Diagnosis
Diagnostic Tests1
There are many blood tests used to diagnose and monitor patients with thyroid conditions.
No single lab test is considered 100% accurate in determining thyroid disease, but a combination of two or more tests may help
to determine if there is a problem with the thyroid gland (primary hypothyroidism) or the pituitary gland (secondary hypothyroidism). The Thyroid Stimulating Hormone (TSH)
and Free Thyroxine Index (FT4I) are the two blood tests used to diagnose patients with hypothyroidism and to
monitor their progress while using a thyroid medication.
Thyroid Stimulating Hormone (TSH)1: TSH, also referred to as thyrotropin, is released by the pituitary gland in response to thyrotropin-releasing hormone stimulation. Just as the name implies, TSH stimulates the thyroid gland to produce more thyroid hormone for the body. In summary, a low level of circulating thyroid hormone results in a release of TRH, which stimulates TSH production, and finally an increased production of thyroid hormones by the thyroid gland. TSH measurement may help the physician to determine if the problem is primary or secondary hypothyroidism. TSH measurement may also be used, in conjunction with other tests, to determine whether the dose of thyroid supplementation medication needs to be adjusted. For example, if the TSH level is high, an increase in thyroid supplement dose may be needed.
Total T4 Immunoassay (Total T4, T4 RIA, thyroxine, or T4)2: This test is used to determine the total amount of T4 in the blood, representing both the bound (to proteins) and unbound portions of T4. The majority of T4 in the blood is bound to protein and therefore has no activity. The amount of free versus bound T4 can be measured with a separate test.
Free Thyroxine Index (FT4I)2: This test is used to determine the amount of free (unbound) T4 in the blood. It is important to look at the amount of free T4 since this is the portion of thyroid hormone available to become active in the body.
Total T3 Immunoassay (Total T3, T3RIA, L-triiodothyronine, or T3)2: This test is used to determine the total amount (both bound and unbound portions) of T3—the more potent and active form of the two thyroid hormones—in the blood. Like T4, if the hormone is bound to protein, it is considered inactive. This test is most often used in the diagnosis of different types of hyperthyroidism, such as Graves’ disease. In hypothyroidism, these levels may often remain within the normal range.
The normal values for the thyroid blood tests may vary with different laboratories.
It is always important to compare your results with the normal values provided by your laboratory.
It is also important to pay particular attention to the units of measure (ie, µg/dL versus mmol/L) to be sure you are comparing similar entities.
The following are average normal values1-4:
| Name |
Normal Value
|
Results in
Hypothyroidism |
Results in
Hyperthyroidism
|
|
Thyroid Stimulating Hormone (TSH) |
0.4–4.0 mIU/L
|
High
|
Low
|
|
Total T4 Immunoassay |
58–160 nmol/L
(4.5–12.6 µg/dL) |
Low |
High
|
|
Free T4 Index
|
9–23 pmol/L
(0.7-1.8 ng/dL) |
Low |
High
|
|
Total T3 Immunoassay
|
1.2–2.7 nmol/L
(80–180 ng/dL)
|
Normal or Low |
High |
|
Free T3 Index
|
3.5-7.7 pmol/L
(0.2-0.5 ng/dL) |
Normal or Low |
High
|
Wide Range of Variation
Within the range of normal for thyroid hormone concentration (0.4 – 4.0 mIU/L),
there is a rather wide degree of variation from individual to individual.1
When a patient has symptoms suggestive of hypothyroidism but is shown to have lower normal values of thyroid hormone, a physician may start a trial of thyroid hormone therapy, hoping to increase these lower normal thyroid hormone values to upper normal values. If there is a sustained favorable response without any evidence of clinical hyperthyroidism, long-term replacement therapy may be justified.
1. WebMD. Health A to Z. Tests & Tools A to Z. Thyroid-Stimulating Hormone (TSH).
Available at: www.webmd.com/a-to-z-guides/Thyroid-Stimulating-Hormone-TSH. Accessed April 25, 2007.
2. WebMD. Health A to Z. Tests & Tools A to Z. Thyroid Hormone Tests.
Available at: www.webmd.com/a-to-z-guides/Thyroid-Hormone-Tests. Accessed April 25, 2007.
3. American Thyroid Association. Thyroid Function Tests. Available at: http://www.thyroid.org/patients/patient_brochures/function_tests.html.
Accessed February 27, 2007.
4. Demers LM, Spencer CA. NACB: Laboratory Medicine Practice Guidelines. LMPG: Laboratory support for the diagnosis and monitoring
of thyroid disease. Available at: http://www.nacb.org/lmpg/thyroid_lmpg_word.stm.
Accessed February 27, 2007.
|