Normal Thyroid Function Tests

Thyroid Function Tests Interpretation

Thyroid function tests interpretation may vary slightly from one laboratory to another, so anyone taking thyroid function tests should consult with their doctor about their scores.

Before discussing normal thyroid function tests interpretation, it's probably a good idea to describe what thyroid function tests are and what they measure. 

The term "thyroid function tests" covers a range of lab tests which evaluate the production of two active thyroid hormones:

• thyroxine, also known as T4, and
• triiodothyronine, which is also called T3

Thyroid function tests may also be used to determine if  production of thyrotropin is normal.  Thyrotropin is the thyroid-stimulating hormone (TSH) secreted by the pituitary gland.

Tests that measure serum thyrotropin and thyroxine are the most widely used and considered to be the best.

TSH test normal range TSH is a laboratory test using a blood sample to measure the amount of thyroid stimulating hormone or TSH in the blood. Normal range of test results is from .3 to 3, which indicates no symptoms of abnormal thyroid function. Lower than normal TSH tests may be an indication of

• hyperthyroidism or overactive thyroid (also called high thyroid)
• TSH deficiency
• Use of certain medications (including dopamine agonists, glucocorticoids, somatostatin analogues, and bexarotene)

High TSH tests may indicate:

• congenital hypothyroidism (also known as cretinism)
• primary hypothyroidism
• TSH-dependent hyperthyroidism
• Thyroid hormone resistance
• Exposure to mice (this happens occasionally with lab workers or veterinarians).

Total T4 / Serum thyroxine test T4 or thyroxine is a hormone produced by the thyroid gland. A laboratory test using a blood sample can be done to measure the amount of T4 in the patient's blood.

Normal thyroid function scores for this test range from 4.5 to 11.2 micrograms per deciliter (mcg/dL). Lower than normal thyroid function test levels of T4 may be a sign of:

• use of certain medications
• some form of illness
• malnutrition or fasting
• hypothyroidism or underactive thyroid (also called low thyroid)

Higher than normal levels of T4 combined with low levels of TSH may result from hyperthyroidism conditions, or overactive thyroid, including:

• toxic multinodular goiter 
• subacute or chronic thyroiditis 
• iodine-induced hyperthyroidism
• high levels of the protein that carries T4 in the blood (this usually occurs during pregnancy, through use of contraceptive pills or estrogen, liver disease, or as part of an inherited condition). See thyroid problems in women. 
• Hashimoto thyroid disease (early stages)
• Graves' disease.

Free T4 / Free Thyroxine - FT4 test This test evaluates thyroid function in those who may have protein abnormalities which could affect total T4 levels. To better understand this test, it's important to know what free thyroxine, or FT4, is. T4 circulates in the blood in two forms:

• T4 which is bound to proteins that prevent it from entering the various tissues that need thyroid hormone and
• so called "free T4," which is not bound to the above mentioned proteins, meaning it can enter various target tissues and have an impact on their performance.

The free T4 fraction plays a key role in determining thyroid function. Normal scores range from 0.7 to 2.0. Scores under 0.7 are may indicate hypothyroidism. A score above 2.0 may indicate hyperthyroidism.

Combining the TSH test with the FT4 accurately evaluates thyroid gland function.

T3 / Serum triiodothyronine test Triiodothyronine, or T3 is a thyroid hormone that plays a significant role in controlling metabolism. The amount of T3 in a patient's blood stream is measured through a laboratory blood test.

A score between 100 and 200 ng/dL (nanograms per deciliter) is considered normal. Low scores could be an indication of

• hypothyroidism or underactive thyroid,
• severe malnutrition or starvation,
• some kind of long-term illness.

Higher than normal levels may indicate:

• high levels of a protein that carries T3 in the blood (this could in pregnancy, through the use of contraceptive pills or estrogen, liver disease, or as part of an inherited condition)
• hyperthyroidism or overactive thyroid
• thyroid cancer, though this situation is rare

To read related information, click on thyroid scan and thyroid uptake tests and thyroid blood tests. 

 

        

 
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