Thyroid Function Tests - Clinical Methods - NCBI Bookshelf
The relationship of serum thyroxine (T4) and 3,5,3′-triiodothyronine (T3) to the Solomon DH, Benotti J, DeGroot LJ, et al: Revised nomenclature for tests of. What Your Hypothyroidism Test Results Mean hormone, or TSH, which tells the thyroid gland how much T4 and T3 to produce. The TSH level. T3 and T4 production is regulated by thyroid stimulating hormone (TSH) Thyroid function is assessed by one or more of the following tests1: there is a relationship between serum free T4 and TSH concentrations in that.
At this juncture TSH bound to labeled TSH antibody is also bound to solid-phase antibody, which allows for separation of TSH with its attendant antibodies and label from the reaction mixture.
After washing, the concentration of label is measured by techniques appropriate for the label. In the case of the immunoradiometric assay, the antibody is labeled with I, and this is counted. The original concentration of TSH is directly proportional to the I activity separating with the bound solid-phase antibody. Published sensitivities of the various assays range from 0. This new methodology appears to be a reliable means of separating hyperthyroid from euthyroid patients.
With very few exceptions, hyperthyroid patients have zero or low values, whereas normal subjects have measurable values of TSH. For these reasons the sensitive TSH determination is increasingly being used as a first-line test to confirm or screen for the diagnosis of hyperthyroidism. In otherwise healthy individuals this approach has merit; however, a disconcerting number of sick, hospitalized euthyroid patients are found to have low or unmeasurable TSH values Table Most have sustained trauma or have a severe illness, often treated with steroids or dopamine; but some have acute psychiatric disorders and others are depressed.
Low or unmeasurable values for TSH and flat TRF—TSH responses have also been found in otherwise well patients with nontoxic nodular goiters, patients recently treated for thyrotoxicois, patients with "euthyroid" Graves" disease or exophthalmous, normal subjects in the first trimester of pregnancy, and patients with pituitary or hypothalomic hypothyroidism.
Finally, a small fraction of apparently well people are found to have low TSH values which sometimes are normal several weeks later but sometimes remain low in the absence of recognizable thyroid disease. Conversely, a fraction of euthyroid hospitalized patients have elevated TSH concentrations. Some of these are in renal failure, whereas others are severely ill with a variety of nonthyroidal diseases.
In the setting of thyroid failure induced by I treatment of hyperthyroidism or external irradiation to the neck for treatment of lymphoma, the concentration of TSH often rises long before T4 falls below normal. In fact, TSH is sometimes elevated in patients who have few or no symptoms, and the question of whether these subjects have minimal hypothyroidism or "compensated" thyroid dysfunction is difficult to resolve.
This latter circumstance has raised a number of issues, the most important of which has been the question of whether to treat. With an elevated TSH, it is reasonable to assume that the T4 concentration is at or below the lowest acceptable limit for that patient.
If the patient is symptomatic, and especially when there are signs of hypothyroidism, treatment is indicated even if the value of T4 is within the "normal" range.
Patients who have normal values of T4 and no symptoms or signs of hypothyroidism, but in whom the TSH is clearly elevated, pose a special problem that may best be resolved by a period of observation. While TSH is an extremely reliable marker for the state of primary hypothyroidism, it is not invariably low in pituitary or hypothalamic hypothyroidism.
If there is discordance between the expected concentration of T4 and TSH, the possibility of pituitary or hypothalamic disease should be considered. Thus, in a patient who has a low or low normal T4 and a TSH that is either normal, low, or just minimally elevated, a TRF challenge should be used to measure pituitary secretory reserve.
A reliable assay would be of great clinical interest as it might provide a powerful means of distinguishing between various levels of dysfunction along the pituitary—hypothalamic axis. Side effects are nausea, a peculiar taste, chest discomfort, urinary urgency, and a modest increase in blood pressure.
Timing of blood samples is predicated on the questions being asked Figure If hyperthyroidism or primary hypothyroidism are diagnostic possibilities, samples are drawn at injection and 20 and 30 minutes afterward.
The "flat line" response of hyperthyroidism and supranormal response of primary hypothyroidism will be evident at these time points Figure If pituitary or hypothalamic hypothyroidism is suspected, samples should be obtained at injection and 20, 40, 60, 90, and minutes. These will illustrate the reduced response of pituitary hypothyroidism and the delayed response of hypothalamic hypothyroidism.
Unfortunately, a fraction of patients with pituitary hypothyroidism have a delayed TSH release, so this finding is consistent with both pituitary and hypothalamic dysfunction. A fraction of multinodular goiters and follicular adenomas are autonomous and make sufficient hormone to suppress TSH production and thereby blunt the TSH response to TRH.
Relationship of Serum T4 and T3 to TSH in Primary Hypothyroidism
In this setting a flat TSH response is not necessarily diagnostic of hyperthyroidism. On the other hand, a normal TSH response excludes hyperthyroidism. Frequently, patients with Graves" ophthalmopathy have a flat response, and they are not necessarily hyperthyroid. A normal response does not exclude the diagnosis of Graves" ophthalmopathy. Other euthyroid nonresponders are patients treated for hyperthyroidism within the last several months, patients with severe illness, and especially those treated with dopamine and steroids.
Finally, some patients with depression and acute psychiatric disorders have a flat response. A delayed and sometimes exaggerated TSH peak suggests either pituitary or hypothalamic disease. Unfortunately, a number of hypopituitary subjects will have a significant TSH increase that cannot be distinguished from a low normal response, and this is especially true in older males.
In this setting, and as isolated TSH deficiency is unusual, a complete evaluation of the pituitary gland will establish the presence of pituitary disease. Radioactive Iodine Uptake The thyroid's avidity for iodide and the ease of measuring it externally give the clinician another indirect measurement of T4 production. By using appropriate columnation and suitable standards, fractional thyroid accumulation of an oral dose of I can be measured.Thyroid In Hindi - T3,T4,TSH Harmon Explained
Activity measured over the thyroid following oral administration of the radio-nudide is characterized by an early, rapid uptake phase followed by a plateau of radioactivity in the gland that is most conveniently measured at 24 hours Figure The fractional uptake of a given amount of I depends on the production rate of thyroid hormone, thyroid iodide stores, and dietary iodide intake.
Despite its susceptibility to these hazards, the radioactive iodine uptake is an excellent adjunctive test. As with the estimation of T4, there is overlap in various states of thyroid function as assessed by the radioactive iodine test.
Major disadvantages are cost and the requirement for at least two visits to the hospital. Because of its reduced use, it is increasingly unavailable. For these reasons, the radioactive iodine uptake is not routinely used to assess thyroid function, but in some circumstances it still has great utility. For example, if lymphocytic thyroiditis with spontaneously resolving hyperthyroidism is suspected, low iodine uptake during the hyperthyroid phase will greatly assist in diagnosis.
In addition, if a patient is hyperthyroid from ingesting pharmacological doses of thyroid hormone, or if the patient has the rare struma ovarii with hyperthyroidism, the radioactive iodine uptake will again be low. In the latter case, scanning of the abdomen will reveal the excessive T4 production site. In patients with symptoms of hyperthyroidism and nondiagnostic studies of thyroid hormone concentration, a significant elevation of the I uptake helps establish the diagnosis.
Basic Science Pyroglutamyl-histadyl-prolinamide, the TSH-releasing hormone, or TRH, is synthesized in anterior hypothalamic neurons and released in the region of the median eminence Figure After circulating down the neurohypophyseal portal plexus, TRH binds to cell membrane receptors on anterior pituitary thyrotropes and causes production and release of TSH.
The number of TRH receptors is in part regulated by T3 nuclear receptor occupancy.
Thyroid Function Tests | American Thyroid Association
High T3 nuclear receptor occupancy is associated with reduced TRH receptor numbers, and low T3 nuclear receptor occupany the converse. TRH released in the hypothalamus stimulates synthesis and release of TSH by anterior pituitary thyrotropes. TSH, in turn, stimulates the thyroid to produce and release T4 and T3, which on entering the extracellular compartment, more TSH, a basic glycoprotein with a molecular weight of approximately 28, daltons, is composed of alpha and beta chains linked by sulfhydryl bonds.
An ice pack or an over-the-counter pain reliever can help ease your discomfort. If you experience a great deal of pain, or if the area around the puncture becomes red and swollen, follow up with your doctor immediately. These could be signs of an infection. The T4 test is known as the thyroxine test. A high level of T4 indicates an overactive thyroid hyperthyroidism. Symptoms include anxiety, unplanned weight loss, tremors, and diarrhea.
Most of the T4 in your body is bound to protein. A small portion of T4 is not and this is called free T4. Free T4 is the form that is readily available for your body to use. Sometimes a free T4 level is also checked along with the T4 test. The TSH test measures the level of thyroid-stimulating hormone in your blood. The TSH has a normal test range between 0. If you show signs of hypothyroidism and have a TSH reading above 2.
Symptoms include weight gain, fatiguedepression, and brittle hair and fingernails. Your doctor will likely want to perform thyroid function tests at least every other year going forward. Your doctor may also decide to begin treating you with medications, such as levothyroxine, to ease your symptoms. The major thyroid hormone secreted by the thyroid gland is thyroxine, also called T4 because it contains four iodine atoms. To exert its effects, T4 is converted to triiodothyronine T3 by the removal of an iodine atom.
This occurs mainly in the liver and in certain tissues where T3 acts, such as in the brain. The amount of T4 produced by the thyroid gland is controlled by another hormone, which is made in the pituitary gland located at the base of the brain, called thyroid stimulating hormone abbreviated TSH. The amount of TSH that the pituitary sends into the blood stream depends on the amount of T4 that the pituitary sees.
If the pituitary sees very little T4, then it produces more TSH to tell the thyroid gland to produce more T4. In fact, the thyroid and pituitary act in many ways like a heater and a thermostat. When the heater is off and it becomes cold, the thermostat reads the temperature and turns on the heater. When the heat rises to an appropriate level, the thermostat senses this and turns off the heater. Thus, the thyroid and the pituitary, like a heater and thermostat, turn on and off.
This is illustrated in the figure below: T4 and T3 circulate almost entirely bound to specific transport proteins, and there are some situations which these proteins could change their level in the blood, producing also changes in the T4 and T3 levels it happens frequently during pregnancy, women who take control birth pills, etc.