Thyroid.org describes the thyroid gland as a butterfly-shaped gland located in the lower front of the neck. It makes hormones that help each cell in the body to work right. Thyroid function tests are used to tell how well the thyroid is working. They are used to help diagnose hypothyroid, or underactive function; hyperthyroid, or overactive function; and to monitor response to thyroid medications.
T3 Blood Test
Triiodothyronine, or T3, plays an important, active role in metabolism. T3 is normally present in very small amounts, but has a significant impact. The range for normal values is 100 to 200 nanograms per deciliter, according to Medline Plus. Greater-than-normal values may be due to hyperthyroidism, thyroid cancer or high levels of a protein that carries T3 in the blood. Lower-than-normal values may indicate hypothyroidism, starvation or a long illness.
T4 Blood Test
Thyroxine is also known as T4 because it contains four iodine atoms, according to thyroid.com. Most T4 is transported in the bloodstream by proteins, and the liver converts T4 into T3. A typical normal range, says Medline Plus, is 4.5 to 11.2 micrograms per deciliter. If higher, it can indicate hyperthyroidism, or Graves disease; toxic goiter; or thyroiditis. Lower levels may involve such conditions as hypothyroidism, malnutrition or use of certain medications.
TSH Blood Test
TSH, or thyroid stimulating hormone, is produced by the pituitary gland and tells the thyroid gland to produce and release T3 and T4, says Medline Plus. Normal lab values are 0.4 to 4.0 milliliters per deciliter; this range is less if under treatment for a thyroid disorder, at 0.5 to 3.0 milliliters per deciliter. A high level means the thyroid gland is failing, or hypothyroidism. In the opposite situation, when the TSH level is low, hyperthyroidism is indicated. Occasionally, a low TSH level may result from an abnormality in the pituitary gland.
Medline Plus reports that “a thyroid scan is a nuclear medicine exam that uses a radioactive iodine tracer to see how well the thyroid gland is working.” A radioactive iodine pill is swallowed; the thyroid gland is scanned four to six hours later to detect the amount of iodine absorbed and to observe a “picture” of the thyroid. In a normal scan, the thyroid appears the correct size, shape and in the proper location. Hypothyroidism is suspected if less than the normal amount of iodine is absorbed, and hyperthyroidism if there is higher-than-normal iodine absorption.
Thyroid Gland Function
A description of thyroid function is given by thyroid.org in that the thyroid gland and the pituitary gland act like a heater and a Thermostat in the body. The amount of TSH that the pituitary sends into the bloodstream depends on the recognized amount of T4 from the thyroid. More TSH is released if very little T4 is recognized. When the T4 in the bloodstream rises above a certain level, the pituitary production of TSH is shut off.