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Free Thyroxine + T4 Blood Test

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Free Thyroxine + T4

Clinical definition of the Free Thyroxine (FT4) test
Thyroxin (T4) is one of the two major hormones (the other called Triiodothyronine) produced by the thyroid gland. In the blood, T4 is either free (not bound) or protein-bound (bound to thyroxin- binding globulin). The free form of T4 is biologically active, hence measuring concentrations of free thyroid  hormone is of diagnostic value. The T4 test measures the amount of thyroxin in the blood and helps control metabolism and growth.

Free Thyroxine + T4 Blood Test

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Free Thyroxine + T4

Clinical definition of the Free Thyroxine (FT4) test
Thyroxin (T4) is one of the two major hormones (the other called Triiodothyronine) produced by the thyroid gland. In the blood, T4 is either free (not bound) or protein-bound (bound to thyroxin- binding globulin). The free form of T4 is biologically active, hence measuring concentrations of free thyroid  hormone is of diagnostic value. The T4 test measures the amount of thyroxin in the blood and helps control metabolism and growth.
Thyroxine (T4) circulates in the blood as an equilibrium mixture of free and serum protein bound hormone. Thyroxine binding globulin (TBG), albumin and pre-albumin bind approximately 75%, 10% and 15% of the total circulating T4 respectively.  The binding of T4 by these proteins is such that less than 0.03% is present in the circulation as unbound, free T4 (FT4).  This small percentage of the total T4 (TT4) represents the physiologically available hormone which is biologically active. Once the FT4 is absorbed by the target cells, the equilibrium reestablishes circulating FT4 levels. The equilibrium results in the maintenance of a constant level of FT4 when alterations occur in either the concentration or affinity of the serum binding proteins. Therefore, in a variety of normal (pregnancy) and abnormal (Familial Dysalbuminemic Hyperthyroxinemia, FDH) states, or as a result of the administration of certain drugs (e.g. furosemide and fenclofenac ), the target tissues are assured of receiving the required amount of hormone. FT4 values may, therefore, provide the best indication of thyroid dysfunction, since FT4 is less sensitive to changes in the serum binding proteins. Historically, the diagnosis of thyroid function has involved performing a total T4 assay in addition to a Thyroxine Uptake (TU) assay of the same sample. The mathematical combination of these two assays produces a Free Thyroxine Index (FTI) which provides an indirect proportional estimate of FT4. Alternatively, direct assays have been developed using equilibrium dialysis, ultrafiltration, RIA, and solid-phase EIA technology to measure FT4. In these methods, separation of free and bound tracer is achieved either with a membrane, or by binding FT4 to a solid phase antibody. This extraction step removes an amount of T4 which is proportional to the original amount of FT4 present in the patient sample. Provided that the extracted T4 is less than approximately 5% of the TT4 in the sample, a true estimation of the FT4 content of the sample can be obtained.
FT4 stands for free thyroxine. It is a thyroid hormone which has different levels at adults, infants, children, and pregnant women. A FT4 blood test is designed to indicate the level of this hormone.
This test measures free thyroxine, or FT4, in your blood.  Thyroxine is one of two major hormones produced by the thyroid gland (the other is called triiodothyronine. Within the blood, most thyroid hormones are attached to a protein, but it is the hormones that are free from these proteins that are able to affect body functions.  This is therefore why we measure the 'free' thyroixine in your blood sample.  The thyroid gland is found in the neck, in front of the windpipe.
Thyroxine (T4) makes up nearly all of what we call thyroid hormone, while triiodothyronine (T3) makes up less than 10%. Thyroid hormones help regulate the body’s metabolism (that is, how the body functions).
Most T4 in blood is attached to a protein; less than 1% is unattached. The blood test can measure either the total (both bound and unattached, TT4) or free (unattached, FT4) T4 hormone in your blood.
Scientists believe that free hormone is responsible for all the effects of thyroid hormone.
Thyroid hormone tests are blood tests that check how well the thyroid  gland is working. The thyroid gland makes hormones that regulate the way the body uses energy.
The thyroid gland is a butterfly-shaped gland that lies in front of your windpipe (trachea), just below your voice box (larynx). The thyroid gland uses iodine from food to make two thyroid hormones: thyroxine (T4) and triiodothyronine (T3). The thyroid gland stores these thyroid hormones and releases them as they are needed.
Thyroid hormones are needed for normal development of the brain, especially during the first 3 years of life. Intellectual disability may occur if a baby's thyroid gland does not produce enough thyroid hormone (congenital hypothyroidism). Older children also need thyroid hormones to grow and develop normally, and adults need the hormones to regulate the way the body uses energy (metabolism). The United States Preventive Services Task Force recommends that all newborns be tested for congenital hypothyroidism.
What is the thyroid gland?
The thyroid gland is a butterfly-shaped endocrine gland that is normally located in the lower front of the neck. The thyroid’s job is to make thyroid hormones, which are secreted into the blood and then carried to every tissue in the body. Thyroid hormone helps the body use energy, stay warm and keep the brain, heart, muscles, and other organs working as they should.
The thyroid gland is located on the front part of the neck below the thyroid cartilage (Adam's apple). The gland produces thyroid hormones, which regulate metabolic rate (how fast calories are consumed to produce energy). Thyroid hormones are important in regulating body energy, body temperature, the body's use of other hormones and vitamins, and the growth and maturation of body tissues.
Diseases of the thyroid gland can result in either production of too much (overactive thyroid disease or hyperthyroidism), too little (underactive thyroid disease or hypothyroidism) thyroid hormone, thyroid nodules, and/or goiter. Thyroid problems are much more common in women than in men.
Production of thyroid hormones: The process of hormone synthesis begins in a part of the brain called the hypothalamus. The hypothalamus releases thyrotropin-releasing hormone (TRH). The TRH travels through the venous plexus located in the pituitary stalk to the pituitary gland, also in the brain. In response, the pituitary gland then releases thyroid-stimulating hormone (TSH, also called thyrotropin) into the blood. The TSH travels to the thyroid gland and stimulates the thyroid to produce the two thyroid hormones, L-thyroxine (T4) and triiodothyronine (T3). The thyroid gland also needs adequate amounts of dietary iodine to be able to produce T4 and T3, the molecules of which contain four and three atoms of iodine, respectively.

Regulation of thyroid hormone production: To prevent the overproduction or underproduction of thyroid hormones, the pituitary gland senses how much hormone is in the blood and adjust the production of hormones accordingly. For example, when there is too much thyroid hormone in the blood, TRH and TSH production are both decreased. The sum effect of this is to decrease the amount of TSH released from the pituitary gland and to reduce production of thyroid hormones from the thyroid gland to restore the amount of thyroid hormone in the blood to normal. Defects in these regulatory pathways rarely may result in hypothyroidism (underactive thyroid problem) or hyperthyroidism (overactive thyroid problem). The most common cause of hypothyroidism and hyperthyroidism occurs due to problems within the thyroid and not the regulatory system.
Thyroid goiter: Thyroid goiter is any enlargement of the thyroid that can occur with hyperthyroidism or hypothyroidism but also with benign and malignant (cancerous) nodules. Worldwide, the most common cause of goiter is iodine deficiency. Although it used to be very common in the U.S., it is now less common with the use of iodized salt. Multiple nodules in the thyroid are very common, but only about 5% of the nodules are a thyroid cancer. Thyroid cancer rates have been increasing steadily by about 6% every year for more than 20 years. It is one of the few cancers whose rate is increasing and whose very low rate of mortality is also rising with time. Although radiation exposure as a child can increase the risk of thyroid cancer, we do not know why the overall rate has been increasing. Thyroid cancer is diagnosed after a thyroid ultrasound exam and a needle aspiration biopsy of the nodule.
Hypothyroidism in Pregnancy
Newly diagnosed hypothyroidism in pregnancy is rare because most women with untreated hypothyroidism do not ovulate or produce mature eggs in a regular manner, which makes it difficult for them to conceive.
It is a difficult new diagnosis to make based on clinical observation. The signs and symptoms of hypothyroidism (fatigue, poor attention span, weight gain, numbness, and tingling of the hands or feet) are also prominent symptoms of a normal pregnancy.

Undiagnosed hypothyroidism during pregnancy increases the chance of stillbirth or growth retardation of the fetus. It also increases the chance that the mother may experience complications of pregnancy such as anemia, eclampsia, and placental abruption.
Probably the largest group of women who will have hypothyroidism during pregnancy are those who are currently on thyroid hormone replacement. The ideal thyroxine replacement dose (for example, levothyroxine [Synthroid, Levoxyl, Levothroid, Unithroid]) may rise by 25% to 50% during pregnancy. It is important to have regular checks of T4 and TSH blood levels as soon as pregnancy is confirmed; and frequently through the first 20 weeks of pregnancy to make sure the woman is taking the correct medication dose. It is recommended that the levothyroxine dose be adjusted to keep the TSH level < 2.5 mIU/L during the first trimester of pregnancy and < 3 mIU/L during the last two trimesters of pregnancy. Usually the increase in thyroid hormone needed during pregnancy disappears after the delivery of the baby and the pre-pregnancy dose of levothyroxine can be resumed immediately post-partum.
Newly diagnosed hyperthyroidism occurs in about 1 in 2,000 pregnancies. Graves' disease accounts for 95% of cases of hyperthyroidism newly diagnosed during pregnancy.
As with hypothyroidism, many symptoms of mild hyperthyroidism mimic those of normal pregnancy. However, anyone experiencing symptoms such as significant weight loss, vomiting, increased blood pressure, or persistently fast heart rate should have blood tests to evaluate whether hyperthyroidism is present.
Mild or subclinical hyperthyroidism defined as a lower than normal TSH and normal Free T4 level is not dangerous to the mother or baby and does not need to treated. Thyroid tests should be checked again in 4 weeks. However, untreated moderate to severe hyperthyroidism does cause fetal and maternal complications including poor weight gain and tachycardia (an abnormally fast heart rate).
There are new recommendations for the treatment of hyperthyroidism during pregnancy Propylthiouracil is used during the first trimester to block the synthesis of thyroid hormone and to bring thyroid hormone levels to borderline or slightly higher than normal levels. Propylthiouracil has a lower risk of some rare fetal malformations compared to methimazole (Tapazole) and is preferred during the critical fetal developmental period during the first trimester. Propylthiouracil is not recommended during the remainder or pregnancy because of the risk of serious hepatitis. During the second and third trimester, propylthiouracil should be switched to methimazole. The incidence rate of side effects for each medication is not increased in pregnancy.
Iodine will cross the placenta, so its use in either a thyroid scan or in treatment with radioactive iodine is prohibited in pregnancy.
One positive note for women with hyperthyroidism is that those with Graves' disease or Hashimoto's thyroiditis may have improvement in their symptoms as the pregnancy progesses.
The thyroid gland is located in the front of the neck, below the larynx (voice box). The small, two-inch gland consists of two lobes, one on each side of the windpipe, connected by tissue called the isthmus.
The thyroid tissue is made up of two types of cells: follicular cells and parafollicular cells. Most of the thyroid tissue consists of the follicular cells, which secrete iodine-containing hormones called thyroxine (T4) and triiodothyronine (T3). The parafollicular cells secrete the hormone calcitonin. The thyroid needs iodine to produce the hormones.

Thyroxine and triiodothyronine contain iodine and are formed from thyronines, which are composed of two molecules of the amino acid tyrosine. (Both iodine and tyrosine are acquired in the diet.) Thyroxine contains four iodine atoms, and triiodothyronine contains three iodine atoms. Because each molecule of tyrosine binds one or two iodine atoms, two tyrosines are used to synthesize both thyroxine and triiodothyronine. These two hormones are the only biologically active substances that contain iodine, and they cannot be produced in the absence of iodine. The process leading to the eventual synthesis of thyroxine and triiodothyronine begins in the thyroid follicular cells, which concentrate iodine from the serum. The iodine is then oxidized and attached to tyrosine residues (forming compounds called iodotyrosines) within thyroglobulin molecules. The iodinated tyrosine residues are then rearranged to form thyroxine and triiodothyronine. Therefore, thyroglobulin serves not only as the structure within which thyroxine and triiodothyronine are synthesized but also as the storage form of the two hormones.
Considerably more thyroxine is produced and secreted by the thyroid gland than is triiodothyronine. However, thyroxine is converted to triiodothyronine in many tissues by the action of enzymes called deiodinases. After thyroxine enters a cell, deiodinases located in the cytoplasm remove one of its four iodine atoms, converting it into triiodothyronine. The triiodothyronine either enters the nucleus of the cell or is returned to the circulation. As a result, all of the thyroxine and about 20 percent of the triiodothyronine produced each day come from the thyroid gland. The remaining 80 percent of triiodothyronine comes from deiodination of thyroxine outside of the thyroid. Most if not all of the action of thyroid hormone in its target tissues is exerted by triiodothyronine. Therefore, thyroxine may be considered a circulating precursor of triiodothyronine.
In serum more than 99 percent of the thyroxine and triiodothyronine is bound to one of three proteins. These binding proteins are known as thyroxine-binding globulin, transthyretin (thyroxine-binding prealbumin), and albumin. The remaining thyroxine and triiodothyronine (less than 1 percent) is free, or unbound. When free hormone enters a cell, it is replenished immediately by hormone attached to the binding proteins. The binding proteins serve as reservoirs of the two hormones to protect the tissues from sudden surges of thyroid hormone production and probably also to facilitate delivery of the hormones to the cells of large, solid organs such as the liver.
Essentially all cells in the body are target cells of triiodothyronine. Once triiodothyronine is inside a cell, it enters the nucleus, where it binds to proteins known as nuclear receptors. The triiodothyronine-receptor complexes then bind to deoxyribonucleic acid (DNA) molecules. This results in an increase in the rate at which the affected DNA molecules are transcribed to produce messenger ribonucleic acid (mRNA) molecules and an increase in the rate of synthesis of the protein (translation) coded for by the DNA (by way of the mRNA). Triiodothyronine increases the transcription of DNA molecules that code for many different proteins; however, it also inhibits the transcription of DNA that codes for certain other proteins. The patterns of activation and inhibition differ in different tissue and cell types.

How does the thyroid gland function?
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. Once the T4 in the blood stream goes above a certain level, the pituitary’s production of TSH is shut off. 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) Another measurement done to assess the thyroid status of patients is the Free T4 measurement. It avoid any change the proteins could have, giving us the really value of the T4 level.
The thyroid plays an important role in regulating the body's metabolism and calcium balance. The T4 and T3 hormones stimulate every tissue in the body to produce proteins and increase the amount of oxygen used by cells. The harder the cells work, the harder the organs work. The calcitonin hormone works together with the parathyroid hormone to regulate calcium levels in the body.
Levels of hormones secreted by the thyroid are controlled by the pituitary gland's thyroid-stimulating hormone, which in turn is controlled by the hypothalamus.
Thyroid hormone blood tests include:
Total thyroxine (T4). Most of the thyroxine (T4) in the blood is attached to a protein called thyroxine-binding globulin. Less than 1% of the T4 is unattached. A total T4 blood test measures both bound and free thyroxine. Free thyroxine affects tissue function in the body, but bound thyroxine does not.
Free thyroxine (FTI or FT4). Free thyroxine (T4) can be measured directly (FT4) or calculated as the free thyroxine index (FTI). The FTI tells how much free T4 is present compared to bound T4. The FTI can help tell if abnormal amounts of T4 are present because of abnormal amounts of thyroxine-binding globulin.
Triiodothyronine (T3). Most of the T3 in the blood is attached to thyroxine-binding globulin. Less than 1% of the T3 is unattached. A T3 blood test measures both bound and free triiodothyronine. T3 has a greater effect on the way the body uses energy than T4, even though T3 is normally present in smaller amounts than T4.
Why is the Free Thyroxine (FT4) test done?
Free thyroxine test is carried out to evaluate thyroid function, to help diagnose hypothyroidism or hyperthyroidism and to screen for hypothyroidism in new borns and aid in the diagnosis of female infertility. This test is more accurate as it is not affected by protein level.
Thyroxine (T4) Free is one of two major hormones produced by the thyroid gland, and it accounts for nearly 90 percent of the all thyroid hormone in your system. This test measures the level of free (not total) T4 hormone in your body. This is an important distinction, since the level of free T4 hormone illustrates how much is immediately available for uptake and use by cells. In addition, this test also measures the level of thyroid-stimulating hormone (TSH) produced by the pituitary gland. The resulting TSH level will indicate if your body is producing too much—or too little—of this critical.
People with a family history of thyroid problems should strongly consider testing. This is even more important if you're pregnant or just had a baby, since proper T4 (and T3) levels are essential to brain development in the early years of a child's life. Additionally, consider testing if you are experiencing symptoms of hypothyroidism (unusual weight gain, lethargy, dry skin, a constant feeling of cold, or constipation) or hyperthyroidism (sudden weight loss, increased heart rate, recurring warm or sweaty feeling, or diarrhea).
FT4 can tell whether the thyroid is performing properly. It aids in the diagnosis of hypothyroidism or hyperthyroidism. The test may also be used to help evaluate a patient with an enlarged thyroid gland, called a goitre. It may also aid in the diagnosis of female infertility problems.
A total T4 or free T4 test is usually used in response to an abnormal TSH test result.

Many medicines including oestrogen, certain types of contraceptive birth control pills, those drugs used to help control epilepsy and large doses of aspirin can interfere with total T4 test results, so tell your doctor about any drugs you are taking. In general, free T4 levels are not affected by these medications.
In addition, total T4 levels may be affected by contrast material used for certain X-ray imaging tests. Total T4 is now rarely measured in the UK having been replaced by Free T4 (FT4).
Thyroid hormone tests are done to find out:

  • What is causing an abnormal thyroid-stimulating hormone (TSH) test. For more information, see the topic Thyroid-Stimulating Hormone (TSH). This is the most common reason for thyroid hormone tests.
  • Check how well treatment of thyroid disease is working. The total thyroxine (T4), free thyroxine (FT4), and free thyroxine index (FTI) values are often used to keep track of treatment for hyperthyroidism.
  • Screen newborns to find out if the thyroid gland function is normal. A condition called congenital hypothyroidism can prevent normal growth and development and cause other severe problems, such as intellectual disability, if it is not treated soon after birth.

Many medicines may change the results of this test. Be sure to tell your doctor about all the nonprescription and prescription medicines you take. If you are taking thyroid medicines, tell your doctor when you took your last dose. Your doctor may instruct you to stop taking thyroid medicines temporarily before having this test.
Talk to your doctor about any concerns you have regarding the need for the test, its risks, how it will be done, or what the results will mean. To help you understand the importance of this test.
What does the test include?
The thyroxine (T4) free plus TSH test involves a simple blood sample taken by a qualified technician. Because this test includes a TSH screening, remember that medications like corticosteroids, aspirin and lithium can interfere with those results. Also, if you've had a recent X-ray that used iodine dye—or other radioactive tests—ask your doctor when you can take an accurate TSH test. Pregnant women in their first trimester should also ask their doctor about the proper time to get a TSH test. The blood tests will be analyzed by an accredited laboratory, and you can access the confidential results on the HealthCheckUSA web site within four business days after testing.
Access 2 (Beckman Coulter) Method:
The Access Free T4 (FRT4) assay is a two-step enzyme immunoassay.  Monoclonal antiThyroxine (T4) antibody coupled to biotin, sample, buffered protein solution, and streptavidin-coated solid phase are added to the reaction vessel.  During this first incubation the anti-T4 antibody coupled to biotin binds to the solid phase and the free T4 in the sample.  After incubation in a reaction vessel, materials bound to the solid phase are held in a magnetic field while unbound materials are washed away.  Next, buffered protein solution and triiodothyronine (T3)-alkaline phosphatase conjugate are added to the reaction vessel.  The T3-alkaline phosphatase conjugate binds to the vacant anti-T4 antibody binding sites.  After incubation in a reaction vessel, materials bound to the solid phase are held in a magnetic field while unbound materials are washed away. Then, the chemiluminescent substrate Lumi-Phos™ 530(see notes #1) is added to the vessel and light generated by the reaction is measured with a luminometer.  The light production is inversely proportional to the concentration of free T4 in the sample.  The amount of analyte in the sample is determined from a stored, multi-point calibration curve. The hormone 3,5,3',5'tetraiodothyronine (L-thyroxine,T4) is the most commonly measured substance in the assessment of thyroid function.  Most of the thyroxine secreted into the bloodstream is bound to a transport protein, thyroxine binding globulin (TBG), and to albumin and pre-albumin.  The small fraction (less than 0.1%) of T4 not bound to these proteins is believed to be the metabolically active hormone as well as the precursor to the physiologically active T3 concentration.  This fraction of total T4 concentration is called free thyroxine (free T4, FT4). The level of total thyroxine in the bloodstream is affected by changes in the concentration of the T4 binding proteins, especially thyroxine binding globulin.  Changes in TBG concentration are usually compensated for by proportionate changes in the total T4 levels, as the negative feedback mechanism of the hypothalamic-pituitary-thyroid axis works to keep the free T4 concentration constant.  Many factors can alter the binding protein levels, such as normal pregnancy and estrogen treatment.  Assessment of thyroid function of such individuals by a determination of total serum T4 alone could result in an erroneous diagnosis, since there has been compensation for the alterations of protein content.  The free concentration of T4 is not altered by such factors, and is therefore a more accurate indicator of the thyrometabolic status of the patient. The hypothalamic-pituitary-thyroid axis controls thyroid hormone synthesis, release, and action. Thyrotropin-releasing hormone (TRH) secreted from the hypothalamus stimulates the synthesis and release of thyrotropin or thyroid-stimulating hormone (TSH). TSH, in turn, stimulates the synthesis, storage, secretion, and metabolism of thyroxine (T4) and triiodothyronine (T3).  Both free and bound forms of T4 and T3 are present in the blood.  More than 99% of the T4 and T3 circulate in the blood bound to carrier proteins, leaving less than 1% unbound. It is this level of unbound or free hormone that correlates with the functional thyroid state in most individuals. Free T4 and free T3 regulate normal growth and development by maintaining body temperature and stimulating calorigenesis.  In addition, free T4 and free T3 affect all aspects of carbohydrate metabolism as well as certain areas of lipid and vitamin metabolism. Fetal and neonatal development also require thyroid hormones. Clearly elevated free T4 levels support the clinical findings of a diagnosis of hyperthyroidism while clearly low free T4 levels coupled with appropriate clinical findings, can establish a diagnosis of hypothyroidism.  Measurement of free T4 levels along with other thyroid tests and clinical findings can establish borderline hyperthyroid and hypothyroid diagnoses. Free thyroxine is used as an indicator of patient thyrometabolic status.  Free thyroxine is a better indicator than total T4 in that it is not affected by levels of T4 binding proteins.
Consider all samples received for analysis potentially positive for infectious agents including HIV and the hepatitis B virus.  Observe universal precautions.  Wear gloves, lab coat, and safety glasses when handling all human blood products and infectious viruses.  Place disposable plastic, glass, paper, and gloves that contact blood in a biohazard bag or discard pan to be autoclaved.  Disinfect all work surfaces with staphene solution.  Dispose of all biological samples and diluted specimens in a biohazard bag at the end of the analysis.  
Do not pipette by mouth.  Do not eat, drink or smoke in designated work areas.  Wash hands thoroughly after removal or personal protective devices used in handling specimens and kit reagents.
A. No special instructions such as fasting or special diets are required.  
B. Serum is the preferred specimen type.  Heparin plasma is acceptable.  If testing is to be done within 48 hours, samples can be refrigerated at 2 to 8°C.  Freeze at -20ºC or colder for longer storage.
C. Blood should be collected aseptically and the serum separated by standard laboratory techniques.  Specimens may be collected by using regular or serum-separator Vacutainers.  Serum should be separated from the cells within 2 hours of collection.
D. The requested sample volume for the assay is 1.0 mL, and the minimum sample volume is 0.3 mL.
E. Specimens may be stored in glass or plastic vials, as long as the vials are tightly sealed to prevent desiccation of the sample.
F. Turbid samples or those with particulate matter should be centrifuged prior to assay.
G. More than one freeze-thaw cycles is not recommended.
Example free T4 Calibration:

Free T4 concentrations are calculated by using a calibration curve. This method utilizes a four parameter logistic curve with an inverse relationship of measured light produced (RLU) to concentration of free T4 in the serum sample.  Serum results are expressed as mg/dL. Calibrators are traceable by to the manufacturer’s working calibrators.  The assigned values were established using representative samples from this lot of calibrator and are specific to the assay methodologies of the Access reagents.  Values assigned by other methodologies may be different. An active calibration curve is required for all tests. For the Access free T4 assay, calibration is required every 28 days or whenever new lot numbers of reagents are placed into use.  Refer to the Operator’s Guide and Reference Manual for complete instructions on calibration procedures.
What are the common signs/symptoms when the Free Thyroxine (FT4) test is done?
Excessive amounts of T4 in the blood results in symptoms associated with hyperthyroidism such as:

  • Nervousness
  • Tremors of the hands
  • Weight loss
  • Insomnia
  • Puffiness around dry, irritated eyes
  • Increased heart rate
  • Anxiety

Insufficient amounts of T4, show symptoms associated with hypothyroidism and a slowed metabolism, such as:

  • Weight gain
  • Dry skin
  • Fatigue
  • Constipation
  • Irregular menstruation

What should I do before the Free Thyroxine (FT4) test?

Specimen type

Specimen collection procedure

Preparatory instructions before the test *

(Blood Sample)

(Collection of blood from a vein, usually from the arm)

No Fasting required.
No other special preparations required.

The health professional taking a sample of your blood will:

  • Wrap an elastic band around your upper arm to stop the flow of blood. This makes the veins below the band larger so it is easier to put a needle into the vein.
  • Clean the needle site with alcohol.
  • Put the needle into the vein. More than one needle stick may be needed.
  • Attach a tube to the needle to fill it with blood.
  • Remove the band from your arm when enough blood is collected.
  • Put a gauze pad or cotton ball over the needle site as the needle is removed.
  • Put pressure on the site and then put on a bandage.

Heel stick
A heel stick is used to obtain a blood sample from a newborn. The baby's heel is pricked with a sharp instrument (lancet) and several drops of blood are collected.
How It Feels
Blood test
The blood sample is taken from a vein in your arm. An elastic band is wrapped around your upper arm. It may feel tight. You may feel nothing at all from the needle, or you may feel a quick sting or pinch.
Heel stick
A brief pain, like a sting or a pinch, is usually felt when the lancet punctures the skin. Your baby may feel a little discomfort with the skin puncture.
Blood test
There is very little chance of a problem from having blood sample taken from a vein.
You may get a small bruise at the site. You can lower the chance of bruising by keeping pressure on the site for several minutes.
In rare cases, the vein may become swollen after the blood sample is taken. This problem is called phlebitis. A warm compress can be used several times a day to treat this.
Ongoing bleeding can be a problem for people with bleeding disorders. Aspirin, warfarin (Coumadin), and other blood-thinning medicines can make bleeding more likely. If you have bleeding or clotting problems, or if you take blood-thinning medicine, tell your doctor before your blood sample is taken.
Heel stick
There is very little chance of a problem from a heel stick. A small bruise may develop at the site.
How do I interpret my Free Thyroxin (FT4) test results?

Reference Range*


0.8 to 1.8 ng/dL


‘*A Reference range is a set of values which helps the healthcare professional to interpret a medical test. It may vary with age, gender, and other factors. Reference ranges may also vary between labs, in value & units depending on instruments used and method of establishment of reference ranges’
The results should always be interpreted in association with TSH.
Free T4 is elevated in hyperthyroidism. Levels are usually depressed in hypothyroidism, but TSH elevation is a more sensitive indicator.
A patient may be hypothyroid, with TSH levels greater than twice the reference interval but free T4 within the reference interval.
Total T4 is occasionally measured (as part of the free thyroxine index) when free T4 results are suspected, on clinical grounds, of being anomalous.
Primary hypothyroidism cannot be excluded by free thyroxine alone.
Free T4 may be reduced or increased in non-thyroidal illness (sick euthyroid syndrome), but TSH levels are not abnormal.
Thyroid hormone tests are blood tests that check how well the thyroid  gland is working.
The normal values listed here-called a reference range-are just a guide. These ranges vary from lab to lab, and your lab may have a different range for what’s normal. Your lab report should contain the range your lab uses. Also, your doctor will evaluate your results based on your health and other factors. This means that a value that falls outside the normal values listed here may still be normal for you or your lab.
Free thyroxine (FT4): 0.7-2.0 ng/dL nanograms per deciliter (ng/dL) or 10-26 picomoles per liter (pmol/L)
High values
High thyroid hormone levels (hyperthyroidism) may be caused by:

  • Diseases of the thyroid gland, such as Graves' disease, thyroiditis, or a goiter that contains one or more abnormal growths (nodules).
  • Taking too much thyroid medicine.

Low values
Low thyroid hormone levels (hypothyroidism) may be caused by:

  • Thyroid disease, such as thyroiditis.
  • Pituitary gland disease.
  • Destruction of the thyroid gland by surgery or radiation.

What Affects the Test
Reasons you may not be able to have the test or why the results may not be helpful include:
Taking certain medicines, such as:

  • Corticosteroids, estrogen, progesterone, or birth control pills.
  • Blood-thinning medicines such as aspirin, heparin, or warfarin (Coumadin).
  • Antiseizure medicines such as Dilantin or Tegretol.
  • Heart medicines such as amiodarone or propranolol.
  • Lithium.
  • Having recently had an X-ray test that uses contrast material.
  • Being pregnant.

What To Think About

  • Other blood tests are often used to check how well the thyroid gland is working.
    • Thyroid-stimulating hormone (TSH) test measures the amount of TSH in the blood and is considered the most reliable way to find a thyroid problem. If the TSH test is abnormal, other thyroid hormone tests such as a T3 or T4 may be done. For more information, see the topic Thyroid-Stimulating Hormone.
    • Thyroid antibodies test measures the presence of antibodies against thyroid tissue. Antibodies may mean that you have an autoimmune disease such as Hashimoto's thyroiditis or Graves' disease.
    • Thyroxine-binding globulin (TBG) test. TBG is an important protein in the blood that carries the thyroid hormones T3 and T4. TBG testing is not done very often.
  • Other tests used to investigate problems with the thyroid gland include thyroid scan, ultrasound, or biopsy. For more information, see the topics Thyroid Scan and Radioactive Iodine Uptake Test, Thyroid Ultrasound and Parathyroid Ultrasound, and Thyroid Biopsy.
  • Because false-positive results can occur when testing a newborn for congenital hypothyroidism, the thyroid hormone tests may be repeated a few days after initial testing. If the results are still abnormal and congenital hypothyroidism is suspected, additional testing is done.

Diseases/conditions related to the Free Thyroxine (FT4) test:

  • Hypothyroidism
  • Hashimoto's disease
  • Graves disease
  • Germ cell tumors
  • Iodine-induced hyperthyroidism
  • Subacute or chronic thyroiditis
  • Toxic multinodular goiter
  • Trophoblastic disease
  • Goiter
  • Thyroid Disorders
  • Acute thyroiditis
  • Cretinism
  • Cirrhosis
  • Malnutrition
  • Hepatitis

Other tests related to the Free Thyroxine (FT4) test

  • TSH
  • T3
  • Thyroid Antibodies


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