these invaders with substances called antibodies produced by blood cells known as lymphocytes. In numerous patients with hypothyroidism or hyperthyroidism, the lymphocytes react beside the thyroid (thyroid autoimmunity) and produce antibodies beside thyroid cell proteins. Two common antibodies are the thyroid peroxidase antibody and the thyroglobulin antibody.
Measuring thyroid antibody levels can help diagnose the cause of the thyroid problem. For example, positive anti-thyroid peroxidase and / or anti-thyroglobulin antibodies in a patient with hypothyroidism result in a diagnosis of Hashimoto’s thyroiditis. Although the detection of antibodies is useful in the initial diagnosis of hypothyroidism due to autoimmune thyroiditis, following their levels overtime is not useful in detecting the development of hypothyroidism or the response to treatment. TSH and FT4 are what tell us about actual thyroid function or levels.
A different antibody that can be positive in a patient with hyperthyroidism is the TSH-stimulating receptor (TSI) antibody. This antibody makes the thyroid overactive in Graves’ disease. If you have Graves’ disease, your doctor may also order a thyrotropin receptor antibody (TSHR or TRAb) test, which detects both stimulating and blocking antibodies. Monitoring antibody levels in Graves’ patients can help assess response to hyperthyroidism treatment, determine when it is appropriate to discontinue antithyroid medication and assess the risk of transmitting antibodies to the fetus during pregnancy.
Thyroglobulin (Tg) is a protein produced by normal thyroid cells and thyroid cancer cells. It is not a measure of thyroid function and does not diagnose thyroid cancer when the thyroid gland is still present. It is most often used in patients who have had surgery for thyroid cancer to control them after treatment. Tg is included in this thyroid function test booklet to communicate that although it is frequently measured in certain settings and individuals, Tg is not a primary measure of thyroid hormone function.
Thyroid function tests without blood
Radioactive iodine consumption
Because T4 contains iodine, the thyroid gland must extract a large amount of iodine from the bloodstream to produce an adequate amount of T4. The thyroid has developed a very active mechanism to do this. Therefore, this activity can be measured by having a person ingest a small amount of iodine, which is radioactive. Radioactivity allows the doctor to track where the iodine goes. By measuring the amount of radioactivity taken up by the thyroid gland (radioactive iodine uptake, RAIU), doctors can determine if the gland is working normally.
A very extraordinary RAIU is seen in individuals whose thyroid gland is overactive (hyperthyroidism), while a low RAIU is seen when the thyroid gland is underactive (hypothyroidism). In addition to radioactive iodine uptake, a thyroid scan may be obtained, which shows a picture of the thyroid gland and reveals which parts of the thyroid have absorbed the iodine (see thyroid nodules brochure).
Medicine Interfering with thyroid function tests
There are many medications that can affect thyroid function tests. Some common examples include:
- Estrogens, such as those in birth control pills or during pregnancy, cause elevated levels of T4 and total T3. This is because estrogens increase the level of binding proteins. In these situations, it is best to request both TSH and free T4 for thyroid evaluation, which will normally be in the normal range.
- Biotin, a commonly taken over-the-counter supplement, can make the measurement of various thyroid function tests appear abnormal when they are actually normal in the blood. Biotin should not be taken for 2 days before blood is drawn for thyroid function tests to avoid this effect.