Test Overview

Thyroid testing

Thyroid hormone tests are blood tests that check how well the thyroid gland is working. The thyroid gland makes hormones that regulate the way the body uses energy.

The thyroid gland is a butterfly-shaped gland that lies in front of your windpipe (trachea), just below your voice box (larynx). The thyroid gland uses iodine from food to make two thyroid hormones: thyroxine (T4) and triiodothyronine (T3). The thyroid gland stores these thyroid hormones and releases them as they are needed.

Thyroid hormones are needed for normal development of the brain, especially during the first 3 years of life. Intellectual disability may occur if a baby's thyroid gland does not produce enough thyroid hormone (congenital hypothyroidism). All newborns are screened for congenital hypothyroidism. Older children also need thyroid hormones to grow and develop normally, and adults need the hormones to regulate the way the body uses energy (metabolism).

Thyroid hormone blood tests include:

Total thyroxine (T4) and free thyroxine (FT4).

Most of the thyroxine (T4) in the blood is bound to a protein called thyroxine-binding globulin. Less than 1% of the T4 is free. Free thyroxine affects tissue function in the body, but bound thyroxine does not. A total T4 blood test measures both free and bound thyroxine but is not available in BC as it has been replaced by the more relevant free thyroxine test. Free T4 can be measured directly.

Triiodothyronine (T3) and free triiodothyronine (FT3).

Most of the T3 in the blood is attached to thyroxine-binding globulin. Less than 1% of the T3 is unattached. Free triiodothyronine (FT3) can be measured directly. T3 has a greater effect on the way the body uses energy than T4, even though T3 is normally present in smaller amounts than T4.

Why It Is Done

Thyroid hormone tests are done to:

• Find out what is causing an abnormal thyroid-stimulating hormone (TSH) test. For more information, see the topic Thyroid-Stimulating Hormone (TSH). This is the most common reason for thyroid hormone tests.
• Check how well treatment of thyroid disease is working. The free thyroxine (FT4) value is often used to keep track of treatment for hyperthyroidism or hypothyroidism, if TSH is abnormal or if TSH is normal and a patient has pituitary disease.

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How To Prepare

If you are taking thyroid medicines, tell your doctor when you took your last dose. You may need to stop taking thyroid medicines for a short time before having this test.

How It Is Done

Blood test

A health professional uses a needle to take a blood sample, usually from the arm.

Heel stick

A heel stick is used to get a blood sample from a baby. The baby's heel is poked, and several drops of blood are collected. The baby may have a tiny bruise where the heel was poked.

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How It Feels

Blood test

When a blood sample is taken, you may feel nothing at all from the needle. Or you might feel a quick sting or pinch.

Heel stick

A brief pain, like a sting or a pinch, is usually felt when the lancet punctures the skin. Your baby may feel a little discomfort with the skin puncture.

Risks

Blood test

There is very little chance of having a problem from this test. When a blood sample is taken, a small bruise may form at the site.

Heel stick

There is very little risk of a problem from a heel stick. Your baby may get a small bruise at the puncture site.

Results

Normal

Each lab has a different range for what's normal. Your lab report should show the range that your lab uses for each test. The normal range is just a guide. Your doctor will also look at your results based on your age, health, and other factors. A value that isn't in the normal range may still be normal for you.

Results are usually available within a few days.

Labs generally measure free T4 (FT4) levels, if the TSH is abnormal. Results of these thyroid hormone tests may be compared to your thyroid-stimulating hormone (TSH) results.

Many conditions can change thyroid hormone levels. Your doctor will talk with you about any abnormal results that may be related to your symptoms and past health.

High values

High thyroid hormone levels (hyperthyroidism) may be caused by:

• Diseases of the thyroid gland, such as Graves' disease, thyroiditis, or a goiter that contains one or more abnormal growths (nodules).
• Taking too much thyroid medicine.

Low values

Low thyroid hormone levels (hypothyroidism) may be caused by:

• Thyroid disease, such as thyroiditis.
• Pituitary gland disease.
• Destruction of the thyroid gland by surgery or radiation.