Thyroxine (T4) is one of two major hormones produced by the thyroid gland, a small butterfly-shaped organ that lies flat across the windpipe at the base of the throat. The other major thyroid hormone is called triiodothyronine (T3) and together they help control the rate at which the body uses energy. Almost all of the T4 (and T3) found in the blood is bound to protein. The rest is free (unbound) and is the biologically active form of the hormone. This test measures the amount of free T4 in the blood.
Pituitary-Thyroid Feedback System
T4 production is regulated by a feedback system. When the level of T4 in the bloodstream decreases, the hypothalamus releases thyrotropin releasing hormone, which stimulates the pituitary gland to produce and release thyroid-stimulating hormone (TSH). TSH then stimulates the thyroid gland to make and/or release more T4. As the blood concentration of T4 increases, TSH release is inhibited.
T4 makes up about 90% of thyroid hormones. When the body requires thyroid hormone, the thyroid gland releases stored T4 into circulation. In the blood, T4 is either free (not bound) or bound to protein (primarily bound to thyroxine-binding globulin). The concentration of free T4 is only about 0.1% of that of total T4. T4 is converted into T3 in the liver or other tissues. T3, like T4, is also mostly bound to protein, but it is the free forms of T3 and T4 that are biologically active. Free T3 is 4 to 5 times more active than free T4 in circulation.
If the thyroid gland does not produce sufficient T4, due to thyroid dysfunction or to insufficient TSH, then the affected person experiences symptoms of hypothyroidism such as weight gain, dry skin, cold intolerance, irregular menstruation, and fatigue. Severe untreated hypothyroidism, called myxedema, can lead to heart failure, seizures, and coma. In children, hypothyroidism can stunt growth and delay sexual development.
If the thyroid gland produces too much T4, the rate of the person's body functions will increase and cause symptoms associated with hyperthyroidism such as increased heart rate, anxiety, weight loss, difficulty sleeping, tremors in the hands, and puffiness around dry, irritated eyes.
The most common causes of thyroid dysfunction are related to autoimmune disorders. Graves disease causes hyperthyroidism and Hashimoto thyroiditis causes hypothyroidism. Both hyperthyroidism and hypothyroidism can also be caused by thyroiditis, thyroid cancer, and excessive or deficient production of TSH. The effect of these conditions on thyroid hormone production can be detected and monitored by measuring the free T4.
How is the sample collected for testing? A blood sample is obtained from a needle placed in a vein in the arm or from pricking the heel of an infant.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Free Thyroxine, FT4
To help evaluate thyroid gland function; to help diagnose thyroid disease; to screen for hypothyroidism in newborns; to monitor effectiveness of treatment.When you have signs and symptoms of thyroid disease, usually after an abnormal result on a TSH test; commonly performed as a screening test on newborns soon after birth; when you are being treated for a thyroid disorder.
A free T4 test may be ordered when a person has symptoms of hyperthyroidism or hypothyroidism, particularly if the TSH test is out of range.
Signs and symptoms of hyperthyroidism may include:
Symptoms of hypothyroidism may include:
Free T4 testing may be ordered along with other thyroid tests on a regular basis when a person is undergoing treatment for a thyroid disorder.
In pregnant women with thyroid disorders, the health practitioner is likely to order thyroid testing early and late in the pregnancy and for a time period following delivery to monitor the mother and baby.
Thyroid hormone screening is commonly performed on newborns in the United States as part of newborn screening programs.
We at iHealthMantra enable patients to book their diagnostics tests from best Diagnostics Labs in India such as Thyrocare, Dr Lal Pathlabs, Metropolis, Suburban Diagnostics, Medall Clumax and many others top quality diagnostic labs. Diagnostics tests booked through us are at low price and patients can avail free home sample collection.
We also provide preventive health checkups, Regular Health check-up, Health checkup packages, Thyroid Profile, PET Scan, MRI Scan, CT Scan, Full body check up, Vitamin D Profile, Lipid Profile, Renal Function Tests and other blood tests. Provided below is a brief description of few of our partner labs.
Thyrocare, with its tag line of Think Thyroid, Think Thyrocare, is India's first fully automated diagnostic laboratory with a focus on providing quality at affordable costs to retail customers, laboratories and hospitals in India and other countries. Thyrocare has strong focus on technology, brand and systems that enables Thyrocare laboratories to give their clients the best of science and technology at an affordable cost. Thyrocare laboratories have NABL and CAP accreditations. Thyrocare supports home sample collection to provide convenience to customers.Thyrocare is a PAN India chain with services across all the major Indian cities such New Delhi, Mumbai, Pune, Bangalore, Chennai, Hyderabad and Kolkata.
Key tests and health profiles supported by Thyrocare are: T3, T4, TSH also called Thyroid Profile, TSH also called Thyroid Stimulating Hormone, Thyrocare Aarogyam Packages such as Mini Aarogyam, Aarogyam A, Aarogyam B, Aarogyam C, Vitamin Profiles such as Vitamin D Profile and Vitamin D3 test.
Metropolis is India?s one of the biggest and multinational chain of pathology labs and diagnostics centre known for excellent customer satisfaction and preventive healthcare. Metropolis has following accreditations, CAP, NABL and RNTCP. Metropolis Healthcare has been honoured for its best practices and its commitment to excellence in customer satisfaction and innovation. The journey of Metropolis started in 1981, when its Founder and Chairman, Dr. Sushil Shah had a vision to introduce specialized testing facilities to the Indian population. Processing over 30 million tests a year, catering to over 20,000 Laboratories, Hospitals, Nursing Homes and 2,00,000 consultants; and with 34 years of experience in delivering accurate reports, Metropolis has earned the reputation of being India's multinational chain of laboratories with a strong presence in Sri Lanka, UAE, South Africa, Kenya, Mauritius & Ghana. Metropolis supports home sample collection to provide convenience to customers. Metropolis is a PAN India chain with services across all the major Indian cities such New Delhi, Mumbai, Pune, Bangalore, Chennai and Hyderabad.
Common Tests supported by Metropolis are Lipid Profile, Thyroid Profile, Vitamin D Profile, Renal function test, Blood Tests, Vitamin D3, TSH, T3, T4, Diabetes Test, ECG test, thalassemia test, preventive health checkups, Regular Health check-up, Health checkup packages and Full body check up.
About Dr Lal Pathlabs
Dr Lal PathLabs is a leading Diagnostic Centre and Pathology Lab for Blood Test in India. Dr Lal PathLabs has a strong focus on quality with NABL accreditation. Dr Lal PathLabs has a qualified team of Pathologists, Radiologists, Microbiologists, Biochemists and specialists. Dr Lal PathLabs supports home sample collection to provide convenience to customers. Dr Lal PathLabs is a PAN India chain with services across all the major Indian cities such New Delhi, Mumbai, Pune, Bangalore, Chennai, Hyderabad and Kolkata.
Common Tests supported by Dr Lal PathLabs are Lipid Profile, Thyroid Profile, Vitamin D Profile, Renal function test, Blood Tests, Vitamin D3, TSH, T3, T4, Diabetes Test, ECG test, thalassemia test, preventive health checkups, Regular Health check-up, Health checkup packages and Full body check up. Few unique health packages supported by Dr Lal PathLabs are Swasth Plus Basic Health Checkup, Swasth Plus Advanced Health Checkup and Swasth Plus Vitamins Package.
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