How to Interpret Thyroid Function Tests?

The thyroid gland plays a crucial role in various processes in the body, including maintaining bone strength, heart health, immune system function, and metabolism. Therefore, it is essential to understand how to interpret thyroid function tests to interpret the results and understand the diagnosis.

How to interpret thyroid function tests?

Understanding the results of thyroid function tests can be challenging for anyone because the diagnosis can vary based on the relationship between different values. Depending on the extent of the elevation or reduction in various values, the doctor can determine the type of problem (hyperthyroidism or hypothyroidism) and the possible causes. Here, we will explain the common types of thyroid tests, as well as how to interpret thyroid function tests clearly and simply.

1. Thyroid-Stimulating Hormone (TSH) Test

This test measures the level of thyroid-stimulating hormone (TSH) in the blood, which is a hormone produced by the pituitary gland in the brain. Its main function is to stimulate the thyroid gland to produce its hormones: thyroxine (T4) and triiodothyronine (T3).

When thyroid hormone levels are low, the pituitary gland produces more TSH to stimulate the production of these hormones. Conversely, when thyroid hormone levels are normal, the pituitary gland stops producing TSH.

The normal range for TSH levels in the blood is 0.4 - 4.5 milli-international units per liter (mIU/L). Low levels indicate hyperthyroidism, while high levels indicate hypothyroidism.

2. Thyroxine (T4) Test

This test measures the level of free or total thyroxine (T4) hormone in the blood, produced by the thyroid gland, and plays a crucial role in regulating energy, mood, and body temperature.

Total T4 refers to the overall amount of hormone in the body, while free T4 refers to the unbound amount that is not bound to proteins (the active form that affects the body). Typically, this test, along with the TSH test, is among the most common thyroid tests.

The normal range for total T4 is 4.5 - 12.5 micrograms per deciliter (mcg/dL), while free T4 ranges from 0.8 - 2.0 nanograms per deciliter (ng/dL). 

Abnormal levels may indicate: 

• High levels: hyperthyroidism, thyroiditis, high iodine levels, high blood protein levels, or excessive intake of thyroid medications.
• Low levels: hypothyroidism, iodine deficiency, or a possible pituitary disorder.

3. Triiodothyronine (T3) Test

This test measures the level of triiodothyronine hormone, which is the active thyroid hormone produced from the conversion of T4 hormone. There are three types of this hormone:

• Free Triiodothyronine (Free T3): This is the active unbound form that enters the body's tissues.
• Total Triiodothyronine (Total T3): This is the total amount of this hormone, whether bound to proteins or unbound.
• Reverse Triiodothyronine (Reverse T3): This is an inactive form, but it blocks the active form from entering tissues.
  

The normal range for total T3 hormone is 80 - 215 nanograms per deciliter (ng/dL), while free T3 ranges from 1.5 - 6 picograms per milliliter (pg/mL). Abnormal levels typically indicate Graves' disease, an autoimmune disorder usually associated with hyperthyroidism.

4. Thyroid Antibodies Test

While this test is not common, doctors may order it to confirm the diagnosis of autoimmune thyroid diseases and determine their underlying causes.

This test measures the level of antibodies produced by the immune system to attack the thyroid gland in certain autoimmune disorders, including:

• Thyroid Peroxidase Antibodies: These antibodies are detected in 95% of individuals with Hashimoto's disease and 70% of those with Graves' disease.
• Thyroid-Stimulating Hormone Receptor Antibodies: These antibodies are present in 90% of Graves' disease cases and 10% of Hashimoto's disease cases.
• Thyroglobulin Antibodies: These antibodies are found in 80% of individuals with Hashimoto's disease, 50-70% of those with Graves' disease, and in 1 out of every 4 individuals with thyroid cancer.

The normal ranges are:

• Less than 34 international units/mL is the normal ratio for thyroid peroxidase antibodies.
• Less than 1.0 international units/mL is the normal ratio for thyroglobulin antibodies.

Interpreting Thyroid Test Results

The diagnosis can vary based on the relationship between different values. Therefore, we will simplify for you in the following table how to interpret thyroid test results based on the different values of the most common tests:

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