Blood is drawn from a vein on the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and an elastic band is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the band to 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 band 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.

For an 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.

• infant test or procedure preparation (birth to 1 year)
• toddler test or procedure preparation (1 to 3 years)
• preschooler test or procedure preparation (3 to 6 years)
• schoolage test or procedure preparation (6 to 12 years)
• adolescent test or procedure preparation (12 to 18 years)

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 made by the hypothalamus.

In people with normal thyroid function, the presence of sufficient T3 and T4 inhibits the release of both TSH and TRH, preventing excess T3 and T4. Most of the thyroid hormone secreted by the thyroid gland is in the form of T4, but T3 is the active hormone, that is, the body's tissues convert T4 to T3.

T4 is the major hormone controlling the basal metabolic rate, or metabolism. 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 the body.

Most of the T4 in the blood is bound to proteins. This provdes a reservoir of available thyroxine because only free T4 can act on cells. T4 appears to be converted to T3 within cells, resulting in the production of various proteins by the cell.

Most of the T3 and T4 is transported by a protein called 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 and hormones, such as estrogen, can change the level of TBG in the body.

• hyperthyroid states
  • Graves' disease
  • toxic multinodular goiter
  • subacute or chronic thyroiditis
  • early Hashimoto's disease
  • iodine-induced hyperthyroidism
  • germ cell tumors
  • trophoblastic disease

• 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

• hypopituitarism
• hypothyroidism; primary
• hypothyroidism; secondary
• thyrotoxic periodic paralysis

• 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