| Definition: |
A test that measures the amount of T4 in the blood.
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| Alternative Names: |
Thyroxine test; thyroid function test
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| How the test is performed: |
Adult or child:
Blood is drawn from a vein (venipuncture), usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and a tourniquet (an elastic band) or blood pressure cuff is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the tourniquet to distend (fill with blood). A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the tourniquet is removed to restore circulation. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.
Infant or young child:
The area is cleansed with antiseptic and punctured with a sharp needle or a lancet. The blood may be collected in a pipette (small glass tube), on a slide, onto a test strip, or into a small container. Cotton or a bandage may be applied to the puncture site if there is any continued bleeding. |
| How to prepare for the test: |
The health care provider may advise you to withhold drugs that may affect the test (see "special considerations").
Infants and children:
The physical and psychological preparation you can provide for this or any test or procedure depends on your child's age, interests, previous experience, and level of trust. For specific information regarding how you can prepare your child, see the following topics as they correspond to your child's age:
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| How the test will feel: |
| When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing. |
| Why the test is performed: |
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This test may be performed as part of an evaluation of thyroid function.
Thyroid function is complex, and depends on the action of many different hormones. Some hormones, such as T3, are active hormones and directly affect the way the body functions. Other hormones regulate hormone production, but do not have physiologic effects themselves.
Thyroid-stimulating hormone (TSH) is a hormone secreted by the pituitary gland. It stimulates the synthesis and secretion of T4 (thyroxine) and T3 (triiodothyronine), by the thyroid gland. TSH is stimulated by another hormone, thyroid-releasing hormone (TRH), which is released by the hypothalamus.
T3 and T4 feedback inhibit the release of both TSH and TRH in people with normal thyroid function. Most of the thyroid hormone secreted by the thyroid gland is in the form of T4, but T3 is the active hormone; i.e., T4 is converted to T3 by tissues in the body.
T4 is the major hormone controlling the basal metabolic rate. The exact mechanisms are not completely known, but it is known that T4 increases the concentrations of numerous enzymes involved in the production of energy in all nucleated cells of the body. Most of the T4 in the blood is bound to proteins; this acts as a reservoir of available thyroxine because only free T4 is active on cells. T4 appears to be converted to T3 within cells, before it enters the nucleus and interacts direction with DNA, eventually resulting in the production of various proteins by the cell.
Only 0.03% of the T4 in plasma is free (soluble and active) and 0.3% of the T3 is free; the rest is bound to protein. Most of the T3 and T4 are transported by TBG (thyroxine binding globulin), but smaller amounts are found on prealbumin and albumin. The available concentrations of T3 and T4 are affected by the level of TBG. Certain conditions can change the level of TBG in the body, such as estrogen.
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| Normal Values: |
Normal values vary among different laboratories. A typical normal range is: 4.5 to 11.2 mcg/dl.
Note: mcg/dl = micrograms per deciliter |
| What abnormal results mean: |
Greater-than-normal levels of thyroxine along with suppressed (lower than normal) levels of TSH may indicate:
Lower-than-normal levels may indicate:
- hypothyroidism (including: later Hashimoto's disease, cretinism, myxedema, goitrous diseases, following radioiodine or surgical thyroid ablation, infiltrative diseases such as scleroderma, amyloid goiter, or hemochromatosis, following neck irradiation for head and neck cancer)
- decreased conversion of T3 to T4, as in:
- malnutrition
- systemic illness
- fasting
- use of certain prescribed medication, including: dexamethasone, propranolol, lithium, iodine, methimazole, propylthiouracil, interferon alfa, interleukin-2, and amiodarone
Additional conditions under which the test may be performed:
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| What the risks are: |
Risks associated with venipuncture are slight:
- excessive bleeding
- fainting or feeling lightheaded
- hematoma (blood accumulating under the skin)
- infection (a slight risk any time the skin is broken)
- multiple punctures to locate veins
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| Special considerations: |
Drugs that can increase T4 measurements include clofibrate, estrogens, methadone, amiodarone, and oral contraceptives.
Drugs that can decrease T4 measurements include anabolic steroids, androgens, antithyroid drugs (for example, propylthiouracil and methimazole), lithium, phenytoin, propranolol, amiodarone, interferon alpha, and interleukin-2.
Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others. |
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