A thyroxine (T4 Test) is a blood test that helps diagnose thyroid conditions. The thyroid is a small, butterfly-shaped gland at the base of the throat. Your thyroid makes hormones that control how your body uses energy. These hormones affect your weight, heart, body temperature, muscle strength, and even your mood. In children, thyroid hormones also affect growth.
| Also Known as | T4, Free Thyroxine, Free T4, Total T4 concentration, FT4 Index, Thyroxine Screen, Free T4 concentration, Thyroxine, Total T4, Bound T4,Thyroxine Test by Equilibrium Dialysis, Free Thyroxine Index, FTI, Free T4 Index, T4 Index , FT4i, T7 |
| Test Purpose | This is a blood test that helps diagnose thyroid conditions. |
| Test Preparations | No Special Preparation Required |
| Test Components | T4, Free T4, Free T4 Index |
| Specimen | 3 ML (1.5 ML Min.) Serum From 1 SST Tube |
| Stability Room | 2 Hrs |
| Stability Refrigerated | 1 Week |
| Stability Frozen | 4 Weeks |
| Method | Chemiluminescent Immunoassay |
| Download Report | Download Report |
- T4: This Test measure free and bound T4 together. A calculation can be done to find out how much T4 is free. This is a less accurate way to check how well your thyroid is working.
- Total T4: This Teest measure free and bound T4 together. A calculation can be done to find out how much T4 is free. This is a less accurate way to check how well your thyroid is working.
- Bound T4: This Teest measure free and bound T4 together. A calculation can be done to find out how much T4 is free. This is a less accurate way to check how well your thyroid is working.
- Free T4: It directly measures the amount of free T4 in the blood. Medical experts believe that this test provides more accurate information than a total T4 test, which is why it is used more often.
- Free T4 Index (FTI): Estimating the amount of circulating FT4 using the Total T4 and thyroid binding capacity (TBC).
Definitions:
Your body makes two different T4 hormones.
- T4 / Total T4 :
Most T4 is bound to proteins, This Test measure free and bound T4 together. Total T4 testing measures the total amount of T4, including free T4 and T4 that is bound to proteins. T4 levels are important, because T4 increases numerous enzymes that produce energy for the body. - FT4/Free T4 :
Free T4 is the thyroxine that is not attached to a protein in the blood.Un T4 libre mide lo que no está unido y puede entrar y afectar los tejidos del cuerpo. - FTI/ FT4 Index:
Estimating the amount of circulating FT4 using the Total T4 and thyroid binding capacity (TBC) The FTI is considered a more reliable indicator of thyroid status in the presence of abnormalities in plasma protein binding. A method used to correct the altered binding capacity, derived from the product of TT4 serum and the absorption ratio of labeled T3 Resin Uptake, is also generally low.
FTI =thyroxine (T4)/thyroid binding capacity
What other tests might I have along with this test?
You may also need these tests:
- T3 uptake.
- Free T3.
- Total T3.
- Total T4.
- Free T4.
- Free thyroxine index.
- TSH.
- Thyroglobulin (Tg)
- Thyroid scan
- Radioactive iodine uptake test
- Thyroid-stimulating immunoglobulin (TSI) test
Why Get Tested T4 Test :
A T4 test is used to check how well your thyroid is working. It is also used to help diagnose and monitor thyroid diseases and conditions that may affect your thyroid, including:
- Hypothyroidism or underactive thyroid, which is when the thyroid gland does not produce enough thyroid hormone to meet your body’s needs.
- Hyperthyroidism or overactive thyroid, which is when the thyroid gland produces more thyroid hormone than your body needs
- Disorders of the pituitary gland, which is a gland at the base of the brain that produces TSH (thyroid-stimulating hormone). TSH controls the amount of T4 that the thyroid gland produces. If the results of a TSH test are not normal, a T4 test can help diagnose whether the problem is with the thyroid or the pituitary gland.
- In certain cases, a T4 test may be done as part of a group of tests called a thyroid panel. A thyroid panel includes a TSH test as well as other thyroid tests.
When to Get Tested:
Our doctor may order a T4 test if a thyroid-stimulating hormone (TSH) test has returned abnormal results. A T4 test will help your doctor determine what type of problem is affecting your thyroid.
Some disorders that affect thyroid function include:
- Hyperthyroidism or overactive thyroid
- Hypothyroidism or underactive thyroid
- Hypopituitarism or underactive pituitary gland
Your doctor may suspect that one of these conditions is present if you have symptoms such as:
- Eye problems, such as dryness, irritation, swelling, and bumps
- Dry skin or swelling of the skin
- Hair loss
- Hand tremors
- Heart rate changes
- Blood pressure changes
You may also experience more general symptoms, such as:
- Weight changes
- Difficulty sleeping or insomnia
- Anxiety
- Fatigue and weakness
- Cold intolerance
- Light sensitivity
- Menstrual irregularity
Sometimes your doctor may also order other thyroid tests (such as T3 or TSH) after you have a T4 test.
Sample Required:
3 ML (1.5 ML Min.) Serum From 1 SST Tube
Preparation for Sample:
No need any Preparation for this test
Before the test
Drugs that can affect the measurements of T4 include:
- Birth control pills
- Clofibrate
- Estrogens
- Methadone
- Certain herbal remedies
- Amiodarone
- Anabolic steroids
- Androgens
- Antithyroid drugs (for example, propylthiouracil and methimazole)
- Lithium
- Phenytoin
- Propranolol
Normal Values:
Source 1
- T4: 5.0 μg/dl – 12.0 μg/dl
- FT4: 0.8 ng/dl – 1.8 ng/dl
- FT4I: 6.0 mcg/dL –11.0 mcg/dL
Source 2:
- T4:
- 0-5 days: 5.0-18.5 mcg/dL
- 6 days-2 months: 5.4-17.0 mcg/dL
- 3-11 months: 5.7-16.0 mcg/dL
- 1-5 years: 6.0-14.7 mcg/dL
- 6-10 years: 6.0-13.8 mcg/dL
- 11-19 years: 5.9-13.2 mcg/dL
- ≥20 years: 4.5-11.7 mcg/dL
- FT4: 0.9 to 2.3 ng/dL (12-30 pmol/L)
- FTI: 1.1 mcg/dL – 4.3 mcg/d
Source 3:
T4 Total (T4)
| Age | µg/dL |
| Cord blood | 7.4 to 13.0 |
| 1 to 3 days | 11.8 to 22.6 |
| 1 to 2 weeks | 9.8 to 16.6 |
| 1 to 4 month | 7.2 to 14.4 |
| 4 to 12 month | 7.8 to 16.5 |
| 1 to 5 year | 7.3 to 15.0 |
| 5 to 10 year | 6.4 to 13.3 |
| 10 to 15 year | 5.6 to 11.7 |
| Adult | Male = 4.6 to 10.5 Female = 5.5 to 11.0 |
| >60 year | 5.0 to 10.7 |
| Maternal serum 15 to 40 weeks | 9.1 to 14.0 |
T4 Free (FT4)
| Age | ng/dL |
| Newborn 1 to 4 days | 2.2 to 5.3 |
| Child (2 weeks to 20 Years | 0.8 to 2.0 |
| Adult 21 to 87 years | 0.8 to 2.7 |
| Pregnancy | |
| 1st trimester | 0.7 to 2.0 |
| 2nd and 3rd trimester | 0.5 to 1.6 |
- To converts SI unit x 12.9 = nmol/L
| TSH | Total and Free T4 | Total and Free T3 | MOST LIKELY DIAGNOSIS |
|---|---|---|---|
| Normal | Normal | Normal | Normal thyroid function (e.g., “euthyroid”) |
| Normal or decreased | Normal or decreased | Decreased | Normal adjustment in thyroid function due to illness (nonthyroidal illness or sick euthyroid syndrome) |
| Increased | Normal | Normal | Subclinical hypothyroidism; in a person with hypothyroidism on treatment, not enough thyroid hormone is being given |
| Increased | Decreased | Normal of decreased | Hypothyroidism resulting from a problem with the thyroid gland itself (primary hypothyroidism) |
| Normal or increased | Increased | Increased | Hyperthyroidism resulting from a problem with the pituitary gland signals (central hyperthyroidism) or from a problem with the thyroid hormone receptor (thyroid hormone resistance) |
| Decreased | Normal | Normal | Subclinical Hyperthyroidism; in a person with Hypothyroidism, too much thyroid hormone is being given |
| Decreased | Normal | Increased | Hyperthyroidism resulting from the thyroid gland making too much active thyroid hormone T3 (uncommon, also known as T3 toxicosis) |
| Decreased | Increased | Increased | Hyperthyroidism resulting from the gland making too much thyroid hormones (primary hyperthyroidism) |
| Decreased | Decreased | Decreased | Hypothyroidism resulting from a problem with the hypothalamus or pituitary signals that govern the thyroid gland (central hypothyroidism) |
Thyroid Function Tests Summary:
| Disease | T3 | T4 (total) | T4 free | TSH | Cholesterol |
|---|---|---|---|---|---|
| Hypothyroidism, | decreased | decreased | low | increased | increased |
| Hyperthyroidism | increased | increased | increased | decreased or absent | decreased |
| Excess Thyroid binding globulin (TBG) | increased | increased | Normal | decreased | |
| Decreased thyroglobulin (TBG) | Low | Low | Normal |
Increase Level of T4, FT4 is seen in :
- Hypothyroidism, which can be caused by Hashimoto’s disease or another medical condition that causes the thyroid to produce too little T4
- Thyroiditis (inflammation of the thyroid)
- Side effects of certain medications
- Congenital hypothyroidism (a condition that affects babies from birth)
- High levels of protein in the blood
- Too much thyroid replacement medication
- Graves disease
- Toxic goiter or toxic thyroid nodules
- Some tumors of the testes or ovaries (rare)
- Euthyroid sick syndrome.
Decrease Level of T4, FT4 is seen in :
- Hyperthyroidism, which can be caused by Graves’ disease or another medical condition that causes the thyroid to make too much T4
- Thyroiditis (inflammation of the thyroid)
- Toxic goiter (an enlarged thyroid with areas that produce extra thyroid hormone)
- Toxic thyroid nodule (a single lump in the thyroid that produces extra thyroid hormone)
- A noncancerous (noncancerous) tumor in the pituitary gland
- Iodine insufficiency.
- Myxedema.
- Advanced cancer.
- Cushing’s disease.
- Cirrhosis.
- Renal failure.
Possible References Used
