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RT3U 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:
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:
How it Feels:
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 .
Risks:
- 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
Why this Test is Performed:
This test is performed as part of a thyroid function test.
T4 (thyroxine) is the major hormone controlling the basal metabolic rate. Most of the thyroxine in the blood is bound to proteins ; this acts as a reservoir of available thyroxine, because only free thyroxine is active on cells. Thyroxine appears to be converted to triiodothyronine ( T3 ) within cells, before it enters the nucleus and interacts directly with DNA, eventually resulting in the production of various proteins by the cell.
RT3U indirectly (that is, RT3U is inversely proportional to TBG levels) quantitates thyroxin binding globulin (TBG), the protein that carries most of the T3 and thyroxine in the blood. Pregnancy or oral contraceptive, for example, may stimulate increased production of TBG by the liver, even though you are with normal thyroid function (euthyroid). Androgens, serious illness, and renal disease may lower TBG even when you are euthyroid. These changes in TBG will affect the level of free thyroxine and thus thyroid hormone functions. It is important to not be diagnosed as having thyroid dysfunction, since it would result in inappropriate treatment.
Normal Values:
24% to 37%
Abnormal Results:
Greater-than-normal levels may indicate: Lower-than-normal levels may indicate: Additional conditions under which the test may be performed:
Cost:
The estimated cost is $15.
Special Considerations:
Drugs that can increase RT3U (that is, lower TBG) include anabolic steroids, heparin, phenytoin, salicylates (high dose), and warfarin.
Drugs that can decrease RT3U include antithyroid agents, clofibrate, estrogen, oral contraceptives, and thiazides.
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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