Los análisis de sangre para la función tiroidea (TSH, T4 total, T3 libre, TSI y otros) son una parte importante para diagnosticar y tratar los trastornos de la tiroides. Si bien se pueden extraer algunas conclusiones de una sola prueba, generalmente se necesita una combinación de los resultados de la prueba para establecer la naturaleza completa de su salud tiroidea. Al comparar los valores de las pruebas de tiroides, un médico puede determinar si una persona tiene hipotiroidismo (función tiroidea baja), hipertiroidismo (tiroides hiperactiva) o una enfermedad tiroidea autoinmune como la enfermedad de Graves o tiroiditis de Hashimoto .
Determinar lo que significan los distintos nombres y números puede ser complicado, pero tomarse el tiempo para aprenderlos puede ayudarlo a controlar mejor su enfermedad.
Types of Tests
The purpose of thyroid testing is to measure the so-called “markers” of thyroid health. These are substances not only produced by the thyroid gland but other organs that regulate thyroid function. For example, the pituitary gland produces a hormone known as thyroid stimulating hormone (TSH), which regulates how much of the hormones triiodothyronine (T3) and thyroxine (T4) are produced by the thyroid gland. The interrelationship of these and other values can tell you a lot about how well or poorly your thyroid gland is functioning.
Thyroid function tests typically look at six key substances in the blood, including hormones, proteins, and immune cells known as antibodies.
Thyroid Stimulating Hormone (TSH)
Thyroid stimulating hormone (TSH) is the pituitary hormone that acts as a messenger to the thyroid gland. If the pituitary gland detects that there is too little thyroid hormone in the blood, it will produce more TSH, prompting the thyroid gland to produce more thyroid hormone. When the pituitary detects too much thyroid hormone, it slows the production of TSH, signaling the thyroid gland to do the same.
Thyroxine (T4) functions as a “storage” hormone. On its own, it is unable to produce energy or deliver oxygen to cells but must undergo a process known as monodeiodination in which it loses an atom of iodine to become triiodothyronine (T3). The T4 test measures two key values:
- Total T4 is the total amount of thyroxine circulating in the blood. The includes T4 that has bonded with protein (interfering with its ability to enter certain tissue) and T4 that has not bonded to protein.
- Free T4 is the type not bonded to protein and is considered the active form of thyroxine.
Triiodothyronine (T3) is the active thyroid hormone created from the conversion of thyroxine into triiodothyronine. Three different tests measure various aspects of T3:
- Total T3 is the total amount of triiodothyronine circulating in the blood, both bound and unbound by protein.
- Free T3 is bound to protein and considered the active form of triiodothyronine.
- Reverse T3 is the inactive “mirror image” of T3 that attaches to thyroid receptors but is unable to activate them.
Thyroglobulin (Tg) is a protein produced by the thyroid gland. It is mostly used a tumor marker to help guide thyroid cancer treatment. The aim of cancer treatment is the eradication of all cancer cells. The elevation of Tg is a sign that cancer cells are still present following thyroid removal surgery (thyroidectomy) or radioactive ablation (RAI) therapy.
By comparing baseline values with subsequent results, the Tg test can tell doctors whether the cancer treatment is working, how durable remissionis, and whether there are signs of cancer recurrence.
There are some thyroid disorders caused by an autoimmune disease. Autoimmune diseases occur when the immune system mistakenly targets and attacks normal cells. It does so by secreting defensive antibodies that are “matched” to receptors (antigens) on the targeted cell.
There are three common antibodies associated with autoimmune thyroid disease:
- Thyroid peroxidase antibodies (TPOAb) are detected in 95 percent of people with Hashimoto’s and around 70 percent of those with Graves’ disease. Elevated TPOAb is also seen, albeit less commonly, in women with postpartum thyroiditis.
- Thyroid stimulating hormone receptor antibodies (TRAb) are seen in 90 percent of Graves’ disease cases, but only 10 percent of Hashimoto’s cases.
- Thyroglobulin antibodies (TgAb) are produced by your body in response to the presence of thyroglobulin. They are detected in 80 percent of people with Hashimoto’s and between 50 percent to 70 percent of those with Graves’ disease. Moreover, one in four people with thyroid cancerwill have elevated TgAb.
Thyroid Binding Proteins
Testing the level of proteins in the blood that binds to T3 and T4 can help doctors characterize the nature of a thyroid problem or explore conditions in which thyroid symptoms develop in people with normally functioning glands. Among the three common measures:
- Thyroid binding globulin (TBG) measures the level of protein, known as globulin, that carries thyroid hormones in the blood. It can be measured either with electrophoresis (which uses an electrical field to measure particles) or a radioimmunoassay (which uses radioactive isotopes to measure particles).
- T3 resin uptake (T3RU) calculates the percentage of TBG in a sample of blood.
- Free thyroxine index (FTI) is an older method of calculation in which the total T4 is multiplied by the T3RU to characterize whether a person is hypothyroid or hyperthyroid
Test Reference Ranges
The results of any blood test will be listed alongside a reference range. The reference range is simply the expected range of values within a population.
Generally speaking, anything between the high and low ends of the reference range can be considered normal. Anything near the upper or lower limit may be considered borderline, while anything outside of the upper and lower limits would be considered abnormal.
In the middle of the reference range is a “sweet spot,” called the optimal reference range, in which thyroid function is considered ideal.
Interpretation of Results
The interpretation of the test results can vary based on the individual and comparative values. The one test that arguably provides the most insight is the TSH. When used in combination with free T3 and free T4, the TSH can also suggest the cause of an abnormality.
TSH values outside of the optimal reference range are suggestive of a thyroid disorder. Values at or near the upper or lower range may suggest a subclinical disorder (or one in which there are no observable symptoms).
According to guidelines issued by the American Association of Clinical Endocrinologists (AACE) and the American Thyroid Association (ATA), a TSH value:
- Between 4.7 and 10 mU/L is considered subclinical hypothyroidism.
- Over 10 mU/L is overt (symptomatic) hypothyroidism.
- Between 1.5 and 2.0 mU/L is suggestive of thyroid dysfunction.
- Between 0.1 and 0.5 mU/L is considered subclinical hyperthyroidism.
- Less than 0.1 mU/L is overt hyperthyroidism.
T3 and T4 Interpretations
By comparing TSH with T4 values, your doctor may be able to better characterize the nature of a thyroid disorder. For example:
- A normal TSH and normal T4 indicates a normally functioning thyroid gland.
- A low TSH and high T4 generally indicates hyperthyroidism.
- A high TSH and low T4 indicates primary hypothyroidism (due to a thyroid disease).
- A low TSH and low T4 suggest secondary hypothyroidism (due to a disease of the pituitary gland or hypothalamus of the brain).
For diagnostic purposes, a low T3 value accompanied by a high TSH value is considered evidence of hypothyroidism. By contrast, a low TSH value accompanied by a high T3 value is considered evidence of hyperthyroidism.
The other thyroid tests may be included as part of a standard panel or ordered when needed. Some have specific aims; others are used for screening purposes or to differentiate between possible causes.
- RT3 tests can help identify dysregulation disorders, such as euthyroid sick syndrome (ESS), in which hormone levels are abnormal, but the thyroid gland does not appear dysfunctional.
- Tg tests, in addition to detecting cancer recurrence, can help predict the long-term outcome of treatment. According to research published in the journal Thyroid, only 4 percent of people with a thyroglobulin level under 1 will experience recurrence after five years.
- TPOAb tests can help confirm Hashimoto’s disease if your TSH is elevated but your T4 is low. By contrast, a TPOAb test can help confirm Graves’ disease if your TSH is normal but your T4 is high.
- TRAb tests, in addition to diagnosing Graves’ disease, may help confirm a diagnosis of toxic multinodular goiter. The test is also commonly performed during the last three months of pregnancy to evaluate the baby’s risk of being born with hyperthyroidism or Graves’ disease.
- TgAb tests, in addition to supporting an autoimmune diagnosis, can help clarify post-cancer treatment results. This is because TgAB can interfere with Tg readings in as many as 15 percent of people with detectable TgAb. If Tg readings are low but TgABb levels are elevated, further evaluation may be needed to avoid misdiagnosis.
- TBG tests can help determine whether the lack of the binding protein is the cause of the thyroid disorder or simply a characteristic. TBG deficiency can sometimes occur as a result of an inherited disorder in which thyroid gland is functioning but symptoms of hypothyroidism persist.
- T3RU tests are another method of assessing TBG deficiency with higher T3RU values corresponding to lower TBG levels (and vice versa).
- FTI tests are a reliable means of assessing thyroid function in the presence of a TBG deficiency. However, they are less commonly used today given the accuracy of newer free T3 and free T4 tests.
There is not always consensus as to what thyroid test results mean, particularly between conventional endocrinologists and integrative medical practitioners. By and large, integrative specialists contend that the diagnostic measures endorsed by the AACE and ATA fall short in diagnosing thyroid disorders, especially in people with subclinical disease.
Even with regards to TSH testing, most integrative doctors will tell you that a TSH under 10.0 mU/L—classified as subclinical hypothyroidism—should be treated and that doing so may prevent the development of overt hypothyroidism. AACE/ATA guidelines suggest a more watch-and-wait approach.
Integrative physicians also believe that the true measure of a person’s thyroid health is the number of active hormones circulating in the blood (free T4 and free T3) and not TSH. They argue that TSH is an inexact value given that it can lie within the normal range with Hashimoto’s disease and that free T3 offers a “real-time” snapshot of thyroid function. For these practitioners, a low free T3 is considered justification for thyroid hormone replacement therapy.
By contrast, many conventional doctors will not test T3 given that there is no direct association between T3 levels and the risk of overt hypothyroidism. Moreover, the T3 replacement drug Cytomel (liothyronine) is not even endorsed for the treatment of hypothyroidism due to the risk of reactive hyperthyroidism, minimizing the value of T3 in directing thyroid treatment.
The same argument has extended to RT3 testing for which integrative practitioners believe an elevated RT3 or an imbalance in the RT3/T3 ratio is a clear sign of hypothyroidism. Research shows that there is little credible evidence that either of these claims is inherently true.
Nor is there evidence that elevated TPOAb warrants preemptive treatment to prevent overt hypothyroidism in people suspected of having Hashimoto’s, as some might suggest.