The thyroid gland is a butterfly-shaped gland in the neck. It has a lobe on each side of the Adam's apple with a thin ridge of tissue between them. 

The two major thyroid hormones are thyroxine (T4) and triiodothyronine (T3). Thyroxine is produced by the thyroid gland when the pituitary gland releases thyroid-stimulating hormone. Almost all of the triiodothyronine is made from thyroxine released by the thyroid gland, with only a small amount produced directly by the thyroid gland itself. T3 and T4 help control the body's use of food for energy (metabolism).

Special cells within the thyroid gland (called parafollicular cells, or “C” cells) also produce a hormone, calcitonin. Calcitonin may help to regulate use of calcium in the bones.

Thyroid hormones are needed for normal development of the brain, especially during the first three years of life. An infant whose thyroid gland does not produce enough thyroid hormone (congenital hypothyroidism) may, in severe cases, become mentally retarded. Older children also need thyroid hormones to grow and develop normally.

The most common blood tests to evaluate thyroid function are:

• Thyroxine (T4). Most of the T4 in blood is attached to a protein (called thyroxine-binding globulin). Less than 1% of the T4 is unattached. This is called free T4 or FT4, and it affects metabolism. The amount of FT4 can be measured; however, it is more common to measure the total T4 amount.
• Triiodothyronine (T3). T3 has a greater effect on metabolism than T4, even though T3 is normally present in lower amounts than T4. Most T3 is made from T4 by body tissues after T4 is released from the thyroid gland. The rest of the T3 is produced directly by the thyroid gland. The total amount of T3 in the blood or the amount of free T3 (FT3) can be measured. Normally, less than 1% of the T3 is free.
• Triiodothyronine uptake (T3U). The T3U test is an indirect measurement of the amount of the protein (thyroxine-binding globulin) that can bind to T3 and T4. The results of this test are helpful only when evaluated along with other thyroid function tests.
• Free thyroxine index (FTI or FT4). The FTI is a measure of the amount of T4 in relation to the amount of thyroxine-binding globulin present. The FTI is calculated from the T4 and T3U values. The FTI value can indicate when an abnormal level of T4 is due to an abnormal level of thyroxin-binding globulin in the blood.

This test is done on a blood sample taken from a vein.

Thyroid Hormone Tests
Results

Normal

Normal values may vary from lab to lab.

Greater than normal values may mean

High values for these tests may indicate an overactive thyroid gland (hyperthyroidism). This can be caused by Graves' disease, thyroiditis, a goiter that contains one or more abnormal growths (nodules), or from receiving too much thyroid hormone medication.

Lower than normal values may mean

Low values for these tests may indicate an underactive thyroid gland (hypothyroidism). This can be caused by thyroid disease (such as thyroiditis), pituitary gland disease, or destruction of the thyroid gland by surgery or radiation.

The T3U value may be useful in interpreting the other thyroid hormone test results.

• A higher-than-normal T4 value combined with a high T3U value usually confirms the presence of an overactive thyroid gland (hyperthyroidism).
• A higher-than-normal T4 value combined with a low T3U value often occurs during pregnancy or in women who take birth control pills.
• A lower-than-normal T4 value combined with a low T3U value usually confirms the presence of an underactive thyroid gland (hypothyroidism).
• A lower-than-normal T4 value combined with an increased T3U value may indicate kidney disease or long-term (chronic) illness. This can also occur normally in some healthy individuals.