Thyroid hormone tests

Thyroid hormone tests are blood tests that check how well your thyroid gland is working. 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 makes hormones that control the way your body uses energy (metabolism).

This test will give your doctor information about your thyroid hormone levels. You may have hyperthyroidism when your thyroid gland makes too much thyroid hormone. You may have hypothyroidism when the gland does not make enough thyroid hormone.

Thyroid hormone tests are done to:

• Find out what is causing an abnormal thyroid-stimulating hormone (TSH) test. This is the most common reason for thyroid hormone tests.
• Check how well treatment of thyroid disease is working. The total thyroxine (T4), free thyroxine (FT4), and free thyroxine index (FTI) values are often used to keep track of treatment for hyperthyroidism.
• Screen newborns to find out if the thyroid gland function is normal. A condition called congenital hypothyroidism can prevent normal growth and development and cause other severe problems, such as intellectual disability, if it is not treated soon after birth.

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. Your baby may have a tiny bruise where the heel was poked.

If your child takes thyroid medicines, tell the doctor when your child took the last dose. Your child may need to stop taking thyroid medicines for a short time before having these tests.

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, but they also may measure total thyroxine (T4) and T3 uptake (T3U). 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.