Thyroid Hormone Tests

T3 (Triiodothyronine) Test, T4 (Thyroxine) Test, Thyroxine (T4) Test, Triiodothyronine (T3) Test

Results

Thyroid hormone tests are blood tests that check how well the thyroid gland is working.

Normal

Normal values may vary from lab to lab. Results are usually available within a few days.

Labs generally measure free T4 (FT4) levels, but 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.

Thyroid hormone tests

Total thyroxine (T4):	9.8–22.6 micrograms per deciliter (mcg/dL) in newborns (1 to 14 days of age) 5.6–16.6 mcg/dL in babies and older children 5–14 mcg/dL in adults
Free thyroxine (FT4):	0.8–2.4 nanograms per deciliter (ng/dL)
Total triiodothyronine (T3):	32–250 ng/dL in newborns (1 to 14 days of age) 83–280 ng/dL in babies and older children 80–230 ng/dL in adults
Free triiodothyronine (FT3):	0.2–0.6 ng/dL
Free Thyroxine Index (FTI):	7.5–17.5 in newborns (1 to 14 days of age) 5.0–12.8 in babies and older children 4.2–13.0 in adults

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 medical history.

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.

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Author:	Caroline Rea, RN, BS, MS	Last Updated: December 3, 2007
Medical Review:	Caroline S. Rhoads, MD - Internal Medicine David C.W. Lau, MD, PhD, FRCPC - Endocrinology & Metabolism
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