thyroid function test
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What is a thyroid function test?
A thyroid function test is a lab procedure that checks how well the thyroid gland is working. It measures the levels of thyroid hormones, thyroxine (T4) and triiodothyronine (T3), and thyroid-stimulating hormone (TSH).
The test helps diagnose thyroid disorders such as hypothyroidism (underactive thyroid) and hyperthyroidism (overactive thyroid). High TSH and low thyroxine levels usually indicate hypothyroidism, while low TSH and high thyroxine levels suggest hyperthyroidism. In cases of hyperthyroidism, a radioactive iodine uptake test can help determine the cause.
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What does the thyroid gland do in the body?
The thyroid gland is an endocrine gland in the lower neck that produces hormones vital for metabolism and growth. The thyroid gland produces the hormones thyroxine (T4) and triiodothyronine (T3). These hormones increase metabolic rate and oxygen consumption and affect growth and maturation. Thyroid hormones are also crucial for neural and skeletal development during fetal life and infancy. The hypothalamic-pituitary-thyroid axis regulates thyroid hormone production. This axis uses a negative feedback system where thyroid hormones inhibit the production of thyrotropin-releasing hormone and thyrotropin, maintaining hormone concentrations within a narrow range.
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Which hormones are measured in a thyroid function test?
A thyroid function test measures the levels of hormones in the blood to assess thyroid gland function. The key hormones measured are:
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Thyroxine (T4) T4 is the thyroid hormone produced in the greatest quantity. It exists in two forms: bound and free. Free thyroxine (free T4) is the metabolically active form that is readily available to the body's cells, so its measurement is preferred.
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Triiodothyronine (T3) T3 is another thyroid hormone, but it is produced in lower quantities than T4.
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Thyrotropin (Thyroid-Stimulating Hormone, TSH) TSH, also known as thyrotropic hormone, is produced by the pituitary gland and regulates thyroid secretion. TSH stimulates the thyroid gland to produce thyroxine (T4) and triiodothyronine (T3).
The test helps diagnose conditions like hypothyroidism (thyroid deficiency) and hyperthyroidism (excessive thyroid hormone production).
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Why might a doctor order a thyroid function test?
A doctor might order a thyroid function test to assess the thyroid gland's hormone production, specifically thyroxine (T4) and triiodothyronine (T3), and to measure thyrotropin (TSH) levels, which regulate thyroid secretion. These tests help diagnose hypothyroidism (thyroid deficiency) or hyperthyroidism (thyroid excess).
The test results can reveal the cause of hyperthyroidism and detect thyroid antibodies related to Hashimoto disease or Graves disease.
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What do high or low levels of thyroid hormones mean for your health?
A thyroid function test assesses thyroid hormone production (thyroxine (T_4) and triiodothyronine (T_3)) and thyroid-stimulating hormone (TSH) production.
- High thyroid hormone levels (hyperthyroidism) often result in a rapid heart rate, weight loss, and anxiety. The diagnosis of hyperthyroidism is based on measurements of high serum total and free thyroid hormone concentrations and low serum thyrotropin concentrations.
- Low thyroid hormone levels (hypothyroidism) can lead to fatigue, weight gain, and slowed mental processes. In hypothyroidism, the usual findings are low serum total and free thyroxine concentrations and high serum thyrotropin concentrations.
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How do thyroid function tests help diagnose conditions like hypothyroidism or hyperthyroidism?
Thyroid function tests diagnose conditions like hypothyroidism and hyperthyroidism by measuring thyrotropin and thyroid hormones in the blood. High thyrotropin levels with low thyroid hormone levels typically indicate hypothyroidism, while the opposite suggests hyperthyroidism. Measuring free thyroxine in serum is a better test for thyroid dysfunction than measuring total thyroxine. The radioactive iodine uptake test helps differentiate the causes of hyperthyroidism.
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thyroid function test, any laboratory procedure that assesses the production of the two active thyroid hormones, thyroxine (T4) and triiodothyronine (T3), by the thyroid gland and the production of thyrotropin (thyroid-stimulating hormone, TSH), the hormone that regulates thyroid secretion, by the pituitary gland. The best and most widely used tests are measurements of serum thyrotropin and thyroxine. The secretion of thyrotropin changes substantially in response to very small changes in thyroxine and triiodothyronine production. For example, small decreases in thyroid hormone production result in relatively large increases in serum concentrations of thyrotropin, and, conversely, small increases in thyroxine and triiodothyronine production result in relatively large decreases in serum concentrations of thyrotropin. Therefore, patients with hypothyroidism (thyroid deficiency) almost invariably have not only low serum thyroid hormone but also high serum thyrotropin concentrations, and those with hyperthyroidism have high serum thyroid hormone and low serum thyrotropin concentrations. An exception is patients with pituitary disease and thyrotropin deficiency, who have low serum thyroid hormone but normal or low serum thyrotropin concentrations. Between the two thyroid hormones, measurements of serum thyroxine are preferred because serum triiodothyronine concentrations are abnormal in many patients with nonthyroid illnesses.
Thyroxine and triiodothyronine exist in serum in two forms, bound and free (or unbound). Over 99 percent of each hormone is bound to one of three proteins—thyroxine-binding globulin, transthyretin (also known as thyroxine-binding prealbumin), and albumin. Serum thyroxine (and triiodothyronine) can be measured as the total hormone, which includes the bound and free fractions, or as free hormone alone. Changes in serum concentrations of these binding proteins occur, with the most common change being an increase in serum thyroxine-binding globulin in pregnant women and women taking estrogen. On the other hand, androgenic hormones and many illnesses decrease production of the binding proteins. These changes alter serum total thyroxine concentrations but not serum free thyroxine concentrations (and, similarly, total and free triiodothyronine concentrations). Thyroid hormone entry into tissues, and therefore hyperthyroidism or hypothyroidism, is correlated with serum free thyroxine and free triiodothyronine concentrations, not serum total thyroxine and total triiodothyronine concentrations. Therefore, measurements of serum free thyroxine are a better test for thyroid dysfunction than are measurements of serum total thyroxine.
The function of the thyroid is sometimes assessed by the radioactive iodine uptake test. In this test the patient is given an oral dose of radioactive iodine, and the fraction of the radioactive iodine that accumulates in the thyroid is measured 6 or 24 hours later. This test is used mostly to distinguish between different causes of hyperthyroidism; radioactive iodine uptake is high in patients with hyperthyroidism caused by Graves disease or thyroid nodular disease, and it is low in patients with hyperthyroidism caused by thyroid inflammation.
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While not a test of thyroid function, another common procedure is to measure several thyroid antibodies found in serum, namely antithyroid peroxidase antibodies, antithyroglobulin antibodies, and antibodies that act like thyrotropin (called TSH-receptor antibodies). Most patients with Hashimoto disease have high serum concentrations of antithyroid peroxidase and antithyroglobulin antibodies. Many patients with Graves disease have high serum concentrations of these two antibodies, as well as high serum concentrations of the TSH-receptor antibodies that cause the hyperthyroidism that characterizes the disease.